Download - Solar Wind Conditions and Composition During the Genesis Mission as Measured by in situ Spacecraft

Space Sci Rev (2013) 175:125–164DOI 10.1007/s11214-013-9960-2

S P E C I A L C O M M U N I C AT I O N

Solar Wind Conditions and Composition Duringthe Genesis Mission as Measured by in situ Spacecraft

Daniel B. Reisenfeld · Roger C. Wiens · Bruce L. Barraclough · John T. Steinberg ·Marcia Neugebauer · Jim Raines · Thomas H. Zurbuchen

Received: 7 August 2012 / Accepted: 19 January 2013 / Published online: 19 February 2013© Springer Science+Business Media Dordrecht 2013

Abstract We describe the Genesis mission solar-wind sample collection period and thesolar wind conditions at the L1 point during this 2.3-year period. In order to relate the solarwind samples to solar composition, the conditions under which the samples were collectedmust be understood in the context of the long-term solar wind. We find that the state ofthe solar wind was typical of conditions over the past four solar cycles. However, Genesisspent a relatively large fraction of the time in coronal-hole flow as compared to what mighthave been expected for the declining phase of the solar cycle. Data from the Solar WindIon Composition Spectrometer (SWICS) on the Advanced Composition Explorer (ACE) areused to determine the effectiveness of the Genesis solar-wind regime selection algorithm.The data collected by SWICS confirm that the Genesis algorithm successfully separatedand collected solar wind regimes having distinct solar origins, particularly in the case ofthe coronal hole sample. The SWICS data also demonstrate that the different regimes areelementally fractionated. When compared with Ulysses composition data from the previoussolar cycle, we find a similar degree of fractionation between regimes as well as fractionationrelative to the average photospheric composition.

The Genesis solar wind samples are under long-term curation at NASA Johnson SpaceCenter so that as sample analysis techniques evolve, pristine solar wind samples will beavailable to the scientific community in the decades to come. This article and a compan-ion paper (Wiens et al. 2013, this issue) provide post-flight information necessary for theanalysis of the Genesis array and foil solar wind samples and the Genesis solar wind ionconcentrator samples, and thus serve to complement the Space Science Review volume, TheGenesis Mission (v. 105, 2003).

Keywords Solar wind · Solar wind composition · Solar wind sample collection · Solarcomposition

Electronic supplementary material The online version of this article(doi:10.1007/s11214-013-9960-2) contains supplementary material, which is available to authorizedusers.

D.B. Reisenfeld (�) · R.C. Wiens · B.L. Barraclough · J.T. Steinberg · M. Neugebauer · J. Raines ·T.H. ZurbuchenUniversity of Montana, Missoula, MT, USAe-mail: [email protected]

http://dx.doi.org/10.1007/s11214-013-9960-2

mailto:[email protected]

126 D.B. Reisenfeld et al.

Definitions, Abbreviations, and AcronymsB Bulk collectors that were at the top of the stack and in the Canister lid.

These were exposed continuously during the science collection periodE Collector array directly below the B array. This array was exposed to

coronal mass ejection flows and questionable flowsH Collector array below the E array. This array was exposed to

high-speed, or coronal hole flowsL Bottom collector array in the stack. This array was exposed to

low-speed, or interstream windS Collectors in the SRC lid, primarily to investigate radioactive nuclei

in the solar windCME Coronal mass ejectionsCH Coronal hole, or fast windIS Insterstream, or slow windS/C SpacecraftSRC Science return capsuleSKM Station keeping maneuvers, which occurred approximately every 2

monthsLOI L1 orbit insertion, which occurred prior to the beginning of the

science collection phase of the missionL1 The Lagrangian point between the Earth and the SunUnshaded position Rotational position of the deployable solar-wind collector arrays

where individual, regime-specific arrays were exposed.Deployed position Rotational position of the deployable solar-wind collector arrays

where the B array remained during collection, and where theregime-specific arrays were positioned when they were not exposedor acting as a contamination barrier

1 Introduction

The Genesis mission was launched in August 2001 to obtain and return a sample of the solarwind for detailed isotopic and elemental analyses (Burnett et al. 2003). The Genesis samplereturn capsule (SRC) returned to Earth on September 8, 2004. Despite a hard landing thatresulted from a failure of the avionics system to deploy the parachute, many samples werereturned in a condition that has permitted analysis. These analyses have been carried out bya number of different techniques used for precise isotopic and elemental abundances. Nearlyall analyses have utilized mass spectrometry in various forms: noble gas mass spectrometry,typically using laser ablation to introduce the sample (e.g., Meshik et al. 2007; Grimberg etal. 2006, 2008; Vogel et al. 2011; Pepin et al. 2012; Heber et al. 2012); secondary ion massspectrometry (SIMS), which uses an ion beam to interrogate the sample (e.g., Marty et al.2011; Huss et al. 2012); a hybrid instrument combining an accelerator, typically used in anaccelerator mass spectrometer, with the main elements of a SIMS instrument (Mao et al.2008; McKeegan et al. 2011); resonance ionization mass spectrometry (RIMS), which useslaser beams to selectively produce excited states and then ionize only the element of interest(e.g., Veryovkin et al. 2004; Crowther and Gilmour 2011); and inductively coupled plasmamass spectrometry (ICP-MS; Humayun et al. 2011). At least one non-mass spectrometrytechnique has been used as well, e.g., grazing-incidence X-ray fluorescence, to determineelemental compositions in solar-wind samples from Genesis (Kitts et al. 2009).

Solar Wind Conditions and Composition During the Genesis Mission 127

A wealth of elemental and isotopic solar wind composition information is present in theGenesis samples. For this reason, the Genesis samples are under long-term curation at theNASA Johnson Space Center, in the same facility that houses the Apollo lunar samples,and we expect analysis of the Genesis samples to continue for many decades to come. Theexpectation of the mission designers is that future advances in sample analysis technologywill lead to even more accurate determination of solar isotopic and elemental abundancesthan what is possible today (Burnett et al. 2003). It is thus important to document in oneplace the collection time periods and the state of the solar wind as measured by in situ in-struments during these collection periods so that current and future sample analysis can beplaced in the proper solar wind context. The solar wind is not by any means an unfraction-ated sample of the Sun, nor is the means by which fractionation occurs fully understood(e.g. von Steiger et al. 2000). Thus, a more comprehensive set of solar wind conditions andcomposition characteristics have been recorded to help elucidate physical processes respon-sible for compositional variations found during the collection period. Ultimately we seek toplace Genesis results into the context of the long-term composition of the solar wind, andthe composition of the Sun and solar nebula.

The Genesis solar wind samples were collected on a variety of substrates, exposed to thesolar wind either continuously throughout the 27-month collection period or only at timesof certain types of solar wind flow (see Fig. 1). In particular, three collector panels werededicated to specific flow types: flow emanating from coronal holes (CH), from the inter-stream region near closed-loop boundaries (IS), and from coronal mass ejections (CMEs)(Neugebauer 1991). All three types of solar wind are elementally and isotopically fraction-ated in different ways and amounts relative to the solar photosphere. Thus not only do weneed to understand the state of the solar wind averaged over the Genesis sample collectionperiod, but also solar wind conditions within these three regimes. Furthermore, since thedetermination of solar wind regimes was performed autonomously by Genesis (Neugebaueret al. 2003), it is important to confirm the validity of the choices made by comparison tocontemporaneous solar wind parameters not accessible to Genesis during the mission.

This article and its online appendices serve as a comprehensive repository of solar-windconditions that we deem relevant to the samples. The conditions recorded here include (a) thetimes during which the various samples were collected, (b) solar-wind conditions recordedby the Genesis Ion Monitor (GIM) (Barraclough et al. 2003), and (c) the solar-wind com-position recorded by the Solar-Wind Ion Composition Spectrometer (SWICS) on board theAdvanced Composition Explorer (ACE), which was also orbiting the L1 point during thisperiod (Gloeckler et al. 1998).

We also explore questions related to the how successfully the Genesis mission met adefining design goal—how to relate Genesis samples to photospheric abundances. In partic-ular:

• How typical was the plasma state of the solar wind during the Genesis sample collec-tion period as compared to the long-term state of the solar wind? Is the Genesis samplerepresentative of the solar wind, or is it somehow atypical?

• How successful was Genesis at collecting solar wind of different origins (interstream,coronal hole, CME) on the different regime arrays. Specifically with regard to the Genesisregime selection algorithm, how good a job did it do at identifying CMEs?

We will also rely heavily on the composition measurements of the ACE/SWICS instru-ment to tell us about the average elemental fractionation in the Genesis samples within theaccuracy of the ACE measurements:


Fig. 1 The Genesis spacecraft viewed in solar wind collection configuration. Three of the five solar windcollector arrays are shown exposed. The array in the Science Canister lid and the top array of the stack (theBulk arrays) continuously collect solar wind particles. One regime-specific array can be see in its unshadedconfiguration. The edges of the other two regime-specific arrays can be seen stowed below a Bulk array. Theinside of the SRC lid (backshell) contains molybdenum foils that were also used for sample collection

• What does ACE/SWICS tell us about how elementally fractionated we expect the differentregimes to be?

• How fractionated are the regimes as compared to the photosphere?

We begin our analysis by describing in Sect. 2 the solar wind instrumentation used tocollect in situ solar wind data, and the nature of the data sets. In Sect. 3, we present the col-lection times for each of the different collectors (Sect. 3.1), and then describe the fluences ofH and He as measured by GIM (Sect. 3.2), and then the plasma state of the solar wind duringthe mission (Sect. 3.3), comparing it to the average state of the solar wind over the past 40years. Section 4 follows, detailing the success of the regime selection algorithm. In Sect. 5we consider the expected elemental fractionation between different regimes (Sects. 5.1–5.2)and between the solar wind and the photosphere (Sect. 5.3). Finally, in Sect. 5.4, we look atthe expected fluences of certain elements, and how they are fractionated with depth in thesamples.

2 Observations

Genesis was a spin-stabilized spacecraft, rotating at 1.6 rpm. During the science mission,the spin axis pointed 4◦ from the spacecraft-Sun line, along the velocity vector of the Earth,so that the collection surfaces would face directly into the oncoming solar wind. The solar


Fig. 2 Position of the Genesis spacecraft during the sample collection period. Genesis was located in a haloorbit about L1 liberation point, 106 km sunward of the Earth. The upper panel gives the GSE y-coordinateof Genesis (solid curve), and the lower panel gives the GSY z-coordinate. The time periods when Genesiswas actively collecting solar wind samples are indicated by the gray bar. The orbital parameters for the ACEspacecraft, also in an L1 halo orbit, are given (dashed curve) for comparison

wind conditions during the Genesis sample collection period that are reported in this articlewere primarily derived from three sources: the GIM instrument, the Genesis Electron Moni-tor (GEM), and the ACE/SWICS instrument. Data from the Genesis monitors (Barracloughet al. 2003) include the following solar wind parameters from GIM: proton density, velocity,temperature, alpha/proton ratio, alpha particle temperature, and from GEM a set of param-eters characterizing electron distribution functions that signal bi-directional electron (BDE)streaming along the solar wind magnetic field direction. All these parameters were collectedat a 2.5-minute cadence. Both GIM were GEM are spherical-section electrostatic analyzers,which sampled the plasma phase space distribution by stepping across energy per chargeand collecting counts. The primary purpose of the Monitor data was to determine in realtime the type, or regime, of solar wind impinging on the collectors. These data were thusfed into an on-board robotic algorithm that automatically determined the solar wind regimeand commanded the proper collector array to be unshaded (Neugebauer et al. 2003).

