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Journal of Atmospheric and Solar-Terrestrial Physics 70 (2008) 226–233

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Monitoring the heliospheric current sheet local structurefor the years 1995 to 2001

J.J. Blanco�, J. Rodriguez-Pacheco, M.A. Hidalgo, J. Sequeiros

Space Research Group, Dpto. Fisica, Univ. Alcala, Alcala de Henares-Madrid, Spain

Accepted 27 August 2007

Available online 1 October 2007

Abstract

We have monitored the heliospheric current sheet (HCS) local structure through a 6.5 year period starting in January

1995. This interval begins with near solar minimum conditions and finishes in solar maximum conditions. We have used

data from Wind mission, mainly the magnetic field instrument (MFI) and the solar wind experiment (SWE). Our work is

focused on the HCS local inclination and the solar wind conditions around the HCS crossings, with a particular interest on

their evolution along the ascending phase of solar cycle 23 and its relationship with solar wind phenomena such as

magnetic cloud and stream interaction regions. We defined a real HCS crossing when a magnetic field minimum, showing a

polarity reversal is observed and QeB (where Qe is the electron heat flux in solar wind) reverses its sign through an interval

no longer than 60min. The results suggest that the HCS local structure is more dependent on the solar wind conditions

than on the solar cycle stage.

r 2007 Elsevier Ltd. All rights reserved.

Keywords: Heliospheric current sheet; Solar cycle; Stream interaction region; Magnetic cloud; Slow solar wind

1. Introduction

The heliospheric current sheet (HCS) is probablythe most relevant feature immersed the solar wind.It is present along all the solar cycle although itsglobal shape suffers important changes, particularlyclose to the solar maximum (Smith, 2001; Rileyet al., 2002). The HCS goes into the heliospherefrom the cusp of the magnetic arcs that form thehelmet streamer belt. So any change in the helmetstreamer should affect to the global shape of theHCS, this is clear during the solar maximum when

e front matter r 2007 Elsevier Ltd. All rights reserved

stp.2007.08.030

ing author. Tel.: +34 91 8855053;

4942.

ess: juanjo.blanco@uah.es (J.J. Blanco).

the helmet streamer fills wide corona regions(Bavassano et al., 1997; Wang et al., 2000). At1AU the HCS is observed as a sharp boundarybetween regions with opposite magnetic polaritydividing the heliosphere in magnetic sectors. Thesesectors are connected with solar regions with thesame polarity (Wilcox and Ness, 1965). Frommagnetic field in situ measurements is possible toinfer the local HCS inclination. The most employedmethod for determining this inclination is theminimum variance analysis (MVA) that uses theeigenvector associated with the minimum eigenvalueas proxy of the normal vector to the HCS plane(Sonnerup and Cahill, 1967). Previous worksapplying MVA support that important variationsof the local HCS inclination are not observed along

.

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Fig. 1. Interplanetary magnetic field and electron data detected

by WIND spacecraft at 1 AU on March 9, 1995. From top to

bottom: 320 eV electron pitch angle, Qe�B sign, GSE magnetic

field components and magnetic field strength. Red represents the

greatest intensity and blue represents zero intensity in the color

scale used in the electron pitch angle graph.

Fig. 2. HCS crossing detected by WIND spacecraft sensors on

March 9, 1995. Squares, circles and triangles are the Bx, By and

Bz magnetic field components respectively. Continuous lines and

the values show in the left panel are the fit lines, the reduced CHI-

square w2, the coefficient of determination R2, and the parameters

obtained using Eq. (2) as fit function.

J.J. Blanco et al. / Journal of Atmospheric and Solar-Terrestrial Physics 70 (2008) 226–233 227

the solar cycle. This result can be explained becauseof the presence of local waviness in the HCS(Villante et al., 1979; Behannon et al., 1981). TheHCS is embedded in a solar wind region character-ized by high density, low temperature, slow solarwind (SSW) velocity and high plasma beta calledheliospheric plasma sheet (HPS) (Winterhalteret al., 1994). Recently, Crooker et al. (2004) warnabout the existence of high plasma beta regionswithout HCS and HCS out of these regions. In thiswork we survey the local HCS properties fromJanuary 1995 to May 2001 focussing our attentionon its local inclination and solar wind structuresclose to the HCS.

