A review of solar proton events during the 22nd solar cycle

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~ Pergamon www.elsevier.com/locate/asr Adv. Space Res. Vol. 30, No. 4, pp. 1033-1044, 2002 Published by Elsevier Science Ltd on behalf of COSPAR Printed in Great Britain 0273-1177/02 $22.00 + 0.00 PII: S0273-1177(02)00497-0 A REVIEW OF SOLAR PROTON EVENTS DURING THE 22ND SOLAR CYCLE D. F. Smart and M. A. Shea Air Force Research Laboratory (VSBS), 29 Randolph Road, Hanscom AFB, MA, 01731, USA. ABSTRACT Solar cycle 22 had significant, large fluence, energetic particle events on a scale reminiscent of the 19th solar cycle. Examination of the characteristics of these large events suggests that some of the old concepts of spectral form, in- tensity-time envelope and energy extrapolations, used to estimate the dose from large events that occurred during previous solar cycles should be re-evaluated. There has also been a dramatic change in perspective regarding the source of solar protons observed in interplanetary space. Very large fluence events are associated with powerful fast interplanetary shocks. The elemental composition and charge state of these events is suggestive of a dominate source in the solar corona and not from a very hot plasma. Furthermore, there is a strong suggestion that the inten- sity-time profile observed in space is dominated by the connection of the observer to an interplanetary shock source rather than to a unique location near the surface of the sun. These concepts will be examined from the perspective of energetic panicles contributing to the dose experienced by an astronaut on an interplanetary space mission. Published by Elsevier Science Ltd on behalf of COSPAR. INTRODUCTION This paper is oriented toward the viewpoint of radiation effects to human blood-forming organs (vital organ dose). We will discuss the energetic particle radiation capable of penetrating the shell of a space vehicle and the human body to reach the blood forming organs (at least 5 cm of tissue). From this viewpoint, the energetic charged pani- cle radiation must have a minimum energy of 30 MeV per nucleon. From a solar astrophysics point of view, the increased energetic panicle sensor sensitivity to very low flux levels that has occurred over the past decades provides for hitherto unavailable information on the particle composition and possible origin. However, from a radiation dose point of view, small events of limited duration with maximum particle fluxes of a few per cm 2 per hour are not very significant. Therefore we will not put much emphasis on the very small events routinely measured by the improved instrumentation. OVERVIEW OF SOLAR PROTON EVENTS DURING SOLAR CYCLE 22 Figure 1 illustrates the >30 MeV solar proton events during solar cycle 22. The horizontal lines on this figure indi- cate one sigma in a log-normal distribution of the solar proton event fluence distribution over the last four solar cy- cles. This one sigma characterization of the fluence distribution derived by Nymmik (1993) from the data of Feynman et al., (1993) seems to be one reasonable way of categorizing solar proton events. Stassinopoulos et al. (1996) adopted an order of magnitude classification system categorizing solar proton events as a matter of conven- ience. The lower limit of this figure, an omnidirectional fluence of 105 per cm 2 is comparable to the daily exposure from the integral flux of galactic cosmic ray protons. The solar cycle 22 solar proton events and fluences at ener- gies of>50, >60 and >100 MeV are shown in Figures 2 - 4. In Figure 5 we display some of the very large events of the last solar cycle from the perspective of the size of a cell in the human body. We have used 100 square microns as the unit of area. The scale on the left of this figure indi- cates the number of proton "hits" per hour during these large events. Also indicated by horizontal lines are the flux levels that correspond to "hits" per day and "hits" per week. 1033 1034 D.F. Smart and M. A. Shea A E 0 n t - o o n > 80 MeV EVENT INTE~tATEO PROTON O~(IDIREC'rtONAL R .UBKE . ' .1 . ' . ' . ' .