During the main sample collection phase of the Genesis mission, the Genesis spacecraftwas located in a halo orbit about the Earth-Sun interior libration point (L1), placing it inpristine solar wind flow. During the course of the mission, Genesis executed five halo orbits(see Fig. 2) having an approximate major axis of 1.6 × 106 km in the ecliptic and a minoraxis of 6 × 105 km perpendicular to the ecliptic plane (Burnett et al. 2003).


The solar wind composition instrument ACE/SWICS measures solar wind speed, massand charge independently by providing a series of three measurements in succession(Gloeckler et al. 1998): an electrostatic analysis, a time-of-flight mass spectrometer withpost-acceleration, and a solid state detector for total energy measurements of the ions. Formany ions, the separation is unambiguous and with negligible background due to the triplecoincidence measurement scheme involved. For other ions, the neighboring ion peaks over-lap in measurement space and a complicated, often statistically limited fitting process isemployed (for details see von Steiger et al. 2000. Although that paper describes the anal-ysis of data from the Ulysses/SWICS instrument, it is relevant to ACE/SWICS, as the twoinstruments are nearly identical). From these measurements the following data products arepublically available: the 4He2+ density; the bulk velocity and thermal velocity of 4He2+,C5+, O6+ and Fe10+; the elemental abundance ratios for He/O, C/O, Ne/O, Mg/O, Si/O, andFe/O on two-hour cadences, and N/O and S/O on one-day cadences; and the charge statedistributions for C, O, Ne, Mg, Si, and Fe (Raines et al. 2005). Typically, the accuracy ofthe SWICS quantities is estimated at 20 % or better for the most abundant ions. However,ions affected by the peak overlaps and fitting are N, Ne, as well as some charge states ofMg, S, and Si, and their error bars are therefore significant and dependent on model retrievalmechanisms and systematic uncertainties of the instrument (see Appendix of von Steigerand Zurbuchen 2011).

Since its launch in 1997 and its delivery and test-out phase, ACE has also been in an L1halo orbit, with somewhat smaller dimensions than the Genesis orbit, having an approximatemajor axis of 500,000 km in the ecliptic and a minor axis of 300,000 km perpendicular tothe ecliptic (see Fig. 2) (Stone et al. 1998).

3 Collection Conditions for Arrays

3.1 Collection Times

Solar wind samples were collected by Genesis during the time period August 24, 2001through April 1, 2004. This corresponds to the declining phase of solar cycle 23, startingjust after solar maximum (see Fig. 3). Table 1 provides a summary of exposure statistics forthe Genesis collector arrays. In addition, Table A1 of the Appendix lists the individual timeintervals for solar-wind collection for the passive collectors. The Science Return Capsule(SRC) lid (or “backshell”, indicated by the column labeled “S” in Table A1; see also Fig. 1)was opened not long after launch (August 24, 2001), but was closed to an opening angle of10.2◦ 28.88 days later because the temperature of the SRC was rising much more rapidlythan expected. It was then re-opened after L1 orbit insertion (LOI) on November 26, 2001(see timeline in Fig. 2). The opening angle of the backshell was between 192.2◦ and 193.2◦during its exposure time, being at the lower value after December 3, 2001. The total exposuretime of the SRC lid foils was 886.84 days. [Note: at an opening angle of 180◦ the contactingportion of the backshell is parallel to its counterpart on the capsule side. An opening angleof 180◦ would be the ideal angle for collection.]

Exposure of the arrays contained within the Science Canister (see Fig. 1) began after LOIon November 30, 2001, about five months after launch. Originally this time was to allowoutgassing of the spacecraft, though the backshell could be open only a few degrees most ofthe duration because of the above-mentioned thermal concerns. The Bulk collector arrays—one in the Canister lid and one at the top of the array stack—were continuously exposed for852.83 days. The angle of the Canister lid during exposure was effectively 180◦, so that the


Table 1 Exposure statistics for the Genesis collector arrays

Collector Duration Number of ExposureIntervals

Mean Exposure Duration

Bulk 852.83 d 1 –

IS 333.67 d 146 2.29 d

CH 313.01 d 94 3.33 d

CME 193.25 d 107 1.79 d

Fig. 3 International sunspot number for solar cycle 23 (SIDC-team, 1996–2008). The Genesis sample col-lection period (bracketed by the vertical dashed lines) occurred during the declining phase of the cycle, justafter solar maximum

exposure angle of its collectors to the Sun and solar wind was identical to the other Canistercollectors. During this time, the regime-specific arrays were cycled in and out of the stack toexpose them during times when data from the GIM and GEM monitors indicated that theirintended solar-wind regime was incident upon the spacecraft. The overall exposure times foreach of these arrays tallied to 333.67 days for the IS collector array, 313.01 days for the CHcollector array, and 193.25 days for the CME collector array (see further details in Table 1).The column in Table A1 labeled “Gap” indicates the time intervals between the end of oneinterval and the beginning of the next entry in the table. In most cases this corresponds to thetime it took to switch between arrays. This fractional time varied depending on the numberof arrays that needed to be moved, but was always less than five minutes.

The array motions result in an uncertainty in the exposure time for each regime-specificcollector. This is all the more true because the hard landing dislodged almost every sampletile on a given array from its holder, and thus samples cannot be unambiguously tracedto a specific location within the array. Each time the array was moved from the deployed(stacked) to the unshaded (exposed) position, one side of the array was exposed first, andthat same side was the last to be shaded when the array was moved back to the deployed


Table 2 Estimated H and He fluences, fluxes, and ratios for the different Genesis sample collector types

Collector Duration H Fluence(cm−2)

H Flux(cm−2 s−1)

He Fluence(cm−2)

He/H FluenceRatio

〈nHe/nH〉

SRC Lid 886.84 d 2.14e16 2.79e8 8.40e14 0.0393 0.0433

Concentrator 803.28 d 1.95e16 2.81e8 7.62e14 0.0391 0.0416

Bulk 852.83 d 2.06e16 2.80e8 8.09e14 0.0391 0.0433

IS 333.67 d 9.15e15 3.17e8 3.19e14 0.0349 0.0388

CH 313.01 d 6.40e15 2.37e8 2.52e14 0.0394 0.0398

CME 193.25 d 4.73e15 2.83e8 2.26e14 0.0478 0.0537

(stacked) position. These motions had an uncertainty of about a minute per array movement.The cumulative effect of these motions is an uncertainty of several hundred minutes for eachcollector, estimated conservatively to be ±0.2 days. The uncertainty on the exposure timesfor the Concentrator, the Bulk collectors, and the SRC lid foil collectors is estimated atonly a few minutes each. Fortunately, the tiles from each array have a unique array-specificthickness, so it was at least possible to associate each tile fragment with the correct array.

3.2 H and He Fluences

The H and He fluences were determined from Genesis Ion Monitor (GIM) data for eachtime interval listed in Table A1 and grouped by collector. The results are given in Table 2and Fig. 4. We determine the H and He fluences from all time intervals in each regime andcalculate their fluence distributions as a function of particle speed, as well as total fluencesfor that solar wind regime. By definition the fluence, Fs , for each particle species s, whetherH or He, is

Fs =∑

i

�Fs,i =∑

i

(∑

j

fs,i,j νs,i�νs,i�tj

)(1)

where �Fs,i is the differential fluence for the i-th speed bin, fs,i,j denotes the 1-D phasespace density, νs,i denotes the speed of the i-th bin for particle s,�νs,i denotes the widthof the speed bin, and �tj denotes the length of the j -th data cycle, which for Genesis istypically ∼150 s. The 1-D phase space density fs,i,j is derived from GIM counts vs. E/q

measurements, summed over angle (Barraclough et al. 2003). As such, it has dimensions ofparticles per unit volume per unit speed.

On occasion the phase space density was not available. Unavailable data are not a sig-nificant issue in calculating the H fluence, as less than 0.5 % of the data is missing for thebulk H fluence. On the other hand, over 11 % of the bulk He data are not available. This isbecause at certain times, such as when the H temperature is high, or when the He abundanceis very low, it is not possible to resolve a He peak in the GIM E/q data.

We estimate the missing fluence by assuming the phase space density fs,i,j can be ap-proximated by Maxwellians defined using proxy alpha particle moments (nHe, νHe, and THe)that are built from a simple predictive model to achieve a first-order approximation for themissing data. The model is based on an analysis of the monitor data for the times when allmoments were valid. In particular, we determine a linear relationship between the knownalpha and proton speeds (both measured in km s−1), which provides missing alpha speedsfor the periods where we have good proton speeds:

νHe = 1.09νH − 15.6 (2)


Fig. 4 Relative amounts of slow, fast (coronal hole), and CME wind by collection time, and by H and Hefluences as measured by GIM during the respective collection periods

Fig. 5 Distribution of alpha particle speeds as a function of proton speeds over the course of the Genesismission. The white line is a linear least-squares fit (Eq. (2)), which is used to predict alpha particle speeds fortimes when the alpha particle moments are missing

The data on which this fit was made are shown in Fig. 5, and supports the observations madeby others that to a good approximation, the difference between the alpha and proton speedslinearly increases with proton speed (see, e.g., Neugebauer 1981; von Steiger et al. 2000).From Fig. 5, it is clear that a linear fit should provide reasonably accurate predictions of themissing alpha speeds.

In Fig. 6, we show that the alpha temperature correlates well with the alpha speed, so wedetermine the mean alpha temperature (in Kelvin) as a function of alpha speed, and fit thisto two linear relations, one for speeds above 315 km s−1, and the other for speeds below:

THe = 1.74 × 103νHe − 4.91 × 105(νHe ≥ 315 km s−1

)

(3)THe = 540νHe − 1.13 × 105

(νHe < 315 km s−1

)


Fig. 6 Alpha particle temperature distribution as a function of alpha particle speeds over the course of theGenesis mission. The white curves represent two contiguous linear fits to the distribution, with a break at315 km s−1 (Eq. (3)), which are used to predict alpha particle temperatures for times when the alpha momentsare missing. Note that the fits are to the mean of the temperature distribution within each alpha speed bin. Thedistribution is significantly skewed below 300 km s−1 toward higher temperatures, thus the mean no longertracks the most probable temperature value. This is why the fit curve below 300 km s−1 falls above the peakof the distribution

In Fig. 6, it can be seen that below ∼300 km s−1, the fit to the alpha temperature falls abovethe peak of the temperature distribution. This is because the distribution forms a broad tailto higher temperatures, and thus the mean no longer tracks the most probable value. Fordetermining the missing helium fluence, the mean temperature is the relevant parameter.

