Using high-resolution mass spectrometers on board the Advanced Composition Explorer (ACE), we surveyed the event-averaged ~0.1-60 MeV/nuc heavy ion elemental composition in 64 large solar energetic particle (LSEP) events of cycle 23. Our results show the following: (1) The rare isotope ^3He is greatly enhanced over the corona or the solar wind values in 46% of the events. (2) The Fe/O ratio decreases with increasing energy up to ~10 MeV/nuc in ~92% of the events and up to ~60 MeV/nuc in ~64% of the events. (3) Heavy ion abundances from C-Fe exhibit systematic M/g-dependent enhancements that are remarkably similar to those seen in ^3He-rich SEP events and CME-driven interplanetary (IP) shock events. Taken together, these results confirm the role of shocks in energizing particles up to ~60 MeV/nuc in the majority of large SEP events of cycle 23, but also show that the seed population is not

dominated by ions originating from the ambient corona or the thermal solar wind, as previously

believed. Rather, it appears that the source material for CME-associated large SEP events

originates predominantly from a suprathermal population with a heavy ion enrichment pattern

that is organized according to the ion's mass-per-charge ratio. These new results indicate that

current LSEP models must include the routine production of this dynamic suprathermal seed

population as a critical pre-cursor to the CME shock acceleration process

Cohen, C. M. S.

Desai, M. I.

Dwyer, J. R.

Gold, R. E.

Krimigis, S. M.

Mason, G. M.

Mazur, J. E.

Mewaldt, R. A.