= ' ;2 ! ; : ;~ . : ' ' . , . ' . ' ' .= . ' . ' . ' .= . ' - . ! ; ' ;= . ' . ' . " ] 101o 10o 10 a 107 LU O Z lo0 u.I ::) .. J u. lOS 86 ii 87 88 8g go gl g2 YEAR LARm= 93 94 95 96 Fig. 1. The >30 MeV solar proton events during solar cycle 22, The horizontal lines indicate one sigma of the log-normal distribution. ,'+" E U W e- o 0 : Z uJ ::) . J i, > IB0 MeV EVENT INTEGRATED PROTON ~ FLUENCE 1010 . . , . . . , . . . , . . . , . . . , . . . , . . . , . .+ , . . . , . . . 10o lo0 107 It~ lo0 10s I . . . . . . I t : ' . 86 87 88 89 g0 91 92 g3 YEAR I i| . . m .+ 94 95 96 Fig. 2. The > 50 MeV solar proton events during solar cycle 22. SO MeV EVENT INTEGRATED PROTON OMNIDBECT1ONAL FLUBCCE o !1. 3 u . l O P O . . . . , . . . , . . . , . . . , , , , , . , , , . , , , . . . i . . . , . 1+ L lOa 107 86 87 88 89 90 91 lil I i .... g2 93 g4 95 96 YEAR Fig. 3. The >60 MeV solar proton events during solar cycle 22. M t- O n w 0 z w . J U. ) 100 MIV LrVENT ~ 1 1 D PIIOlrON ~ ~ 100 . . . , . . . , . . . , . . . , . . . , . . . , . . . , . . . , . . , . . . lol 107 1. ,lll 1~ "" . . . . . . . 86 87 88 89 I I I .L . I |= . . J . | . . . 90 91 92 9 -'1 94 95 96 YEAR Fig. 4. The > 100 MeV solar proton events during solar cycle 22. 1 0 0 ~ o . . . . . . . 1.0 Proton per Day 1 0-4 1 9 20 21 22 23 24 25 26 27 28 29 30 31 OCTOBER 1989 (UT DAY) Fig. 5. The very large events of October 1989 displayed from the perspective of the size of a cell ( -100 square microns). > 30 MeV > 50 MeV > 60 MeV > 100 MeV Review of Solar Proton Events 1035 THE PARTICLE SOURCE Both the classical solar flare source scenario and the shock acceleration scenario have enthusiastic advocates. Many of the "large" solar particle events are associated with "large" solar flares. However, many of the proton events observed at the Earth cannot be unambiguously and confidently time associated with specific solar flares. Furthermore, measurement of the elemental and isotopic composition of solar particles during large flux and flu- ence events was found to be consistent with the particles having passed through less than 30 mg cm -2 of matter from the acceleration site to their detection location (Mason, 1987). Thus, the solar photosphere is eliminated as the source region of the particles observed in space since there is no evidence of fragmentation due to interaction with the significant mass of the solar atmosphere. The intensity-time profiles of solar energetic particles leave no doubt that interplanetary shocks accelerate ions. The fast coronal mass ejection (CME) generated interplanetary shock is now considered to be the principal source of the ions observed in space (see Reames, 1995, for a review). There is also the concept of a composite solar particle event to which both sources contribute flux (Cliver, 1996). A relatively recent grouping of solar particle events is to classify them according to the associated solar flare X-ray activity (see Table 1). This results in a classification of either "impulsive" (those associated with impulsive soft X- ray events) or "gradual" (those associated with long duration soft X-ray events). The "impulsive" solar particle events are the group that appears to be associated with the solar flare process. The long duration (i.e. "gradual") X- ray event is believed to be a proxy indicative of a coronal mass ejection, and hence this class of events is assumed to be the result of the shock acceleration process. An extensive study of the "impulsive-flare" associated energetic particle events by Cane et al. (1986) and Reames et al. (1994) shows that these events are generally small flux and fluence events measurable in space over a restricted heliolongitudinal range, about one radian of heliolongitudinal distance centered on the most favorable propagation path from the solar flare location. The charge state of solar energetic ions that are associated with the impulsive soft X-ray flare is consistent with a multi-million degree hot (-2 x 107 OK) plasma source. Table 1. Properties of Solar Proton Events Associated with Impulsive and Gradual soft X-ray events. (Adapted from Reames, 