In Fig. 7, we show that the logarithm of the alpha density (in cm−3) also correlates bestwith alpha speed, and we determine a logarithmic relation between alpha density and speed:

log10(nHe) = −1.09 × 10−3νHe − 0.136 (4)

In this case, the distribution is so broad that the correlation not particularly good; thus, theuse of Eq. (4) to predict individual missing densities would not be particularly accurate.However, our purpose is to approximate the missing helium fluence, so an average trend isstill a meaningful proxy for missing data.

Finally, we note that Genesis went into safe mode from DOY 296.67–307.93 during thesolar super storms of late-October/early-November 2003 (also known as the “HalloweenEvent of 2003” (Skoug et al. 2004)). For this period, all instrument voltages were turned tozero and no array operations occurred. The SRC Lid, Bulk and CME collector arrays wereexposed, but phase space densities for both H and He were unavailable from GIM. For thisinterval we used ACE/SWEPAM (McComas et al. 1998a) data to estimate the fluence atGenesis. These data are provided in the form of ion moments, so to accurately determine thefluences, these moments were used to define Maxwellian flux distributions from which thedifferential fluences (see Eq. (1)) were calculated. At times SWEPAM could only provide


Fig. 7 Alpha particle density distribution as a function of alpha particle speed over the course of the Genesismission. The white line represents a linear fit to the logarithm of the density distribution (Eq. (4)), which isused to predict alpha particle densities for times when the alpha moments are missing

proton data, and the method described above was used to recover the alpha fluences. TheCME array was unshaded for the entire interval.

For several of the highest-speed points during this period, the high-energy part of thesolar wind distribution exceeded the energy sweep range of SWEPAM. During these times,from DOY 302.906 to 304.029, estimates of the H speed and density were obtained fromMaxwellian fits to the velocity distribution rather than from moment integrals (R. Skoug,private communication; cf. Skoug et al. 2004). The He velocity distribution is not resolvedduring this time interval.

Although the Genesis safe-mode period accounts for only 0.5 % of the proton fluence(0.7 % of the alpha fluence), it is nevertheless important to account for the fluence contri-bution from this period. The highest solar wind speeds of the entire Genesis mission wererecorded during this period, exceeding 1850 km s−1 (Skoug et al. 2004)! Thus the deepestembedding of solar wind particles occurred at this time. It is quite probable that laboratorysample analysis has isolated solar wind from this event (e.g., Grimberg et al. 2006, 2008),which explains why differential fluences are given up to speeds as high as 1800 km s−1 inTables A2–A7 (see Sect. 5.4).

Table 2 gives the proton and alpha fluences in each regime as well as the fluences on theBulk collector (including the gold and aluminum foils on the deck of the Science Canister),and the SRC lid collector. We also present the total proton and alpha fluences incident on thesolar wind ion concentrator while it was in operation, that is, when ions were being steeredtoward the concentrator target. This does not necessarily mean the concentrator targets col-lected all of this flux. In order to determine whether proton or alpha fluxes were able toreach the target requires modeling of the concentrator performance, and this is consideredin Wiens et al. (2013, this issue).


3.2.1 He/H Fluence Ratios vs. Density Ratios: A Warning

The last two columns of Table 2 give the He/H fluence ratio and the average density ratios(〈nHe/nH〉), respectively. The density ratio is the average of a set of many short durationmeasurements (∼2.5 minutes in the case of Genesis), all equally weighted, whereas the flu-ence ratio is a ratio of the total amount of helium to the total amount of hydrogen collectedover a long time interval. Both of these ratios are often referred to as the “helium abun-dance”. A comparison of the two columns in Table 2 shows that for a given regime, thefluence and density ratios are not necessarily the same. For the CH period, the ratios werevery close, but for all others, they differ, by up to 12 % in the case of the CME regime.This comparison is made to point out the difference between a fluence ratio, as is typicallyreported in the cosmochemical sample analysis literature, and a density ratio, as is typicallyreported in the solar wind literature. (In this paper, we use the term abundance ratio to meana density ratio.)

For all regimes, the density ratio is greater than the fluence ratio. We can explain this inthe following way: Since the proton flux tends to increase with decreasing proton speed (seeTable 2; see also Feldman et al. 1977; Schwenn 1990), the He/H density ratio of slower windis more heavily weighted in a fluence measurement. Conversely, below about 500 km s−1,the average He abundance drops with speed (see Kasper et al. 2007), and so the fluencemeasurements in Table 2 that contain a substantial amount of material with incident speedsless than 500 km s−1 are weighted by the lower He abundance of the slower wind. Thecoronal hole sample, which contains material that was flowing at speeds mostly greater than500 km s−1 show little difference between the fluence and density ratios because the Heabundance is very uniform above this speed.

3.2.2 GIM Measurement Accuracy

The GIM instrument calibration is described in Barraclough et al. (2003). Estimated fluxeswere adjusted at the beginning of the mission to match ACE/SWEPAM and WIND obser-vations. The ACE/SWEPAM fluxes were also tied to WIND observations when its mis-sion began. The WIND ion sensor is a Faraday cup arrangement (Ogilvie et al. 1995),so it is considered very reliable as an absolute flux monitor (Maksimovic et al. 1998;Kasper et al. 2006). We can compare the GIM H and He fluence measurements over thecourse of the Genesis mission to those made by the ACE spacecraft, and in the case of He,also to Genesis regime-specific noble gas sample analyses recently reported by Heber et al.(2012). (We cannot compare total Genesis fluences to WIND directly, because the orbit ofWIND took it in and out of the solar wind during the Genesis sample collection period.)

To compare H and He fluences to ACE, we have utilized the 12-minute SWEPAM/SWICSmerged proton data set and the SWICS one-hour averaged helium data set available fromthe ACE Science Center (www.srl.caltech.edu/ACE/ASC/), separated according to the threesolar-wind regimes as determined by Genesis. Table 3 presents the fluences for each regime,as well as a computation of the differences. The derived H fluence values are all within a fewpercent between spacecraft, with the exception of the CME regime fluences, which differby 11 %. Even this difference, though, is within the expected measurement uncertainty of10–15 %. We recommend the adoption of the H fluence values reported here as a baselinefor comparison to Genesis sample analysis results. Due to the good agreement betweenthe GIM and ACE values, and their traceability to WIND Faraday cup measurements, wesuggest a measurement uncertainty on the H fluence values of ±10 % (1σ).

The situation with He fluences is more complicated. Between spacecraft, differences varyfrom 0 % for the CME regime to 28 % for the IS sample. We are inclined to attribute this to

http://www.srl.caltech.edu/ACE/ASC/


Table 3 Comparison of H and He fluences to ACE and to sample analysis values

RegimeCollector

GIM Hfluence(×1015

cm−2)

ACE Hfluence(×1015

cm−2)

% diff GIM Hefluence(×1014

cm−2)

He fluencefromsamples(×1014

cm−2)a

% diff ACE Hefluence(×1014

cm−2)

% diff

Bulk 20.6 21.3 3.4 8.09 8.29 −2.5 8.58 6.1

IS 9.15 9.35 2.2 3.19 3.15 1.3 4.08 27.9

CH 6.40 6.33 −1.1 2.52 2.57 −2.0 2.12 −15.9

CME 4.73 5.26 11.2 2.26 2.53 −11.9 2.26 0

a From Heber et al. (2012)

the experimental challenges and potential inaccuracies associated with deriving helium den-sities from energy-per-charge analyzers such as GIM and others. Normally, alpha particlesshow up as a well-resolved peak on the high-energy shoulder of the proton distribution in anE/q measurement at approximately twice the average E/q of the protons. However, whenthe solar wind is hot, highly non-Maxwellian or the alpha abundance is low, the alpha peakcannot be easily and unambiguously separated from protons (for example, Gosling et al.2005). Thus E/q measurements of the He abundance can be inaccurate under certain cir-cumstances. In contrast, the ACE SWICS instruments uses triple coincidences to determinealphas in a fashion that totally separates them from protons and other ions in measurementspace; thus no modeling is required to eliminate potential overlaps.

Nevertheless, when we compare to the laboratory sample analysis results of Heber et al.(2012), we find quite good agreement. For most cases, the GIM monitor and the samplesagree to within a few percent, with the exception of the CME regime, which differs by11 %. Again, these are within expected measurement uncertainty. Deviations should mostlikely be attributed to specific intricacies of the analysis of SWICS, GIM or laboratory dataor combinations thereof. With less likelihood, differences can also be caused by small orbitdifferences between ACE and Genesis mentioned previously (see Fig. 2). In any case, theagreement seen here is comparable to solar wind measurements between spacecraft anddifferent instrument types.

3.3 Plasma State of the Solar Wind During the Genesis Collection Period

In this final section on collection conditions we document the state of the solar wind duringthe Genesis sample collection period, for each regime as well as the Bulk sample collectionperiod. It is important to archive the plasma characteristics of the solar wind associated withthe elemental and isotopic composition in these periods. If we hope to gain understanding ofthe mechanisms responsible for fractionating the solar wind, information about the plasmastate can serve as inputs into fractionation models. We also ask the question: how does thesolar wind state during the Genesis mission compare to the solar wind state over longer peri-ods of time? To this end, we compare the Genesis collection period to solar wind conditionsmeasured over the course of a single solar cycle as well as over the past four solar cyclessince the beginning of regular in situ measurements of solar wind near Earth.

Solar cycle 23 reached maximum during 2000 (see Fig. 3) (SIDC-team 1996–2008). TheGenesis collection period lasted for 2.33 years, occurred shortly after maximum and intothe declining phase of the cycle. The preponderance of a given regime is well-known to


Fig. 8 Monthly averages for thefractional amount of time a givensolar wind regime was present.The sample collection period isindicated by the range betweenthe vertical dashed lines

change within a given 11-year cycle. The fast wind originates in the central regions of largecoronal holes, whereas the slow wind is believed to originate on the boundaries of coronalholes adjacent to magnetically closed regions (Bravo and Stuart 1997; Zurbuchen et al. 2002,2012; Neugebauer et al. 2002). Near solar activity minimum large coronal holes predominatenear the solar magnetic poles and often extend down to low heliographic latitudes. Thusquasi-stationary high speed streams are common in the ecliptic at these times. Near solaractivity maximum, however, magnetically closed regions dominate the magnetic structureof the Sun, and coronal holes exist at all latitudes and are more transient, often only lastingfor 100 days or less (Miralles et al. 2004). Thus, even though the fastest solar wind tends toall but disappear during solar maximum, coronal hole-associated wind remains an importantcontributor at all latitudes (Zurbuchen et al. 2002; Zhao et al., 2009).