2. Methodology and data analysis

A systematic search of HCS crossings detected byWIND spacecraft has been done from January 1995to May 2001. We assume that a HCS crossinghappens when a magnetic field minimum, showing apolarity reversal is observed and Q

!e � B!

(where Q!

e

is the electron heat flux in solar wind) reverses itssign through an interval no longer than 60min(Fig. 1). The latter can also be observed in the lowenergy electron pitch angle distribution (Kahleret al., 1998). We have used data from magnetic fieldinstrument (MFI) (Lepping et al., 1995) for themagnetic field, solar wind experiment (SWE)(Ogilvie et al., 1995) for solar wind (proton andelectron) properties and 3-D plasma instrument forthe electron pitch angle (Lin et al., 1995). Unfortu-nately, valid electron data from SWE are onlyavailable until May 2001. During the observedperiod the WIND spacecraft was at 1AU and itsorbit crossed the Magnetosphere many times. Theintervals inside of Magnetosphere have been re-moved in our study. Each HCS crossing has beengrouped into categories depending on the solar windconditions measured in the HCS crossing neigh-bourhood. Three categories have been established:SSW if the solar wind velocity is lower than 450km/sand it is far from transition regions between fastsolar wind and SSW, stream interaction region (SIR)if the solar wind velocity grows quickly developingan interaction region and magnetic cloud (MC)if a structure in the solar wind shows a smoothrotation of the magnetic field, simultaneously to alow temperature and a relatively high magneticfield strength (Burlaga et al., 1981). Once time anevent has been grouped in one of the previouscategories the mean density and temperature of

electrons and protons, solar wind velocity and localinclination of the HCS crossings have been calcu-lated in an interval 2 times longer than the HCSwidth. If multiple HCS crossings are detected, they

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are considered as a single one and the mean valuesestimated in the region containing the multiplecrossings are used for solar wind properties andHCS local inclination. We have catalogued two ormore consecutive HCS crossings as a multiple crossingwhen the interval between two consecutive events isless than 30min (Blanco et al., 2006). Around 10% ofthe HCS crossings have been considered as multipleone. The HCS local inclination is estimated using themethod described inBlanco et al. (2003). This method,based on a modified Harris field, assumes that theHCS is plane locally and the magnetic field, in a localsystem where the y direction coincides with the vectorperpendicular to the plane and the other two are in theHCS plane, can be written as:

Bx ¼ Bx0 tanhy� y0

L

� �; By ¼ Byo; Bz ¼ Bz0.

(1)

Where L is the HCS semi width and y0 marks thepoint where the magnetic field reversal happens. Bx0,By0 and Bz0 are the magnetic field components justout of the HCS plane. Nevertheless, when a spacecraftcrosses the HCS its magnetometer detects a magneticstructure that it should be the previous one (Eq. (1))but rotated with respect the spacecraft trajectory (seeFig. 1 in Blanco et al., 2003). If our initial assumptionabout the local shape of the HCS is correct, only twoconsecutive rotations applied to Eq. (1) are necessaryfor obtaining the magnetic configuration measured bythe spacecraft magnetometer. Considering a firstrotation around the z axis by a a angle and a secondone around the x0 axis, i.e. the rotated x axis, by a bangle the Eq. (1) can be written as:

Bx ¼ Bx0 tanhy� y0

L

� �cos a� By0 sin a, (2)

By ¼ Bx0 tanhy� y0

L

� �sin a cos bþ By0 cos a cos b

� Bz0 sinb, ð3Þ

Table 1

Resume of heliospheric current sheet crossings grouped by their associ

Solar wind Events Ne ðe=cm3Þ Np ðp=cm3Þ Te (K) Tp

SSW 71 11� 1 12� 1 118 000� 4000 30 0

SIR 67 19� 2 20� 2 150 000� 5000 43 0

MC 27 16� 2 16� 2 110 000� 5000 41 0

All 165 15� 1 16� 1 130 000� 3000 37 0

Slow solar wind (SSW), stream interaction region (SIR) and magnetic cl

mean of electron density, proton density, electron temperature, proton

the normal vector to the HCS plane and heliospheric current sheet wid

By ¼ Bx0 tanhy� y0

L

� �sin a sinbþ By0 cos a sin b

þ Bz0 cos b. ð4Þ

Using this magnetic field as fit function to magneticdata the normal vector to the HCS plane can beestimated from:

n!¼ ð� sin a; cos a sin b; cos a sin bÞ. (5)