1995) Impulsive X-ray event and associated particle event characteristics Gradual X-ray event and associated particle event characteristics Particles Electron rich 3He/4He Fe/O H/He Charge state of Iron DurationHours Heliolongitude range Associated Solar Radio type Associated Solar X-ray emission CME association Associated Interplanetary shock Events per year (At solar maximum) Proton rich ~1 ~1 ~ 10 ~20 Days 1036 D.F. Smart and M, A. Shea Large solar particle events seem to be associated with the occurrence of a fast coronal mass ejection or, in the absence of actual CM]~ observations, CME proxies. This association is complicated by the fact that large fast coronal mass ejoctions also seem to have an association with "big" solar flares. The long duration solar X-ray event is considered to be an excellent CME proxy. Figure 6 illustrates the fast CME associated with the 24 October 1989 solar cosmic ray event observed at the Earth. Large solar particle events tend to have a "normal" composition that is relatively consistent with abundances found in the solar corona or in the solar wind, although there are event to event variations, particularly in the helium/hy&ogen ratios. The class of solar particle events associated with a CME/shock source seems to be detectable at very large heliocentric angles from the centroid of the CME/sourcc heliolongitudc position. The largest flux values are observed at locations in space well connected via the interplanetary magnetic field topology to the presumed centroid of the CME/shock source heliolongitude. GOES CORRECTED INTEGRAL FLUX " ; " 1 0 $ . . . . . , . . . . . , . . . . . , . . . . . , . . . . . , . . . . . , . . . . . , . . . . . , . . . . . i . . . . . , . . . . 24 5 25 5 26 5 27 5 28 5 29 5 Fig. 6. The fast CME associated with the 24 October 1989 solar cosmic ray event. OCTOBER 1989 (UT Day) Fig. 7. The "classic" solar proton event as illustrated by the 24 October 1989 event. The charge state of energetic ions in "large" particle events appears to be consistent with coronal temperatures (-106 K plasma). The initial charge state measurements were made at low energies, of the order ofa McV (Luhn et al., 1985). The charge states have now been measured at energies of 10s o fMcVs (Octliker ctal., 1995). These higher energy charge states are generally consistent with the low energy charge states with an indication that the charge state may be a function of energy. Tylka ct al. (1995) found an ionic charge state of-14 for iron nuclei at -200 MeV per nucleon. The intensity-time profile observed at a specific location in space depends on the "connection" of the observer to the particle source (Cane, 1986). Wc will use the terms "near-sun-injection" and "extended interplanetary shock source" to distinguish between types of solar proton events observed in interplanetary space. The term "ncar-sun- iniection" will refer to the class of solar proton events in which the intensity-time profile at the observer's position is dominated by the particle flux injected near the sun onto the interplanetary field lines passing by the observer. This scenario represents the "classic" solar proton event illustrated by the 24 October 1989 proton event shown in Figure 7. This class of solar particle event often has a relatively "hard" (hard meaning more than an average number of particles at high energies) spectra. The term "extended intcmlanetarv shock source" will refer to the class of solar particle events that does not conform to the "classic" solar proton intensity-time profile, but instead has a characteristic that the flux continues to increase until a maximum flux is observed as a powerful and fast interplanetary shock overtakes the observer, aRcr which the flux decreases. An example of this type of event is the March 1991 solar proton event as observed at the Earth and illustrated in Figure 8. This example is probably typical of this type of event which usually has a relatively "soft" (soft meaning fewer than an average number of particles at high energies) spectra. Review of Solar Proton Events 1037 FLUX ENHANCEMENTS ASSOCIATED WITH INTERPLANETARY