For the Genesis collection period, Fig. 8 shows the relative fraction of the time a givenregime was identified each month based on the onboard algorithm. The plot shows inter-stream wind (green) dominating at the beginning of the collection period (demarcated bythe vertical lines) and again beginning to dominate at the end of the period, with coronalhole wind (blue) dominating for essentially all of 2003 (the accuracy of the regime selectionalgorithm will be discussed in Sect. 4.1). The distribution of solar wind regimes behaved asexpected through the end of 2002, but the preponderance of fast wind in 2003 is not typi-cal of ecliptic solar wind conditions during the declining phase of the solar cycle; thus theGenesis fast wind sample is larger than the pre-launch expectations (Wiens et al. 2003).

3.3.1 Comparison of GIM Measurements to the OMNI 2 Solar Wind Archive

The average state of the solar wind as measured by the GIM during the Bulk collectionperiod is given in Table 4, column 2, with OMNI 2 data for the Genesis collection periodand over the long term given in columns 3 and 4, respectively. In addition to the principalproton plasma moments, np,Vp, and Tp, we provide the proton flux npVp, the proton momen-tum flux (or dynamic pressure) npmpV

2p , the average magnetic field strength B , the average

Alfvén speed VAlfvén, and the ion (protons and alphas) beta β , the ratio of thermal pressureto magnetic field pressure. For quantities containing magnetic field data, we obtain B fromthe OMNI 2 archive. OMNI 2 is a multi-source data set beginning in November 1963 thatis maintained by the National Space Science Data Center (NSSDC) (King and Papitashvili2004). We use OMNI 2 as the source of solar wind data for the time period 1966–2006, aperiod over which the data set is mostly continuous (column 4). Beginning in 1998 with thelaunch of ACE, the OMNI 2 archive consists primarily of data from ACE, with gaps filled in


Table 4 Statistical properties of the bulk solar wind, comparing Genesis to solar wind data for the period1966–2006 (OMNI 2 dataset), and IMP 6/7/8 data from 1971–1974 (Feldman et al. 1977)

Parameter Genesis Period(GIM)

Genesis Period(OMNI 2)

1966–2006(OMNI 2)

% difference 1971–1974(Feldman et al.)

Vp (km s−1) 470 (119)a 487 (115) a 446 (105) a 8.4 % 468 (116) a

np (cm−3) 5.91 (5.30) 5.86 (4.77) 7.02 (5.68) −19.9 % 6.8 (4.6)e

Tp (×105 K) 1.26 (0.94) 1.40 (1.08) 1.13 (1.02) 19.2 % 1.2 (0.9)

npVp

(×108 cm−2 s−1)

2.57 (2.06) 2.64 (1.89) 2.90 (2.15) −9.8 % 2.7 (1.7)

npmpV 2p

(×10−8 dyne cm−2)

1.99 (1.64) 2.10 (1.74) 2.11 (1.64) −0.5 % 2.0 (1.3)

B (nT) –b 7.52 (3.17) 6.64 (3.24) 11.7 % 6.2 (2.9)

VAlfvén (km s−1)c 71.2 (36.7)b 70.9 (37.8) 63.5 (35.8) −10.5 % 60 (27)

Ion βd 0.53 (0.60)b 0.57 (0.64) 0.75 (0.99) −26.7 % 0.49 (0.63)

a Numbers in parentheses indicate one-σ variation of stated parameter (not error)

b Genesis had no magnetometer. Magnetic field data taken from OMNI dataset

c VAlfvén = B/√

4π1 + 4(nαnp

)mpnp

d β = 1.2npkBTp

B2/8π, where the factor of 1.2 accounts for the alpha particle temperature

e Feldman et al. (1977) quantities involving np have been scaled by a factor of 0.70 reflecting the results ofcross-calibration with Helios (see Schwenn 1990)

by WIND or IMP 8, where needed. We stop at 2006 because the four years to follow repre-sent the deepest and most extended solar minimum of the last century. This may or may notturn out to be a typical period, so we chose not to bias our comparison with this time period.

In order to make the comparison as self-consistent as possible, we also calculate thesolar wind parameters for the Bulk collection period using OMNI 2, given in column 3, tocheck against the GIM data. Comparing the two data sets, it is comforting to see that mostparameters agree to within a few percent. This agreement is not entirely fortuitous, sincethe calibration of GIM was tied to ACE (see Sect. 3.2.2). The largest discrepancy is thetemperature, which is about 11 % higher in the OMNI 2 data set. From a data reductionpoint of view, temperature, which is derived from the second moment, is sensitive to non-Maxwellian characteristics and also to statistical uncertainties of the observed distributions.

3.3.2 Genesis Collection Period in the Context of the Long-Term Solar Wind

The global structure of the heliosphere is radically reorganized between solar minimumand solar maximum. At solar minimum, the slow wind is confined primarily to an equa-torial region surrounding a roughly planar heliospheric current sheet. At higher latitudes,the heliosphere is dominated by flow emanating from large polar coronal holes. As solarmaximum approaches, the heliospheric current sheet begins to smoothly warp toward in-creasingly higher latitudes (Gazis 1996; McComas et al. 1998b) and becomes increasinglydisorganized. Tracking this evolution, at high latitudes, the solar wind density and velocityexhibit strong shifts—from steady high speed/low density flow at solar minimum to muchlower speed, denser, and highly variable flow at solar maximum (Ebert et al. 2009).

Despite the radical reconfiguration that the heliosphere undergoes over the course of thesolar cycle, the character of the ecliptic wind slows little consistent periodic change, aside


from the cyclic variation of the CME frequency (Riley et al. 2006). The variations of mostsolar wind parameters in the ecliptic (e.g. n,V,T , and B) show no obvious organization bysolar cycle. As Gazis (1996) points out, long-term (>1 year) changes in ecliptic solar windparameters are observed, but these do not necessarily repeat from cycle to cycle. Richardsonet al. (2001) show that a notable exception is the dynamic pressure (nmV2), which consis-tently varies by about a factor of two from maximum to minimum over the course of multiplesolar cycles. Furthermore, by comparing measurements from Ulysses, Voyager and IMP 8,they show that this variation is essentially independent of latitude.

We expect the solar wind sample collected by Genesis to be reasonably representative ofthe ecliptic solar wind average, so long as the collection period is long enough to averageover short-term solar source variability. To confirm this, we compared the Genesis periodto the forty-year span of ecliptic solar wind observations from 1966–2006. We do in factfind that the Genesis period was rather comparable to the forty-year average. In column 5 ofTable 4, we explicitly calculate the percent differences. The spread in most parameters is onthe order of 10–20 %. In truth, considering this comparison enlists data sets from over 15spacecraft, any differences are well within systematic error.

If we were to attribute any significance to the differences, we could say that during theGenesis period, the solar wind was slightly faster (8 %), less dense (20 %), and of highertemperature (19 %) than the long-term average. This is likely due to the fact that an un-expectedly large fraction of the collection period that contained coronal hole flow (recallFig. 8). The other difference of significance is that the magnetic field strength during theGenesis period was about 12 % higher than average. This can be traced to the fact that therewere a large number of CMEs during the Genesis period, typical of the declining phase ofsolar maximum (see, e.g. Riley et al. 2006). Interestingly, the dynamic pressure, which isthe parameter that has shown the clearest solar cycle variation (Richardson et al. 2001), isthe same to better than 1 % between the Genesis period and the forty-year average. We thusconclude that the 2.3-year Genesis sample is in fact representative of the long-term averageecliptic solar wind.

The last column of Table 4 gives the solar wind description of Feldman et al. (1977)compiled from IMP6/7/8, which is one of the most oft-quoted sets of solar wind parameters.We included it here for comparison because it for so long has defined the “standard” solarwind. In addition, the period it represents, 1971–1974, coincides with nearly the same phaseof the solar cycle as the Genesis period. The 1971–1974 period was remarkably similar to theGenesis period, the largest difference being in the average magnetic field, which was about20 % stronger during the Genesis period. Otherwise, the parameter differences between theperiods were <15 %. Note that all IMP-based quantities involving the proton density thatappear in Table 4 reflect a scaling of the proton density by a factor of 0.70 from valuespresented in Feldman et al. (1977), as suggested by Schwenn (1990), resulting from an in-flight cross calibration between IMP and the Helios plasma instruments. [We do not do adirect comparison to the Helios data because Schwenn (1990) does not present a completeset of the parameters to which we wish to compare.]

3.3.3 Comparing Solar Wind Regimes for the Genesis Period to a Complete Solar Cycle

We next consider the solar wind state specifically during the different Genesis regime col-lection periods compared to the same type of flow, but accumulated near the ecliptic over asolar cycle. To accurately make such a comparison, we need to process the OMNI 2 solarwind through the Genesis regime algorithm. This is not difficult to do for selecting IS andCH regimes, but in order to identify CMEs, the Genesis algorithm relies on identification


of bi-directional electron streaming, a parameter that is not available in the archived solarwind data. To overcome this, we need to rely on compilations of near-Earth CMEs to serveas a proxy for the CME identification portion of the algorithm. Ideally we would compareto the same forty-year period as above, but the only continuous CME compilation availablefor a full solar cycle is the Richardson and Cane (R&C) list (Cane and Richardson 2003)described in Sect. 4.2.1. Their list covers the entirety of solar cycle 23: 1996–2008. Becauseof the desire to avoid any contamination of the quasi-stationary regimes by CME material,the criteria of the Genesis algorithm are less restrictive than the criteria used by Richardsonand Cane, so the hypothetical CME sample we arrive at will contain fewer CMEs than if theexact criteria of the Genesis algorithm were applied. We are at least able to invoke part ofthe Genesis CME determination logic: we extend the ‘duration’ of the R&C CMEs at least6 hours beyond the R&C stop time, and ensure that the duration of a CME interval is at least18 hours.

The average solar wind parameters for the regime periods resulting from applying theGenesis algorithm to a complete solar cycle are compared to the Genesis period regimes inTable 5. For consistency, the data for the Genesis period are also from the OMNI 2 archive,appropriately sorted according to the Genesis periods. For each regime we also compute thepercent difference between the Genesis period and the full solar cycle. Perhaps the mostimportant observation is that the coronal hole regimes differ by only a few percent for allparameters, except the ion plasma β .

There are a few additional observations from this analysis: A review of Table 5 indicatesthat the solar wind parameters of the Bulk period show larger differences than any singleregime. This is likely due to the fact that the relative amounts of the three solar wind typesin a full solar cycle differ from that of the Genesis period. As already mentioned, the Genesisperiod contained an unusually high fraction of CH wind, and this is reflected in the differ-ences in Table 6, as most of the differences go in the direction expected if the Genesis periodhad a larger fraction of CH wind. The last point we comment on is that despite the fact thatwe had to rely on the manually-prepared R&C CME list (Cane and Richardson 2003), thedifferences between the Genesis period CME regime and that for the full cycle are consid-erably smaller than what we expected. The largest differences are that the full cycle sampleis significantly cooler (24 %), higher in helium abundance (6 %), and has a stronger mag-netic field (8 %) on average than the Genesis period. These are all consistent with the notionthat the full-cycle CME period has a higher fraction of true CME plasma than the Genesisperiod. This is to be expected, since the Genesis algorithm is less restrictive.