The used fit procedure is based on the minimizationof the CHI-square function where the theoreticalfunction is the rotated Harris field and Levenberg–Marquardt is the employed algorithm. As for theselected data interval, it has to be greater than theHCS crossing width because our method needs to testthe interplanetary magnetic field conditions close toboth sides of HCS. An example of this procedure isshown in Fig. 2.

3. Results: HCS crossings and solar wind

A total of 165 HCS crossings have beenconsidered, 71 were in SSW regions, 67 close toSIR and 27 were associated with a MC passage. Theresults have been summarized in Table 1. The solarwind density in SIR and MC is higher than in SSW,probably related with the presence of high pressureregions. The highest mean velocity is associatedwith MC events. Here we have to point out that theSIR events are located closer to the SSW stream forwhat it should not be rare to find the previousresult. Electron and proton temperatures are quitedifferent by almost an order in magnitude, as itcould be expected for two particle populations thatdo not interact. The HCS width is calculated fromthe time of crossing, the normal vector to the HCSand the solar wind velocity. The obtained values arecloser to those found by Winterhalter et al. (1994)for the HPS than the HCS. In this point, a briefcommentary on how the values were obtained in

ated solar wind phenomena

(K) VSW (km/s) f (1) y (1) L (km)

00� 2000 366� 6 28� 3 17� 2 600 000� 40 000

00� 4000 393� 9 29� 3 14� 2 1 300 000� 1 200 000

00� 5000 400� 10 5� 6 18� 3 320 000� 80 000

00� 3000 382� 5 25� 2 16� 1 900 000� 500 000

oud (MC). The columns show the mean values and the error of the

temperature, solar wind speed, longitude and elevation angles of

th.

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Table 1 is needed. Each quantity is the mean valueand the uncertainty is the statistical error. For all themagnitudes, the uncertainty is reasonably low but thisis not truth for the HCS width. In our opinion, thiscan be due mainly to two factors: the existence of nineextremely wide crossings ð41� 106 kmÞ and anoverestimation of the time of crossing for these HCScrossings. As for the HCS local orientation, thelongitude angle of the normal vector to the HCS planelies closer to the ortho-spiral (i.e. 45�) in SIR thanSSW according with a greater solar wind velocity. ForMC events, this angle is away from the ortho-spiral.This can be due to an important effect on the HCSshape of the MC propagation. The HCS deformationis clearly observed in Fig. 3. Histograms of thelongitude and elevation angles are presented for SSW,SIR and MC. In this figure we observe how the HCSassociated with MC show the highest dispersion in thelongitude angle while that for SIR and SSW this angle

Fig. 3. Histograms of the longitude (upper row) and the elevation (dow

been grouped in three categories: SSW events first column, SIR events

is around 45� and 30� respectively. As for theelevation angle, the three categories show a quasi-perpendicular HCS local inclination, nevertheless, theHCS crossings close to MCs seem to be uniform until30� and those associated with SIRs are concentratedin the range of 0�–10�. We interpret these results witha strong HCS dynamic deformation due to the MCand SIR presence.

4. Results: HCS along the solar cycle

The studied period begins at the last part ofdescending phase of cycle 22 (1995) and finishes atthe beginning of descending phase of cycle 23(2001). In 1996 an extremely low number of HCScrossings were detected (12 events) and in 2001 HCScrossings were only considered until May becauseof the electron detector of SWE left to work atthat time.

n row) angles of the normal vector to the HCS plane. Events have

second column and MC events third column.