SHOCKS Particle flux enhancements associated with the passage of powerful and fast interplanetary shocks is a concept still being defined. A common misconception is that there will always be a flux enhancement with the passage of an interplanetary shock. This is true only at low energies (~1 MeV and below). At higher energies (>30 MeV) it is the exception when a flux enhancement is observed with the passage of an interplanetary shock. Kallenrode (1993) and Kallenrode et al. (1993) examined the entire HELIOS particle data base and found at 20 MeV, the correlation between the shock jump plasma parameters and the flux increase was approximately zero. Because there are no definitive analytical rules for predicting the flux increase associated with the passage of an interplanetary shock we will use an illustrative example. This example is the last significant particle event of solar cycle 22, which began on 20 February 1994, and shown in Figure 9. This is an event having both an initial near-sun injection and a flux profile dominated by an extended interplanetary shock source. The time associated M4/3B solar flare occurred at 0141 UT on 20 February at a heliographic position (as viewed from the Earth) of North 07 , West 02 , near the solar central meridian. This solar event had all of the CME proxies, and it is presumed that a fast CME was launched toward the Earth even though there was no equipment in space to observe the actual CME. The initial particle flux was detected at the Earth shortly after the solar flare electromagnetic emissions were observed. The particle onset times at the Earth were in a time sequence consistent with a near-sun energetic particle injection, the components of which propagated along the interplanetary magnetic field lines from the sun to the Earth at speeds consistent with the particle energy. Inspection of Figure 9 shows that only the highest energy particles recorded at the Earth have the intensity-time profile that is dominated by a near- sun injection. At energies below 30 MeV, the observed intensity-time profile indicates that an extended interplanetary shock source dominated the flux profile observed at the Earth. The maximum low energy particle flux was observed as the fast interplanetary shock passed over the spacecraft and the Earth, initiating a SSC (storm sudden commencement) magnetic storm which began at 0901 UT on 22 February. The hypothesis is that a fast CME propagating toward the Earth was generating a shock wave which was accelerating particles. We can hypothesize that when this shock wave was within 1/2 AU of the Earth, the low energy particle flux observed at the Earth was dominated by the particles accelerated at the shock. At higher energies (>20 MeV), the increased flux due to the extended interplanetary shock source is not apparent until the shock is within 0.2 AU of the observer. i u) O4 E O v X J 14. . . . . i . . . . . i . . . . . i . . . . . i . . . . . i . . . . 1 0 s 1 04 ~ ,o L .o 1 01 ~ P=.50 , ,.J ; t . . t~ e P>100 1%30 2&5 245 250 255 MARCH 1991 (UT DAY) Fig. 8. The extended interplanetary shock type of event as i l lustrated by the March 1991 solar proton event as observed at the Earth. > :S (D - J LL COMPOSITE SOLAR PROTON EVENT GOES 7 data (February 1994) 105 ~. . . . , . . . . . , . . . . . , . . . . . , . . . . . , . . . . ! Near Sun Inteq~netary Shock 104 P~=~ ~=~,.source Source .'~, m ~ ~v ^/] \;. ~, P4 =uu,v 1 02 /~/" "~', ~ =o v,~. ,oo r_ ' / ,o2 10-4L . - - . J . . . . . , . . . . . I . . . . . J . . . . . J . . . . . " 51.0 51.5 52.0 52.5 53.0 53.5 54.0 Day of Year (1994) Fig. 9. The February 1994 event as observed at the Earth, This "compos i te" solar particle event had both a "near sun inject ion" and a f lux enhancement associated w i th an interplanetary shock. 