4 Regime Selections and Accuracy

The Genesis regime selection algorithm processed data from the Genesis electron and ionspectrometers, determining the solar wind regime in real time on board the spacecraft. Thealgorithm is fully described in Neugebauer et al. (2003). Here we focus on those aspects ofthe algorithm relevant to solar wind composition. Figure 9 shows the decision flow chartused to determine one of three cases: whether the observed solar wind is (1) fast windoriginating from a coronal hole (CH), (2) slow wind from an interstream source (IS), or(3) CME plasma. Based on this determination, collection array changes are made withinminutes of determination of a regime change.

The following assumptions underlie the algorithm:


Tabl

e5

Stat

istic

alpr

oper

ties

ofso

lar

win

dre

gim

es,c

ompa

ring

gene

sis

colle

ctio

npe

riod

toa

com

plet

eso

lar

cycl

e(1

996–

2006

)

Para

met

era

Bul

kC

oron

alH

ole

Inte

rstr

eam

CM

E

Gen

esis

Peri

odFu

llC

ycle

diff

(%)

Gen

esis

Peri

odFu

llC

ycle

diff

(%)

Gen

esis

Peri

odFu

llC

ycle

diff

(%)

Gen

esis

Peri

odFu

llC

ycle

Dif

f(%

)

V(k

ms−

1)

487

441

9.5

587

570

2.9

408

386

5.5

461

471

−2.1

n(c

m−3

)5.

866.

66−1

3.7

3.76

3.76

0.0

7.30

7.76

−6.4

6.72

6.73

−0.1

Tp

(×10

5K

)1.

401.

0624

.32.

061.

993.

80.

970.

7522

.91.

040.

7924

.3

npV

p

(×10

8cm

−2s−

1)

2.64

2.74

−3.6

2.16

2.12

1.9

2.92

2.92

0.1

2.93

3.04

−3.7

npm

pV

2 p

(×10

−8dy

necm

−2)

2.11

1.98

6.1

2.12

2.04

3.8

2.01

1.87

6.9

2.24

2.42

−8.0

Nα/N

p(%

)3.

583.

414.

72.

762.

92−5

.83.

803.

2713

.94.

825.

10−5

.8

B(n

T)

7.52

6.53

13.2

6.89

6.45

6.3

7.47

6.04

19.1

8.68

9.37

−8.0

VA

lfvé

n(k

ms−

1)

70.9

59.2

16.5

74.7

70.7

5.5

61.9

49.0

20.8

79.7

91.3

−14.

6

Ion

β0.

570.

73−2

80.

690.

81−1

60.

550.

77−4

00.

410.

3320

aA

llda

tais

from

OM

NI

2da

tase

t


Table 6 Fraction of time spent by Genesis collectors in each flow type, as determined by O7+/O6+ criteria

Collector Percentage of Time Spent in Flow Type

IS CH CME

IS 61.6 34.4 4.0

CH 8.7 90.0 1.3

CME 41.4 25.2 33.4

Fig. 9 Flow chart showing theGenesis on-board logic fordetermining the solar windregime in real time

(1) CH and IS flow (collectively referred to as quasi-stationary flow) originate from solarsources that exhibit distinct abundance signatures. Such flows are reasonably character-ized by speed. We therefore identify CH and IS material based on solar wind speed anduse the speed to determine which collector array to unshade.

(2) Elemental abundances in CMEs are highly variable and are not necessarily related toCH or IS abundances. We therefore identify and collect CME material on a separatecollector.

(3) The algorithm is biased toward minimizing the contamination of the CH sample byCME or IS wind. This is because the composition of the CH wind is expected to be lessfractionated with respect to the photosphere than the CME or IS winds.

(4) Although solar wind speed is related to the outflow source (CH or IS), the solar windundergoes hydrodynamic evolution which can obscure the source signature. The recenthistory of the solar wind can be used to partially recover some of this information. Inparticular: (a) on the leading edge of a stream interface (where high-speed flow ramsinto lower-speed flow), initially slow wind has been accelerated; thus, although at the


Fig. 10 Proton fluence per 25 km s−1 bin measured by GIM during the regime and bulk collection periods.The velocity distribution for the SRC lid and the Concentrator collection periods are essentially identical tothe Bulk curve shown here, except for the slightly higher (SRC lid) and lower (Concentrator) fluences

location of observation the wind is fast, it will have an abundance expected of slowerwind. Thus when a slow-to-fast transition occurs, the speed set point for retracting theIS collector and unshading the CH collector is set relatively high: 525 km/s. (b) Onthe trailing side of a fast stream, originally fast wind has been decelerated due to therarefaction caused by fast wind outrunning slow wind; thus, although at the locationof the observation the trailing wind is slow, it will have the abundance of faster flow.Thus when a fast-to-slow transition is observed, the speed set point for retracting theCH collector and unshading the IS collector is set relatively low: 425 km s−1.

(5) Interplanetary shocks often precede stream interactions and CMEs, as a result of thepile-up of originally slow, dense wind ahead of overtaking fast wind. At 1 AU, if theambient wind was slow to begin with, the wind following a forward shock is likely tostill have a composition typical of the slow wind, even though it may now be movingfast enough to be classified as fast wind. Shocks are such good indicators of acceler-ated slow wind, that the algorithm simply assumes that the twelve hours following aninterplanetary shock consists of solar wind having an IS composition. Thus when thealgorithm detects a shock, if the IS array is already unshaded, it remains exposed for 12hours regardless of the following flow type.

(6) Regardless of the flow type, rapid non-persistent excursions in solar wind parameterscould possibly be interpreted as a change in regime. Therefore, for a regime change totrigger an array change, it must persist for at least half an hour. In addition, the hysteresislogic described in point 4 above also serves to keep intermittent speed fluctuations fromactivating too many array changes.

Figure 10 shows the results of the solar-wind regime selection in terms of the velocitydistribution of the protons measured by GIM. Supporting data for both H and He are givenin Tables A2 and A3. There is a clear overlap between the CH and IS regimes, as would beexpected from the algorithm settings discussed above, while the CME regime reaches overthe entire velocity range. One can see the extension of IS to speeds as high as 600 km s−1;this is a result of the 12-hour hold following a shock detection. The overlap begs the questionof the effectiveness of the regime selection algorithm at separating the solar wind based


on the solar source. This, as well as the accuracy of the onboard CME selection, will bediscussed in the next section.

4.1 CH and IS Regime Selection Accuracy

The Genesis regime selection algorithm is designed to separate coronal hole from inter-stream material, not simply by setting a speed boundary, but also by using the recent his-tory of the solar wind to decide the origin of solar wind at intermediate speeds. A possiblymore accurate determinant of the origin of the non-transient solar wind is the relative chargestates of O7+ and O6+, the two most abundant charge states of oxygen in the solar wind (vonSteiger et al. 1997). Similarly, a ratio of C6+ and C4+ has been suggested as a very success-ful quantity to separate solar wind regimes (Landi et al. 2012). These ionic charge statesreflect the electron temperature near the freeze-in point, where recombination times becomelonger than the expansion time-scales. A “freeze-in temperature”, the electron temperatureat the freeze-in point, can be approximated from the observed ionic charge state ratios underthe assumption of local equilibrium. Thus, observations of the charge distribution at greatdistances from the sun can yield a measure of the electron temperature at the coronal source.The coronal temperature is significantly lower in coronal holes than above coronal loops ornear the edges of streamers, where the low speed wind is thought to originate. Thus themeasured charge state distribution serves as an excellent marker for the solar source of theplasma. To this end, Zurbuchen et al. (2002), and more recently, Zhao et al. (2009) haveused the O7+/O6+ ratio to separate CH and IS flows. Unfortunately for Genesis, this diag-nostic was not well established at the time the Genesis mission was developed, but we cancarry out an after-the-fact comparison to see how well the implemented Genesis algorithmperformed compared to the charge state separation method.

To make the comparison, we have sorted the ACE/SWICS observations made duringthe Genesis collection period by solar wind regime as determined in flight by the Gen-esis regime selection algorithm. We then binned the O7+/O6+ data within each regimeby solar wind proton speed as measured by the GIM. The results of this are shown inFig. 11. A remarkably monotonic decline of the O7+/O6+ ratio with speed is observed,across the full range of speeds, by over an order of magnitude. Note there is no sharpboundary at a specific speed, as has been pointed out by others (von Steiger et al. 2001;Zurbuchen et al. 2002). Following Zhao et al. (2009), we indicate in Fig. 11 the criterionfor CH wind to be O7+/O6+ < 0.145, which was determined as the ionic ratio at whichO7+/O6+ had the smallest sensitivity to the actual solar wind speed. The vast majority(90 %) of observations that Genesis attributed to CH flow are seen to be below this O7+/O6+level. This indicates that the algorithm was largely successful at isolating coronal hole ma-terial on the CH sample (CMEs will be addressed in the next section). On the other hand,there are a considerable number of observations (34.4 %) that Genesis considered to be ISflow that may in fact originate from coronal holes, according to the charge state criterion.See Table 7 for a tabulation of the percentages of each solar wind type on each collector asdetermined by the O7+/O6+ ratio analysis.

From a first look at Fig. 11, it does not seem that the hysteresis algorithm successfullyseparated flows in the overlap region between 425 and 525 km s−1. It appears that mostof the overlap flow was in fact of CH origin. However, a statistical analysis of the overlapregion shows that the algorithm was at least partially successful—the average O7+/O6+ ratioin the IS period is in fact 5.5 (±3.5) % higher than in the CH period (see Table 8). This doesnot necessarily mean that the same is true for elemental composition. We will return to thispoint in Sect. 5.1.


Table 7 Mean abundances and charge state ratios vs. regime

IS CH CME Bulk IS/CH Ratioa Photospherica,b

He/O 77.0 84.0 93.3 83.2 0.917 (0.005) 186 (4.29)

C/O 0.672 0.702 0.625 0.672 0.957 (0.004) 0.589 (0.068)

N/O 0.139 0.154 0.138 0.145 0.901 (0.011) 0.148 (0.020)

Mg/O 0.131 0.118 0.200 0.141 1.106 (0.010) 0.087 (0.0182)

Si/O 0.166 0.168 0.180 0.170 0.991 (0.009) 0.071 (0.0049)

S/O 0.062 0.037 0.064 0.053 1.691 (0.053) 0.029 (0.0020)

Fe/O 0.116 0.080 0.146 0.109 1.447 (0.015) 0.069 (0.0064)

C6+/C5+ 1.189 0.609 1.491 1.042 1.950 (0.031) –

O7+/O6+ 0.249 0.087 0.526 0.253 2.854 (0.058) –

fFIP 1.852 1.509 2.413 1.846 1.226 (0.009) –

a Uncertainties are given in parentheses

b From Asplund et al. (2009)

Fig. 11 O7+/O6+ charge state ratio as a function of solar wind proton speed and Genesis solar wind regime.The points indicate the mean values of the parameter within bins 25 km/s wide. The thin lines indicate thevariability over the measurement period, spanning the 10- to 90-percentile ranges. Points are only includedif there are at least 16 hours of measurements for the given speed bin. The statistical uncertainty in the meanis typically a few percent or less. The horizontal dashed line indicates the criterion of Zhao et al. (2009) forthe division between coronal hole (<0.145) and interstream flow (>0.145). The diagonal line indicates theirthreshold for CME identification (see text for further explanation)

4.2 CME Regime Selection Accuracy

The Genesis mission marks the first time that a spacecraft determined the solar-wind regimein real time on-board the spacecraft. Of particular significance was the ability of the regimeselection algorithm to determine the passage of an interplanetary coronal mass ejection.Referring to Fig. 9, CMEs were identified via the presence of three indicators in a weightedcombination: an enhanced alpha to proton density ratio, a depressed proton temperature(that is, lower than expected based on the measured proton velocity), and the presence ofbi-directional electron (BDE) streaming. Roughly, the CME state was triggered if at leasttwo of these three indicators were simultaneously present. See Neugebauer et al. (2003) fordetails on the exact weighting function employed by the onboard algorithm.