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All the considered HCS crossings have beengrouped by year and categories in Table 2. It isclear that the solar wind conditions around the HCScrossings vary along this period (Fig. 4). Around1998, a minimum in solar wind density appears whilea minimum and a maximum in speed are observedin 1996 and 1999 respectively. The temperaturefollows the speed trends throughout the wholeperiod. Nevertheless, the electron temperature

Table 2

Number of heliospheric current sheet crossings grouped by year

and categories

Year Slow solar

wind (SSW)

Stream interaction

region (SIR)

Magnetic

cloud (MC)

HCS

crossings

1995 10 14 5 29

1996 4 4 4 12

1997 15 4 6 25

1998 25 8 6 39

1999 5 14 2 21

2000 8 16 3 27

2001 4 7 1 12

Fig. 4. Annual mean values of solar wind in HCS crossings.

From top to bottom: HCS width, solar wind velocity, proton

temperature, electron temperature, proton density and electron

density. The error bars are the statistical errors.

seems to be slight shifted with respect to theproton temperature. In 1995 during the last stageof solar cycle 22, the solar wind conditionsmeasured near HCS crossings seem to be balancingbetween SSW regions and SIRs. In 1997 and 1998,in coincidence with the initial ascending phase ofsolar cycle 23, the HCS crossings were majoritydetected in SSW regions. During these years thenumber of HCS crossings associated with MCswas higher than that associated with SIRs. Thistrend changes in 1999 and 2000 where the dominantconditions near the HCS crossings were thoseof SIR.

The local orientation of the HCS does not seem tohave a clear dependence on the solar cycle (Figs. 5and 6). This is not in contradiction with a globalshape of the HCS which evolves along the solarcycle if assuming a local waviness (Villante et al.,1979; Behannon et al., 1981). The HCS is mainlyoriented along to the nominal Parker spiral duringthe whole period. ‘‘Pure’’ signatures, i.e. SSW, SIRor MC, are not observed in any year but in 1996(Fig. 5). Higher elevation angle were estimated incoincidence with a higher relative number of MCevents (Fig. 6 and Table 2). Our results seem tocorroborate a local HCS orientation depending onthe ratio among SSW, SIR and MC events morethan solar cycle dependence.

5. Conclusions

In this work we have followed the HCS localstructure from January 1995 to May 2001. Afterrejecting intervals inside magnetosphere, data gapsand crossings showing highly fluctuating magneticfields a total of 165 HCS crossings have beenconsidered. They have been grouped into threecategories depending on the observed solar windconditions during the crossings. These categorieswere SSW, SIR or MC. Our results can besummarized in the following points:

�

The density of protons and electron in crossingsnear MCs and SIRs is higher than SSW. This factis probably due to the presence of high pressureregions. � There seem to be strong deformation on the HCS

local shape when it is detected close to MCs.Otherwise; SIRs seem to push the HCS localplane towards quasi-perpendicular elevation an-gles. Both phenomena exert a strong dynamicdeformation on the HCS local structure.

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Fig. 5. Annual histograms of the longitude angle of the normal vector to the HCS plane.

J.J. Blanco et al. / Journal of Atmospheric and Solar-Terrestrial Physics 70 (2008) 226–233 231

�

During the early ascending phase of solar cycle23 (1997–1998) the HCS is mainly located inSSW regions. This trend changes in 1999 and2000 when the HCS is principally detected nearSIRs. � The solar wind characteristics change along the

studied period showing a density minimumaround 1998 and a velocity minimum in 1996and a maximum in 1999.

� The local HCS inclination does not seem to be

directly related to the solar cycle but the relative

number of MC, SIR and SSW. Nevertheless, theHCS is mainly oriented along the Parker Spiral.

The studied period (6.5 years) is too short forestablishing conclusive results about the HCSlocal structure along the solar cycle. We will extendour study to the whole solar cycle and comparewith the available data of previous cycles with thegoal to confirm or not the results presented inthis work.

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Fig. 6. Annual histograms of the elevation angle of the normal vector to the HCS plane.

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Acknowledgements

The authors wish to thank the WIND/MFI,WIND/SWE and WIND/3DP teams for the use oftheir data and especially to Dr A. Vinas for hiscomments on electrons in the solar wind. This workhas been supported by Spanish Ministerio deEduacion y Ciencia into the project with referencecode: ESP2005-07290-C02-01 and by Universidadde Alcala into the project with reference code:ESP2006-08459/.

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