1038 D.F. Smart and M. A. Shea SOLAR ENERGETIC PARTICLE FLUX ANISOTROPY The particle flux anisotropy is generally defined as the ratio of the maximum particle flux divided by the average of the particle flux in all directions. In the isotropic case, there is equal particle flux from all directions. In the anisotropic case, there is an excess of particle flux in one specific direction. If it is assumed that the energetic charged particles are constrained to travel along an interplanetary magnetic field line with little scattering across field lines, then the particle flux anisotropy also reflects the rate at which particles are injected onto the observer's interplanetary field line with respect to the rate at which the particle flux is transported away from the observer. In the case where there is a "near sun" injection of energetic charged particles, then the initial particle flux observed "downstream" will be initially anisotropic, with the degree of anisotropy decreasing as particles that have passed the observer scatter and are reflected back by subsequent scattering in the interplanetary magnetic field. Particle flux anisotropy is a transient phenomena, and at the energies important to dose calculations, at radial distances from the sun between the Earth and Mars, a duration of a few hours is typical. In the example shown in Figm~ 10, there was no significant anisotropy remaining after 0.1 day. This figure illustrates the proton flux anisotropy at -30 MeV observed by the HELIOS spacecraR at 0.95 AU during a large anisotropic solar particle event in June 1980 (day of year 173). The anisotropy amplitude (denoted by "A" in Figure 10) ranges from extreme (~1) to low anisotropy (-0.2). An artifact of fitting a cosine function to coarsely sectored data results in large anisotropies having values >1 as shown in the first panel of Figure 10. A=0.2 (1980) 173.077 173.126 173.175 Fig. 10. The proton flux anisotropy at -30 MeV observed by the HELLOS spacecraft. The distance from the center indicates the relative particle flux. The line labeled "B" indicates the magnetic field direction, and the line to the right of the center indicates the anti-sun direction. COMPOSITION OF SOLAR ENERGETIC PARTICLE EVENTS The "impulsive-flare-associated" solar energetic particle events seem most closely related to the solar flare particle acceleration process. Initially, these events were considered as "composition anomalies" since there were distinct differences from "large" solar particle events (see Table 1). They were sometimes called "3He rich" or "iron rich" events. In general, the fluxes in the impulsive events are orders of magnitude smaller than "significant" solar particle events such as those illustrated in Figures 1-4. The analysis of Reames et al. (1994) concluded that in the -I - 20 MeV energy range, large solar particle events tended to have a "normal" composition that was relatively consistent (perhaps within a factor of 3), although there are event to event variations, particularly in the helium/hydrogen ratios. The ratio of the abundances of the elemental composition observed in large solar energetic particle events to the abundances of the elemental composition observed in the photosphere appears to be a function of the first ionization potential of the individual elements. Some researchers contend that large solar energetic particle events are a reasonable sample of the solar corona (Breneman and Stone, 1985, Mewaldt and Stone, 1989). The available data reflect the composition at relatively low energies (a few MeV to perhaps 10 MeV). The analysis of Mazur et al. (1992) suggested that there was relative consistency of the elemental composition during an event (up to energies of ~30 MeV). However, the latest analysis of the heavy ion composition at high energies (>100 MeVANuc) for very large flux events (Tylka et al., 1997) suggests that the iron to oxygen ratio is a function of energy with variations larger than those reported by Mazur et al. (1992). SOLAR ENERGETIC PARTICLES - CHARACTERISTICS OBSERVED AT 1 AU The majority of the solar particle data that exists has been collect at the Earth by ground-based sensors or by Earth- orbiting spacecraft. Prior to 1967, most of the solar proton data were derived from analysis of the response of the Earth's ionosphere to solar particle events; the spectral information derived lacks the accuracy of the spacecraft Review of Solar Proton Events 1039 data. Most of the