Table 8 Interstream-to-coronal hole regime abundance and charge state ratios between 425 km/s and525 km/s

IS/CH Ratio Uncertainty

He/O 1.026 0.010

C/O 0.987 0.005

N/O 0.961 0.021

Mg/O 1.092 0.017

Si/O 1.040 0.019

S/O 1.260 0.077

Fe/O 1.032 0.019

C6+/C5+ 1.031 0.026

O7+/O6+ 1.055 0.035

f 1.083 0.016

From the post-mission perspective, the CME identification algorithm worked remarkablywell. Given that the CME material was expected to be the most fractionated relative to theSun, the regime selection was biased towards exposing the CME array whenever there wasthe possibility of being in a CME regime. The Genesis spacecraft did not have a magne-tometer, an instrument considered to be very helpful in identifying CMEs. We have goneback over the data and compared the Genesis conditions with ACE/SWEPAM moments,including the ACE magnetometer data. The results of this review on the regime selectionare given in the last column of Table A1. A check mark indicates agreement with the regimethat was selected. Disagreement or questionable results are indicated by a question mark orother comments.

4.2.1 Comparison to Richardson and Cane CME List

The identification of CMEs can be a subjective process (for review, refer to Zurbuchenand Richardson 2006). In fact, one can argue that CMEs lie on a continuum from ma-jor solar events down to minor perturbations nearly indistinguishable from the “normal”quasi-stationary solar wind. Thus to more rigorously test the Genesis CME regime selec-tion, we compare to the interplanetary CME list compiled by Richardson and Cane (Caneand Richardson 2003). From 1996 through 2008, these researchers have maintained a com-prehensive list of interplanetary CMEs observed in the near-Earth environment (hereafter,the “R&C list”). This list is based on a compilation of numerous solar wind markers (avail-able online at: http://www.ssg.sr.unh.edu/mag/ace/ACElists/ICMEtable.html). We mentionthat the CME list compiled by Jian et al. (2006) also spans the Genesis mission, and for thisperiod the two lists have about 80 % overlap. In this analysis, we have compared the Gene-sis CME selections to only the R&C list. For the 107 times Genesis entered the CME state,we have divided the events into 4 categories: intervals containing events from the R&C list(35),1 intervals not on the R&C list but, based on review of the ACE SWEPAM data, weconfirm are in fact intervals containing CMEs (22), intervals not on the R&C list that weidentify as CMEs but with less confidence (12), and episodes that are definitely not CMEs

1These 35 intervals contain 51 entries from the R&C list. Due to the persistence condition of the Genesisregime algorithm (see Sect. 4.2.3), multiple CMEs were often collected during the same regime interval, incases where CMEs followed in rapid succession.

http://www.ssg.sr.unh.edu/mag/ace/ACElists/ICMEtable.html


Fig. 12 Accuracy statistics on CMEs identified by Genesis regime selection algorithm. Events are catego-rized as: (blue) those that appear on Richardson and Cane (R&C) CME list (Cane and Richardson 2003),(red) those not on the R&C list, but identified here as CMEs with high confidence, (purple) those identifiedhere as possible CMEs but with less confidence, and (red) those that are definitely not CMEs

(38). Note that only ∼1/3 of the events that triggered the Genesis CME state were actuallyon the R&C list. The number of accurate CME regime triggers grows to slightly more thanhalf of all CME regime intervals when we add the additional events we identify as CMEs.The results are also summarized in Fig. 12a.

We can also organize the events in terms of total exposure time (Fig. 12b). When donethis way, we see that nearly half of the time that Genesis was in the CME state was duringevents on the R&C list, and only one quarter of the time that Genesis was in the CME statewas definitely not during CMEs.

One may ask how Richardson and Cane seemed to have “missed” so many CMEs thatwe consider correctly identified. Genesis CME regime selections that fall on the R&C Listtend to have longer durations than events not on the R&C list. On average, Genesis dwelledin the CME state for 2.75 days for R&C events, and 1.65 days for confirmed CMEs noton the R&C list, and 1.29 days for “possible” CMEs (see also Fig. 12c). Looking at thisanother way, we can determine the number of Genesis CME events that lasted less than oneday (Fig. 12d). Only 2 of the 35 R&C events were shorter than 1 day (6 %), whereas 7 of


22 “positive” CMEs not on the R&C list were this short (32 %). Thus events we identify asCMEs but that are not on the R&C list tended to be shorter and/or weaker than R&C events.

The 25 % of the Genesis CME regime exposure time classified as ‘not CMEs’ is oftenassociated with BDEs. This is because although BDE streaming is one of the primary in-dicators for the CME state, there are other causes for BDEs. One cause is reflection off ashock following the entry into a high-speed regime (Steinberg et al. 2005). This was recog-nized following the incorrect identification of a classic high-speed stream in January, 2002,early in the collection period, and resulted in a slight modification of the regime selectionalgorithm so that BDEs were less heavily weighted in the selection in cases where either thealpha-proton ratio or the temperature could not be determined. This could happen duringhot flows immediately following the onset of high-speed wind. Nevertheless, some BDEsassociated with high-speed streams were still able to trigger the CME state. BDEs associ-ated with high speed streams accounted for 25.5 days or 13 % of the CME exposure time,or over half of the erroneous CME regime exposure.

Another source of BDEs is reflection of electrons off of the Earth’s bow shock. The mostlikely time for this to occur is when the spacecraft is leading the Earth in its orbit aroundthe Sun, as the spacecraft is most likely to be lined up with the Earth along the magneticfield lines in the typical Parker spiral. Genesis led the Earth by >100 RE during 2002 DOY86-120 and 268-301, 2003 DOY 79-111, 260-292, and in 2004 it neared 100 RE for severaldays around DOY 79. The spacecraft got as far as ∼120 RE ahead of the earth at a sunwarddistance of ∼210 RE. During these periods there were false CME identifications on 2002DOYs 91-94, 97, and 117, and in 2003 on DOYs 283-287, totaling 6.7 days or 3.5 % ofthe CME exposure time. The regime-selection algorithm included a branch to determinewhether the Earth was in the approximate direction from which the sunward electrons wereflowing during BDEs, but this flag was set to be ignored because it could not be determinedhow to implement it properly.

4.2.2 Contamination of CH and IS Regimes with CME Material

As mentioned above, the regime selection algorithm is designed to give special protectionto the CH sample in particular. Thus the fact that, for example, our CME sample tendsto collect non-CME material is due to the conservative threshold designed to avoid CHcontamination. However, there are instances where the CH and IS samples are unavoidablyexposed to CME material. Because the CME detection algorithm requires the presence ofmultiple CME signatures, it is likely that a CME may have begun to pass Genesis before thealgorithm triggers. Even if the selection algorithm otherwise works perfectly, to avoid toomany array actuations, a regime change must persist for at least half an hour before arraysare switched. To determine how much time the other arrays were exposed to CME material,we compared the tabulated start times of the R&C CMEs to the Genesis CME state entrytimes. We find that on average, the entry into the CME state lags the R&C start time by 4.6hours. Taking all of the R&C events, and considering which regimes preceded the entry intothe CME state, this amounts to an accumulated exposure of the IS array to CME material of4.23 days, and of the CH array to CME material of 1.03 days.

One can also look at how often Genesis came out of the CME state too soon. For the 35R&C events, this happened only 3 times, and only once into the CH state, where it cameout 1.0 days too early. Note that usually, Genesis was programmed to stay in the CMEstate past the end of the actual CME, by 1.0 days on average. By number, the 35 R&CCMEs constitute about half of the 69 confirmed and possible CMEs detected by Genesis.Therefore, by extrapolating the statistics for CME late entry and early departure, the CH


array was exposed to CME material for at most 4 days, or 1.3 % of the 313 days the CHarray was unshaded.

Interestingly, three of the 54 CMEs on the R&C list during the Genesis period, amount-ing to 2.75 days of exposure, did not trigger the Genesis CME state. Looking at the ACESWEPAM data, these seem to be cases where all three CME triggers were present, but notwo were present simultaneously, thus not triggering Genesis. For all of these, Genesis wasin the IS state, so none of these events contaminated the CH sample. Adding up all thepieces, we conclude that the IS array was exposed to CME material for ∼15 days, or 4.5 %of the 334 days the IS array was exposed.

4.2.3 CME Regime Selection vs. Oxygen Charge States

Before leaving the topic of CME selection accuracy, we refer back to Fig. 11, which showsthe ACE O7+/O6+ ratios vs. solar wind speed and regime type. It is obvious that theO7+/O6+ ratio for CMEs is very well separated from the quasi-stationary wind except atthe lowest speeds. However, there is a consistent skew of some fraction of the material onthe CME collector to lower charge state ratios, indicated by the longer red lines below themean values, which suggests that some of the material captured in the CME sample is quasi-stationary. The O7+/O6+ ratio has been used as a marker for CME identification in the solarwind (Richardson and Cane 2004). Following Zhao et al. (2009) who performed R&C’sanalysis on an updated data-set, we apply a criterion that the material is of CME origin ifO7+/O6+ ≥ 6.008e(−0.00578vsw), where vsw is the solar wind proton speed. This is indicatedby the diagonal line in Fig. 11. The measurements above this line correspond to 33.4 % ofthe CME regime sample, and the rest quasi-stationary solar wind, according to the chargestate analysis (see last row of Table 6).

That such a large fraction of the CME sample is of non-transient origin is not surpris-ing. From the analysis just discussed, we know that the contamination is mostly due to theconservative design of the regime selection algorithm. To minimize the risk that the CHand IS arrays are exposed to CME material, the algorithm maintains the CME regime statefor either 18 hours, or 6 hours beyond when CME signatures are last detected, whicheveris longer. In this way, because CME signatures are often intermittent, enough persistenceis designed into the algorithm to keep it from leaving the CME decision state prematurely.As a consequence, the CME array is unavoidably contaminated with quasi-stationary flow.Nevertheless, the clear distinction between the composition of the CME regime and thequasi-stationary regimes in Fig. 11 is another confirmation that the algorithm successfullyisolates CME material.