spacecraft data collected have been at energies 100 MeV. There are a number of formulations to model the spectra of small or medium size flux or fluence events, but, in general, these forms do not adequately describe the flux profile encountered in large and very large solar particle events. Particle Flux Evolution and Radiation Dose The possibility that a large very energetic solar cosmic ray event could generate a significant radiation dose to as- tronauts in a short time interval is a matter of concern. Some of the original radiation dose estimates from the large energetic proton events of the 1950's and 1960's were many rads per hour (Foelsche, 1965). However, more recent dose calculations for these same events (Wilson, et al., 1991) are considerably lower. One of the reasons for these differences is the use of new standards for converting radiation types to dose (ICRU Report 71, 1998). Another reason is a better definition of the solar particle spectra for the more recent events and a subsequent re-evaluation of the spectral characteristics of some of the earlier events (Wilson et al., 1991). Inherent in the early dose estimates was the assumption of "explosive" particle release from the sun and a subsequent "explosive" increase in dose from a very fast increase in high energy particle flux. The spacecraft observations of the large energetic solar proton events of solar cycle 22 do not support the "explosive" particle release and flux increase scenario. Typically, in the 30 to several hundred MeV energy range, the time scale of hours from particle onset to flux maximum was ob- served. From the analysis of the large events that occurred in the 22nd solar cycle we have learned it is a serious mistake to use one simple value to characterize the spectral characteristics of a large high energy solar cosmic ray event. This is perhaps one of the major reasons that such high dose values were calculated for the events in the 19th solar cycle. Lacking accurate measurements at a number of energies, the early dose calculations were made using one spectral slope determined from the known energies (usually either very high in the GeV range or in the low MeV range), and extrapolating this spectral slope to all other energies. This method typically results in a considerable overesti- mate of the flux at the other end of the energy spectra thus leading to higher dose calculations. When the 29 Sep- tember 1989 event occurred, the NOAA Space Environment Center radiation dose prediction was significantly lar- ger than the actual measurement on the Concorde or by subsequent calculation by a number of researchers (O'Brien et al., 1996, 1998; Wilson, et al., 1991; Wilson and Nealy, 1992). The most likely reasons for these differences were the utilization of an outdated radiation transport model and the use of an unrealistic exponential model of the solar particle spectrum to extrapolate from low to high energies. The Flux and Spectral Evolution of the 29 September 1989 event. The 29 September 1989 solar cosmic ray ground-level event was large and significant by every measurement; Smart and Shea (1991) evaluated this as the third largest high energy solar proton event since the first ground-level solar cosmic ray event was identified in 1942. Particles with energies >20 GeV were present (Swinson and Shea, 1990); the event was observed at every operating cosmic ray detector on the Earth. The 29 September 1989 event occurred in the "modem" spacecraft era, and there are direct spacecraft measurements of the particle flux up to energies of about 800 MeV. The particle spectrum of this event is complex and not described by simple parameters. The initial reports of particle energy spectra for this event appeared to be inconsistent. Mathews and Venkatesan (1990) determined the spectral slope at the mean response of the high energy detectors (5 to 7 GeV) whereas Hum- ble et al. (1991) and Smart et al. (1991) determined the spectral slope at 1 GV rigidity. Subsequent analyses of this event (e.g., Cramp et al., 1993) show that the spectral slope is a function of energy and time. The slope is ex- tremely hard at low energies; the magnitude of the slope increases (i.e. becomes "softer") with increasing energy and time. An intensity-time plot of the GOES spacecraft data obtained for the 29 September 1989 event is pre- sented in Figures 11 and 12. (The GOES proton detectors has a serious side penetration problem whereby the high energy protons register in the low energy proton channels. Error flags and corrections for this problem removed the early part of the event for protons below 200 MeV.) The time-evolving spectra observed by the GOES spacecraft has been published by Smart and Shea (1997). 1040 D.F. Smart and M. A. Shea 0 (~ E 0 X U_ 2 102 2 lOl 1 oo 10-1 2 10-= 2 1 o --~ 2 ..... , ..... i ..... , ..... , . " , : . - " , - . - , . "~ .~,, , ,v .~p-..,"-- ~-~ s0.s ~v ~ 142 MeV S92 MeV ~ 620 MoV 670 MeV 10-4 . . . . I . . . . . I . . . . . I . . . . , I . . . . . I . . . . . I . . . . . I . . . . . -29 .00 29 .50 30 .00 30 .50 SEPTEMBER 1989 (UT day) Fig. 1 I . The 29 September 1989 solar proton flux from 11 to 800 MeV as observed by the GOES 6 and 7 Spacecraft. 10o ! ,-7 4 ~ S ~ 2 Z ~ 10"I $ t~ 10-2 X s ~ 2 -1 IJ. 1 0-s S 4 $ , , , . l . , . , , l , , , . , lUU, . , l , , , , , l , , , , ,U , , , , . l . , , , , 41 :z MoV :..,.~,:"~" ....."~'--%.r"" - / : ~' " ~ "l . j 440 MoV " - /~ . ,~ .':.: -",,.,,,%. "~ ! t4 2 . Alpha particle f lux 1 "4 , ,= ,1 . . . . . I . . . . . I . . . . . I . . . . . I . . . . . I . . . . . I . . . . . 029.00 29.50 30,00 30.50 SEPTEMBER 1989 (UT Day) Fig. 12. The 29 September 1989 solar alpha particle flux from 41 to 440 MeV/Nucleon as observed by the GOES 6 and 7 Spacecraft. Fluence Distributions Analysis of solar proton events measured at the Earth has led Feynman et al. (1993, 1997) to conclude that solar proton fluence during the "active years" of the solar cycle is well approximated by a log-normal function. Other analysis of the same data using exlreme value statistics (Xapsos et al., 1996) suggests that there may be two distributions; however, the application of either method gives similar results for the probability of a very large flux value being encountered during a long duration interplanetary mission. These models can be used to estimate the probable free space exposure to solar protons for various space missions beyond the Earth's geomagnetic shield as shown in Figure 13. Similar models for the heavy ion component of the large solar particle events observed during solar cycle 22, have been published by Tylka et al., (1997). Application of these models can be used to estimate possible solar particle exposure to interplanetary spacecraft from which risk assessments can be derived. w > 2 w w [-. o z w o 0.1 U. z 0 4 ~ 0.01 0 a. 0.001 10 o9 1 N'M Z I "~ "~ 1',2"'% YR~_ I .4 ;.,,J/ ] J)'% XI ~J "N 2YH--YI I/'P~I,Z" ",Z T',, r'..r' YR,F4 I ~ "~"~ ~, 13 ? f i 1010 FLUENCE ( ern -2 sr-1) '1 i 1011 Fig. 13. The probable exposure to >30 MeV protons during interplanetary missions. From Feynman et al. (1993). Reproduced by permissions of the American Geophysical Union. Review of Solar Proton Events 1041 Estimations of the fluence distribution of solar proton events over a very long period of time involves measuring the induced radioactivity in moon rocks and meteorites. Analysis of these data, combined with the Earth measurements over the past four solar cycles indicates that the very long term solar proton event fluence distribution may be represented by a broken power law (Reedy, 1996) as illustrated in Figure 14. The fluence of solar proton events observed at the Earth from 1954 through 1991 is in the left part of the figure. The right side of the figure shows the solar proton fiuences estimated from the analysis of induced radioactivity in moon rocks which encompasses a time span of perhaps one million years. The increasing slope of the fiuence distribution for very large events suggests that there may be limits to the acceleration process, and extraordinarily large events arc very rare. Flux Distributions Using the dam from the IMP 4 and IMP 5 spacecraft, van Hollebeke et al. (1975) determined that the proton event peak flux