Finally, we can use the Zhao et al. (2009) oxygen charge state ratio CME criterion asanother means of estimating the amount of contamination of the IS and CH arrays to CMEmaterial. Impressively, we get the same answer as in Sect. 4.2.2 above: the O7+/O6+ cri-terion indicates that 4.0 % of the IS and 1.3 % of the CH sample are composed of CMEmaterial, thus lending a strong confirmation to our belief that these arrays are relativelydevoid of CME-associated plasma.

5 Solar Wind Elemental Composition Measurements

The Genesis samples allow for very precise determinations of the isotopic and elemen-tal abundances in the various collector arrays, but with no time or energy resolution. On


the other hand, the ACE/SWICS instrument provides temporal- and energy-resolved abun-dance measurements, but only for the most abundant elements, and with an absolute ac-curacy of 20 % or so. (With the notable exception of 3He, the SWICS team does notroutinely publish isotopic abundances.) Nevertheless, the value of SWICS elemental com-position measurements for providing context for the Genesis sample collection cannot beoverstated. First, they can be used to determine the degree to which the regime samplesare expected to be compositionally distinct. Second, the trends that are revealed in theACE compositing data can provide guidance in directing future Genesis sample analysisobjectives. And third, the ACE abundance data can be used to calculate detailed energy-resolved fluence distributions that are invaluable for modeling the degree and depth of solarwind implantation in the samples. Depth profiles derived from ACE/SWICS data have al-ready been used by a number of sample analysis teams to refine their data analysis (e.g.Grimberg et al. 2006, 2008; Meshik et al. 2007; McKeegan et al. 2011; Heber et al. 2012;Huss et al. 2012) and preliminary discussions of ACE/SWICS composition data compiledover the Genesis sample collection periods are found in Reisenfeld et al. (2003, 2005, 2007).

5.1 Elemental Abundance Variations Across Regimes

Applying the same procedure as was used to sort the O7+/O6+ ratio, we have analyzed theabundances of various elements (as measured by ACE) as a function of solar wind protonspeed and regime type (as measured by Genesis). Figure 13 shows this data for the abun-dance ratios: Mg/O, Fe/O, Si/O, S/O, C/O, N/O, and He/O. All are reported on a 2-hourcadence, except for S/O and N/O, which are reported on a 1-day cadence. Typically, a givenregime is held for 2–3 days, so we expect some amount of regime blending for the sulfurand nitrogen data. We nevertheless find these useful to report, so long as the overlap is keptin mind. Neon is also measured by ACE/SWICS, however we choose not to present it at thistime because of the challenges in extracting accurate Ne abundances. The neon signal peaksoverlap significantly with much more abundant peaks (oxygen and others) in the energy (E)

vs. time-of-flight (TOF) analysis space. Furthermore, the Ne abundance in the photospherecan only be estimated from coronal abundances (see, e.g., Asplund et al. 2005).

To complement the elemental abundance plots, we have calculated the average abundanceratios for each regime and for the full mission (aka the “Bulk” regime), as well as the ratiosof the average IS to CH abundances. These are given in Table 7.

A cursory look at Fig. 13 clearly shows that the Genesis solar wind sample is elemen-tally fractionated as a function of regime and solar wind speed. Furthermore, the degree offractionation depends on species. This is no surprise, as it is well known that elemental frac-tionation of the solar wind depends on the time it takes for an element to become ionizedin the chromosphere, which in turn, depends strongly on the first ionization potential (FIP).Low-FIP elements (EFIP < 10 eV) tend to be enhanced over their photospheric abundancerelative to high-FIP elements (Geiss 1982), an effect which is more strongly observed in ISflow than in CH flow (see, e.g., von Steiger et al. 2000).

We now examine the abundances in quasi-stationary flows in more depth. Figs. 10a–10dshow the ratios of iron, magnesium, silicon and sulfur (all low-FIP elements), respectively,to oxygen (a high-FIP element), and Figs. 13e–g show the ratios of carbon, nitrogen andhelium (all high-FIP elements), respectively, to oxygen (von Steiger and Geiss 1993). Forthe low-FIP elements the abundance with respect to oxygen decreases with increasing solarwind speed, except for silicon. The variations for Fe/O and S/O show significant decreases,dropping by a factor of ∼2.5 each from 250 km s−1 to 800 km s−1. The Mg/O ratio shows


Fig. 13 Panels (a–g): Elemental abundances relative to oxygen as a function of solar wind proton speedand Genesis solar wind regime. Horizontal dot-dashed lines indicate the photospheric ratios derived fromAsplund et al. (2009). Panel (h): FIP enhancement factor. See Fig. 11 for further explanation of the plotconstruction

less decrease over this speed range, dropping by only a factor of ∼1.3 (neglecting the low-statistics extrema). This is all consistent with the tendency for the fractionation of low-FIPelements to be stronger in IS flow than in CH flow.


Contrary to the relatively steady abundance decline of most low-FIP elements with speed,the Si/O ratio remains relatively flat across most of the speed distribution, despite Si havinga FIP comparable to Mg and Si. Yet, there is a gradual drop in abundance above 625 km s−1,such that by 750 km s−1, the abundance has dropped by a factor of ∼1.2 from the meanspeed of the IS regime, which in fact agrees with Ulysses observations (von Steiger et al.2000), noting that the high speed sample considered by Ulysses had an average flow speedof 760 ± 30 km s−1 (McComas et al. 2001). However, why the Si/O ratio remains flat acrossmost of the speed distribution remains a mystery.

For the high-FIP elements helium (EFIP = 24.59 eV), carbon (EFIP = 11.26 eV), andnitrogen (EFIP = 14.53 eV) a different trend is observed, namely these abundances tendto increase relative to oxygen with increasing solar wind speed. The He/O ratio increasesthe most, by a factor of 2, with most of the increase between 250 and 450 km s−1. This isconsistent with the findings of Kasper et al. (2007) who investigated the He/H abundanceratio versus proton speed and phase of the solar cycle. They find that the He/H ratio isnearly constant above 500 km s−1, but drops steeply below this, trending toward zero at asolar wind speed of about 260 km s−1. This is probably not a FIP-related effect, but rathera demonstration of the unique role of helium in coronal dynamics. Although much lessabundant than hydrogen by number, it comprises about 15 % of the mass of the extendedcorona, and thus is a non-negligible participant in the plasma processes leading to solarwind acceleration. In fact, Kasper et al. (2007) show that the solar wind itself cuts off at260 km s−1, from which they surmise that helium has a throttling effect on the solar wind.

The C/O ratio shows a similar but weaker trend, rising by about 40 % between 250and 350 km s−1, and climbing slightly but steadily thereafter. Even though the increase isonly about 5 % between 350 and 400 km s−1, the variability of the C/O ratio is so small,the increase is nevertheless statistically significant. The low variability and the remarkableflatness as a function of solar wind speed above 400 km s−1 may be attributed to the factthat carbon and oxygen have nearly identical FIPs and also comparable mass per chargeratios, and thus presumably experience a very similar fractionation and acceleration history.The same may be said for nitrogen, which has a FIP that is also close to oxygen, and has aN/O ratio that also shows a gentle rise above 400 km s−1. As the solar wind speed decreasesbelow 400 km s−1, the N/O ratio rises slightly but significantly; thus, there is an abundanceminimum. The physical origin of this is not obvious, and may be an artifact of the dataanalysis method, as the nitrogen signal can be challenging to resolve on the shoulder of thecarbon and oxygen mass peaks. It should be said, though, that the N determination also isplagued by some experimental challenges, as discussed above and has a larger error thanboth C or O.

Figure 13 also shows that for certain species, CME material consistently exhibits abun-dance ratios higher than either the fast or slow wind for a given speed, a point first made byReisenfeld et al. (2003) and further elaborated by Richardson and Cane (2004). The separa-tion is strongest for Fe/O and Mg/O, but is also present for S/O and He/O. Interestingly, thespecies that show the least fractionation with speed, silicon, carbon and nitrogen, are alsothe ones showing little difference between CME and quasi-stationary flow.

5.2 Abundance Evidence for Separation of IS and CH Flow in the Overlap Region

For all elements and regimes, we see a smooth abundance variation with speed. This demon-strates that, as has been pointed out by Zurbuchen et al. (2002), there is not a sharp abun-dance boundary between high and low speed. Rather, at least to first order, fractionation isa continuous function of proton speed. Importantly, where the IS and CH regimes overlap,


we do see some separation in the elemental ratio, which we attribute to the hydrodynamicprocessing at flow boundaries described above. We have quantified the mean separation be-tween the slow and fast winds across the speed interval for which they overlap, and theresults are shown in the Table 8. Tabulated are the elemental ratios for the IS period dividedby those for the CH period over the overlapping velocity range. The uncertainty is reportedat the 1-σ level and is weighted by the number of samples for each ratio. Some statisticallysignificant separation is present for almost all species, ranging up to an IS/CH ratio of 26(±7.7) % for S/O.

In summary, by applying the Genesis algorithm results to ACE/SWICS elemental abun-dance data, we end up with compositionally distinct populations. Furthermore, we show thatthe solar wind speed history does discriminate between the IS and CH flow types. By usinga lower speed threshold for fast-to-slow than for slow-to-fast regime transitions, the Genesisalgorithm partially compensated for effects that are due to transit to 1 AU. Furthermore, weshow that on average, CME composition is distinct from the quasi-stationary flow. The algo-rithm successfully isolated the CME population, and thus protected the CH and IS samplesfrom contamination by CME material.

5.3 Fractionation Relative to the Photosphere

A key objective of Genesis is to derive solar abundances from solar wind abundances.The above discussion clearly shows that elemental fractionation occurs between solar windregimes, but how much fractionation do we expect between the regimes and the photo-sphere? On each of Figs. 13a–g the horizontal dashed line indicates the photospheric abun-dance of the respective species to oxygen (X/O) using the photospheric abundance data ofAsplund et al. (2009). The photospheric abundance ratios are also given in Table 7, col-umn 7. In most cases, the CH sample is closest to the photospheric abundance, which isconsistent with previous observations (von Steiger et al. 2000; Gloeckler and Geiss 2007;von Steiger and Zurbuchen 2011). This in fact validates a design objective of Genesis tokeep the CH sample as pure as possible.

The degree of fractionation in the solar wind (SW) with respect to the photosphere (Ph)can be quantified in a single parameter, the FIP enhancement factor fFIP, which gives the ra-tio of low-FIP to high-FIP elements in the solar wind relative to the same ratio computed forthe photosphere. In the literature, one finds slightly different definitions of the enhancementfactor. It is defined here as:

fFIP = ([Mg]+[Si]+[S]+[Fe]

[C]+[N]+[O] )SW

([Mg]+[Si]+[S]+[Fe]

[C]+[N]+[O] )Ph

, (5)

where the square brackets around elements symbols denote elemental abundances. In addi-tion to providing a single parameter to describe fractionation, fFIP is less sensitive to certainSWICS analysis systematics. For example, the Si and S ion peaks considerably overlap oneanother in the E vs. TOF matrix used to analyze SWICS counts, which may lead to somemisidentification of these ions. However, in fFIP, these species are just summed together,so the identification ambiguity is circumvented (they also have nearly identical detectionefficiencies, so it is okay to simply sum counts without any need for weighting). The samegoes for N, which can be obscured by C and O, but since these are all high-FIP elements,the ambiguity becomes irrelevant when computing fFIP.