distribution could be fitted to a power law having a slope of -1.15. Cliver et al. (1991), analyzing the solar particle events detected by the IMP 8 spacecraft from November 1977 to October 1983, also fitted the event frequency distribution with a similar power law function. A more recent analysis of solar proton event data (Smart and Shea, 1997) concluded that there has been an unconscious exclusion of the very large events in the previous work due to instrumental limitations or restricted data samples. This more recent analysis of all of the events observed during solar cycles 20, 21 and 22 found two distinct particle flux populations There was a distribution of the most often occurring events, which for >10 MeV proton events can be described by a functional form having a slope o f - 0.47. This population extends from very low fluxes to flux values of approximately 1000 (cm2-s-sr) -l. Then there is a population of rare, very large >10 MeV proton events that can be described by a functional form having a slope of-1.4 as shown in Figure 15. LL A I@ 0 r- g: .~ 10" 10 L,_ N el) Z Cumulative Probabilities. Large Solar Particle Events 10' 10' 10" 10 ~ 10" 10 r' ~ ,..~ ,,..~ ,,,.,,,~ ,,.,,.I ,..,.I ,,,...,I , , . 101 i 1954"1991 ~ ' F'" 10" f t l l l l l l~ IH I I I~ l l l i t f l l~ J r f l~ l ld l l l l l l~ I I I I I I I~ J l t l t l l l~ i l l i l lB~ I I t / l l l d [ f l J l~ 10 .7 10' ld 10" 10 ~3 10 ~' 10" F, Event Fluence > 10 MeV (protons/cm 2) 10 .3 PROTON EVENT PEAK FLUX DISTRIBUTION Data from Cycles 20, 21, 22 w 1 o= , . J ! ~ 2 O UJ "o. 10 i r~O 4 I - 3 Z LIJ 2 > U.I 1 O0 = i UJ m 4 Z 2 ~' l " ' ' ' ' ' " ' ' ' . ' , ! ' ' ' ' " , ' ' ' ' " ! ' ' ' ' " ' ' ' ' "1 OTONS >10 MeV ~. ,A . I . . . . . . . , , , . . . I . . . . . . . . , , , . , , I . . . . . . . . . . . . . . I 01 2 a45 ' 102 2 a=5 t0= = a45 104 PEAK >1 0 MeV FLUX (Crn2 S Sr)-I Fig. 14. The fluence distribution of Earth-sensed proton events (heavy line) and proton fluence es- timates from the analysis of induced radioactivity in moon rocks (Adapted from Reedy, 1996) Fig. 15. The expected number of proton events during a solar cycle with the > 10 MeV proton flux exceeding a specified flux threshold 1042 D. E Smart and M. A. Shea SUMMARY From the viewpoint of solar particle events observed at the Earth, solar cycle 22 was quite active, having a total proton fluence slightly below the 19th solar cycle. Table 2 gives a summary of the solar proton events observed during the last four solar cycles. Table 3 lists the fluence of the major proton events during the last solar cycle. Table 2. Summary of Solar Proton Fluence for Solar Cycles 19 - 22 No. of Solar Cycle No. of No. of D iscrete Integrated Omnidirectional Months Discrete Proton* Solar Proton Flucnce Cycle in Proton* Producing No. Start End Cycle Events Regions >10 MeV>30 MeV (era -2) (cm -2) 19 May 54 O~ 64 126 65 47 7.2x10 lo 1.Sx10 E0 20 Nov 64 Jun 76 140 72 56 2,2x10 l0 6.9x109 21 Jul 76 Sep 86 123 81 57 1,8xlO 10 2.8x109 22 O~ 86 Sep 96 120 74 45 5.8xi010 1.0xl01 *Proton events with >10 MeV peak flux >I0 (cm2-s-sr) "1 Table 3. Summary of the Largest Solar Proton Fluence Occurrences for Solar Cycle 22 Cycle Begin End No. Yr Mo Dy Mo Dy Event Integrated Omnidirectional Solar Proton Fluence >10 MeV >30 MeV (cm 2) (cm 2) 22 89 03 09 03 24 0.12 x 89 08 12 - 08 18 0.76 x 89 09 29 - 10 02 0.38 x 89 10 19 - 10 30 1.9 x 89 11 27 - 12 03 0.51 x 89 12 30 - 01 02 (90) 0.21 x 90 05 21 - 05 31 0.035x 91 03 22 - 03 26 0.96 x 91 06 04 - 06 21 0.32 x 91 07 07 - 07 12 0.11 x 92 05 09 - 05 11 0.066x 92 10 30 - 11 05 0.35 x 94 02 20 02 22 0.099x 1010 0.03x 109 1010 1.4 x 109 1010 1.4 x 109 10 I 4.2 x 109 10 l 0.16x 109 10 l0 0.13x 109 101 0.14x 109 1010 1.8 x 109 1010 0.79x 109 1010 0.01x 109 101 O.02x 109 10 l 0.43x 109 101 O.02x 109 Totals: 5.82 xl01 1.Oxl010 Review of Solar Proton Events 1043 The measurements made by the new instrumentation developed during the 22nd solar cycle has resulted in a dra- marie change in perspeerive regarding the source of solar proton events observed in interplanetary space. 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