The enhancement factors for the different solar wind regimes are given in the last rowof Table 7. Not surprisingly, the CME regime has the highest value of fFIP = 2.41 ± 0.36,


while for the coronal hole regime, fFIP = 1.51 ± 0.23. This can be loosely interpreted tomean that the coronal hole material collected by Genesis is fractionated by 50 % relative tothe photosphere. Here, we use the SWICS measurement uncertainties recently evaluated byvon Steiger and Zurbuchen (2011). These uncertainties do not include the uncertainties inthe photospheric abundance values because we are using them to make relative comparisons,to which the photospheric uncertainties do not contribute.

We present fFIP as a function of solar wind speed and as a function of regime in Fig. 13h.We see a smooth decline in fFIP with speed for all regimes, with a distinct and consistentoffset for the CME regime by about 0.5. There is also a clear and consistent separationbetween the IS and CH regimes in the speed overlap region. Within the CH regime, althougha decline is present, the drop of fFIP with speed is fairly shallow.

This last point brings up an interesting question: Could the regime selection algorithmhave been configured differently such that Genesis collected a solar wind sample withelemental abundances significantly closer to that of the Sun? The answer is, “yes, butnot too much closer”. The least fractionated solar wind flow ever observed in the helio-sphere was by the Ulysses spacecraft during its polar passes at the time of solar min-imum. Ulysses flew a version of SWICS nearly identical to that on ACE (in fact, theACE instrument was the Ulysses flight spare). During these polar passes, Ulysses ob-served a solar wind proton flow at latitudes above |36◦| that varied between 700 and800 km s−1 (McComas et al. 2000). This fast, steady wind originated deep within largepolar coronal holes. Using the recently published photospheric data of Asplund et al.(2009) and the elemental abundances measured by Ulysses/SWICS (von Steiger et al. 2000;von Steiger and Zurbuchen 2011), we derive a value of fFIP = 1.37 ± 0.20 for polar coronalholes. Note this is lower than the coronal hole enhancement factor published in von Steigeret al. (2000) of fFIP = 1.8 simply because they used older photospheric abundance values(Grevesse and Sauval 1998). Another caveat is that von Steiger et al. (2000) included neonin the sum of high-FIP elements for their definition of fFIP. We choose not to include neonnot only because of analysis challenges for ACE/SWICS, but also because of the large un-certainty in the photospheric abundance. Not including neon makes a negligible differencein the calculation of fFIP due to the low abundance of neon compared to the other high-FIPelements.

We observe that the Genesis coronal hole sample enhancement factor of fFIP = 1.51is only 10 % greater than the Ulysses polar coronal hole value of fFIP = 1.37, which iswithin the measurement uncertainty. Interestingly, if we isolate that subset of the GenesisCH collection period where the solar wind speed was greater than 700 km s−1, (the last fourspeed bins in Fig. 13h) we find from ACE/SWICS that fFIP = 1.35 ± 0.20, or essentiallythe same as for polar coronal holes. This implies that the very fast wind (vp ≥ 700 km s−1)in low-latitude coronal holes has the same composition as polar coronal holes. (This alsoimplies that any relative error between the two SWICS instruments is small.) Thus in answerto the question whether Genesis could have done better at collecting the most unfractionatedsolar wind possible, the answer is: only if it limited itself to sample collection for windspeeds above 700 km s−1. Of the 313 days Genesis collected coronal hole flow, only 40days, or 13 %, had such high speeds. This would have significantly degraded the accuracyof the sample analysis, and would not have been worth the modest gain in sample purity.

One final comparison of interest is to the Ulysses low-latitude FIP enhancement factor.von Steiger et al. (2000) report two sets of low latitude abundances, collected near solarmaximum and solar minimum, respectively. The solar maximum set was collected duringthe declining phase of the previous solar cycle, at nearly an identical phase of the cycle as theGenesis period. For this period, the Ulysses value is fFIP = 1.93. All data over the course


of 300 days were included in the set, so it is most appropriate to compare to the GenesisBulk sample, for which fFIP = 1.85, in almost perfect agreement with the Ulysses value,and certainly within measurement uncertainty.

5.4 ACE/SWICS-Derived Elemental Fluences

We next present the 1 AU fluences for certain species accumulated over the course of theGenesis mission, based on analysis of ACE/SWICS data. This quantity is particularly usefulbecause it can be directly compared to the amount of solar wind material extracted fromthe collection samples in the laboratory, and thus assists in the sample analysis validationprocess. We also report the differential fluence (see Eq. (1)), as this allows for a quantitativeprediction of the depth profile of the solar wind material within the collection samples, whencoupled with stopping range calculations (Olinger and Wiens 2010). Already, this data hasbeen used to validate certain Genesis sample analysis results (e.g., Grimberg et al. 2006,2008; Meshik et al. 2007; McKeegan et al. 2011; Heber et al. 2012; Huss et al. 2012).

To determine the differential fluence for each element, we use the procedure described inSect. 3.2 for reconstructing the helium phase space density. We use the mean helium speedand (where possible) the element-specific thermal speed to specify a normalized Maxwellianspeed distribution. Although speeds for different species are available in the ACE/SWICSdata products, the observed deviation from the helium speed is typically only a few km s−1

(by at most 10 km s−1; see also Hefti et al. 1998; von Steiger and Zurbuchen 2006), whichis not significant enough to warrant their use in the fluence derivation. For the Mg, Si, S andN thermal speeds, we use the oxygen thermal speed as a proxy, as the thermal speeds forthese species are not produced as a data product from ACE. A given particle density is thenmultiplied by the Maxwellian distribution of the appropriate thermal width to determine thefraction of particles to assign to each speed bin (see Eq. (1)). The distributions are thensummed for all measurements for a given solar-wind regime and speed, and the data so ob-tained are given in Tables A4–A7. Plots of normalized velocity distributions of each of theseSWICS-derived elemental compositions sorted by the Genesis regime-selection algorithmare shown in Figs. 14 and 15. We see that there are significant differences in the shape of thefluence distributions, which is a reflection of FIP fractionation as demonstrated in Fig. 13.For the IS and Bulk collectors, at the lowest speeds we see that the low-FIP elements Fe,Mg, and S are enhanced relative to the other species. For the CH collector, the distributionsare much more uniform, reflecting the relative lack of fractionation in coronal hole material.Note this implies that any fractionation with depth observed between elements in the anal-ysis of CH samples is due to differences in the stopping range, not the solar wind energyprofile.

Figures 16 and 17 give the same fluence information as Figs. 14 and 15, but on a logarith-mic scale. This is to highlight the variations in fluence at the highest velocities, particularlyfor the Bulk and CME samples. Above a speed of ∼900 km s−1, the fluence is almost entirelydue to the highly unusual Halloween Event of 2003, described in Sect. 3.2. We present flu-ences up to 1800 km s−1 (above this speed the solar wind data become unreliable) becauseit should be possible to isolate this material in the laboratory (e.g., Grimberg et al. 2006,2008). It is clear that whereas the fluence distributions are quite close below 900 km s−1, theelements are highly fractionated in the Halloween Event (Skoug et al. 2004), with a spreadof almost an order of magnitude.


Fig. 14 Normalized fluence per speed bin (�v = 25 km s−1) as a function of solar wind proton speedfor heavy ions during Genesis collection periods, as measured by ACE SWICS. Panel (a) shows the Bulkcollector fluence distribution, and panel (b) is for the CME collector

6 Summary

We have presented here a listing of the collection times for each of the Genesis mission so-lar wind collections and a description of the solar wind conditions covering the period whenthe Genesis samples were collected. We also explored in detail how successfully the regimeselection algorithm was at separating flow types and isolating compositionally distinct sam-ples on the different regimes. The Genesis sample was collected during the declining phase


Fig. 15 Same as Fig. 14, except the panel (a) is for the IS collector and panel (b) is for the CH collector

of solar cycle 23, starting just after solar maximum. We have shown that the average stateof the solar wind during this period was similar to the long-term average state, albeit biasedslightly toward faster, kinetically hotter conditions due to the larger-than-average fraction oftime the solar wind dwelled in the coronal hole state.

The plasma states of the regimes themselves were consistent with the state of theseregimes averaged over a solar cycle, with deviations in the parameters of no more than25 %, but typically much less. In particular, the plasma conditions for the CH sample werevery close to the cycle-averaged CH flow conditions (within 6 % for most parameters). In


Fig. 16 Normalized fluence per speed bin (�v = 25 km s−1) as a function of solar wind proton speed forheavy ions during Genesis collection periods, as measured by ACE SWICS, presented on a log scale. Thishighlights the presence of high-velocity solar wind material in the Bulk (this figure) and CME (Fig. 11)regimes, collected during the solar super-storms of the period October 23–November 3, 2003. Panel (a) is forthe bulk collector, and panel (b) is for the CME collector

addition, comparison to the ACE/SWICS O7+/O6+ charge state ratios indicated that the CHregime sample contained a nearly pure sample of true CH flow. In terms of CMEs, the vastmajority (95 %) of CME material was collected on the CME array.


Fig. 17 Same as Fig. 16, except panel (a) is for the IS collector and panel (b) is for the CH collector

We have also shown that the three regime samples are elementally fractionated with re-spect to one another. Using ACE/SWICS elemental composition data for the Genesis period,we showed that the different regimes exhibit FIP fractionation. We also showed that in thespeed overlap region, the samples were at least partially separated in composition by theselection algorithm, validating the hysteresis logic employed on board.

The FIP fractionation that occurred during the Genesis period was very similar to thatreported by Ulysses. In fact, the FIP enhancement factor fFIP = 1.53 for the CH regime isclose to that found by Ulysses in polar coronal holes (fFIP = 1.37). An important finding


is that if we compute fFIP for those times when the CH flow was at or above 700 km s−1,the value agrees almost exactly with the polar coronal hole value, indicating that fast low-latitude coronal holes have the same composition as polar coronal holes, as suggested byZurbuchen et al. (2002).

In addition to the analysis of the solar wind plasma and composition state, we have pre-sented fluence distributions for H and He based on GIM observations, and fluence distri-butions for the abundant heavy elements based on SWICS observations. These data areprovided to aid in the interpretation of Genesis sample analysis by current and future gener-ations of scientists.

Acknowledgements The authors wish to acknowledge the NASA Laboratory Analysis of Returned Sam-ples (LARS) program (Grants NNX10AH57G and NNH10A046I) and the International Space Science In-stitute (ISSI) for supporting this work. The OMNI data were obtained from the GSFC/SPDF OMNIWebinterface at http://omniweb.gsfc.nasa.gov. The authors thank the ACE science team for making their dataavailable for this study. T.H.Z. and J.R. were supported in part by NASA grant NNX08AI11G.

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