English

Caltech Authors - Main

Seed Populations for Large Solar Particle 
Events Of Cycle 23 
M. I. Desaf, G. M. Mason^ R. E. Gold^ S. M. Krlmlgls^ C. M. S. 
Cohen", R. A. Mewaldf, J. R Dwyer'', and J. E. Mazur ' 
'^Southwest Research Institute, 6220 Culebra Road, San Antonio, TX 78238 
.Johns Hopkins University/Applied Physics Laboratory, Laurel, MD 20723 
'California Institute of Technology, Pasadena, CA 91125 
Florida Institute of Technology, Melbourne, FL 32901 
e 
The Aerospace Corporation, Chantilly, VA 20151 
Abstract . Using high-resolution mass spectrometers on board the Advanced Composition 
Explorer (ACE), we surveyed the event-averaged -0.1-60 MeV/nuc heavy ion elemental 
composition in 64 large solar energetic particle (LSEP) events of cycle 23. Our results show the 
following: (1) The rare isotope 'He is greatly enhanced over the corona or the solar wind values 
in 46% of the events. (2) The Fe/O ratio decreases with increasing energy up to ~10 MeV/nuc in 
-92% of the events and up to ~60 MeV/nuc in -64% of the events. (3) Heavy ion abundances 
from C-Fe exhibit systematic M/g-dependent enhancements that are remarkably similar to those 
seen in 'He-rich SEP events and CME-driven interplanetary (IP) shock events. Taken together, 
these results confirm the role of shocks in energizing particles up to -60 MeV/nuc in the 
majority of large SEP events of cycle 23, but also show that the seed population is not 
dominated by ions originating from the ambient corona or the thermal solar wind, as previously 
believed. Rather, it appears that the source material for CME-associated large SEP events 
originates predominantly from a suprathermal population with a heavy ion enrichment pattern 
that is organized according to the ion's mass-per-charge ratio. These new results indicate that 
current LSEP models must include the routine production of this dynamic suprathermal seed 
population as a critical pre-cursor to the CME shock acceleration process. 
Keywords: Solar Energetic Particles, — Particle Acceleration - Sun — Solar Cycle 
PACS: 96.50.-e; 96.50.Pw; 96.50.sb 
INTRODUCTION 
Through much of the 1970's - 1990's, the near-Earth observations of energetic 
particle events from the Sun (SEPs) were grouped into two classes - impulsive and 
gradual. In this two-class picture the gradual events occurred as a result of diffusive 
acceleration of ambient coronal or solar wind material at CME-driven coronal and 
interplanetary (IP) shocks, while the impulsive events were attributed to stochastic 
acceleration of coronal material heated up to ~I0 MK during magnetic reconnection in 
solar flares (see review [I]). The gradual or CME shock-accelerated events lasted 
several days and had larger fluences, while the impulsive or flare-accelerated events 
lasted a few hours and had smaller fluences. Impulsive events were observed when the 
observer was magnetically connected to the flare site, while ions associated with large-
CP1039, Particle Acceleration and Transport in the Heliosphere and Beyond— 7^^ Annual Astrophysics Conference 
edited by G. Li, Q. Hu, O. Verkhoglyadova, G. P. ZanJi, R. P. Lin, and J. Lulimann 
O 2008 American Institute of Pliysics 978-0-7354-0566-0/08/$23.00 
124 
Downloaded 05 Sep 2008 to 131.215.225.9. Redistribution subject to AIP license or copyright; see http://proceedings.aip.org/proceedings/cpcr.jsp
scale CME-driven shocks populate magnetic field lines over a broad range of 
longitudes [2]. The impulsive events were electron-rich and associated with Type III 
radio bursts. These events also had ^He/''He ratios enhanced between factors of 10^-
lO"* and Fe/0 ratios enhanced by up to a factor of 10 over the corresponding solar 
wind values, and had Fe with ionization states up to -20. In contrast, the gradual 
events were proton-rich, were associated with Type II bursts, had average Fe/0 ratios 
of -O.I with Fe ionization states of -14, and were assumed to have ^He/'He ratios 
similar to the solar wind value [I, 3]. 
Since these earlier studies, instruments with greater sensitivity and resolution on 
board the Advanced Composition Explorer (ACE) [4] have provided major 
observational advances in terms of measuring (I) the solar wind ion composition and 
its variations [5-6] and (2) the energy-dependence and event-to-event variability of the 
ionic charge states, elemental, and isotopic composition in SEP events over a broad 
energy range [7-12]. These new observations have made it possible to re-examine the 
origin of the seed populations and probe details of the acceleration and transport 
mechanisms for individual large SEP events. In this paper, we discuss key 
observations that have spawned a re-evaluation of a key aspect of large SEP events -
that of the origin of the seed populations. 
OBSERVATIONS 
We use data from the Ultra-Low-Energy Isotope Spectrometer (ULEIS, [13]) and 
the Solar Isotope Spectrometer (SIS, [14]) on board ACE. We selected 64 large SEP 
(LSEP) events from NOAA's list of 85 solar proton events (SPEs) that affected 
Earth's environment between November 1997 and January 2005. For details of the 
event selection and their sampling intervals see [10]. 
He Enhancements 
Figure la shows the 0.5-2.0 MeV/nuc ^He and ''He time-intensity profiles in a large 
SEP event that occurred on June 4, 1999 (from [15]). The temporal profiles of the two 
species are remarkably similar, which indicates that they probably have the same 
acceleration and transport history. In this event, the ^He is enriched by a factor of 16 ± 
3 while the Fe/0 ratio (not shown) is enhanced by about a factor of 10 over the 
corresponding solar wind values. 
In this survey, we find that the 0.5-2.0 MeV/nuc ^He/''He ratio in 29 of the 64 
events (-46%) is enhanced between factors ^ - 1 5 0 over the corresponding solar wind 
value. Large enrichments in the ^He/''He ratio are also observed above -10 MeV/nuc 
[16-17]. Figure lb shows the - I MeV/nuc He mass histogram from several such 
events. Notice that the ^He is clearly resolved from ''He and the background. 
125 
Downloaded 05 Sep 2008 to 131.215.225.9. Redistribution subject to AIP license or copyright; see http://proceedings.aip.org/proceedings/cpcr.jsp
ACE / ULEIS 0.7 MeV/nucleon 
Mass (AMU) 
FIGURE 1: (a) Temporal profiles of-0.7 MeV/nuc ^He and "He ions in a large SEP event, (b) 0.5-2.0 
MeV/nuc He mass histogram obtained during several large SEP events. The right scale corresponds to 
the open histogram (taken from [15]). 
Energy-Dependence of Fe/O 
We compared (see [12]) the average LSEP abundances with the average slow solar 
wind values as a function of M/Q or First Ionization Potential (FIP), and found that the 
LSEP abundances at -0.4 MeV/nuc were poorly correlated with the solar wind values. 
In particular, we found that elements with similar mass-per-charge (M/Q) values like 
C, N, O, Ne, and Mg exhibited highly unsystematic enhancements and depletions 
relative to the corresponding solar wind values, with the C/0 ratio in particular being 
depleted by about a factor of 2. 
Figure 2 shows the energy-dependent behavior of Fe/0 in 37 of the 64 large SEP 
events in this survey. The Fe/0 ratios are 0.11-0.14 MeV/nuc from ULEIS [12]; 3.3-
10 MeV/nuc from LEMT [18]; and 12-60 MeV/nuc from SIS [12]. Note that the Fe/0 
ratio either decreases or remains constant with increasing energy in 34 of the 37 events 
(-92%) up to -10 MeV/nuc and in 26 of the 36 events (-64%) up to -60 MeV/nuc. 
However, an unexpected and puzzling feature of Fig. 2a is that the low-energy Fe/0 
ratio between 0.11-0.14 MeV/nuc in most of the LSEP events (55 of the 64 events; 
see [12]) is enhanced when compared with the average solar wind value. 
Mass-per-Charge Dependent Enhancements 
In order to examine whether the large enhancements and the event-to-event 
variability seen in the low-energy Fe/0 are systematically organized by the ion's M/Q 
ratio, we plot in Fig. 3: (a) the S/0 ratio and (b) the Ca/0 ratio versus the Fe/C ratio at 
-0.38 MeV/nuc measured in the 64 LSEP events in the survey. We chose to plot the 
event-averaged abundances of S, Ca, and Fe (i.e., low FIP elements) relative to C and 
O (i.e., high FIP elements) to minimize the so-called FIP fractionation effect seen in 
large SEP events [19]. 
Figure 3 also compares the LSEP abundances with those seen in IP shock events of 
[20] and ^He-rich SEP events of [21]. The dashed line represents a linear fit to the 
LSEP data with slope m and intercept c. We remark that the slopes and intercepts of 
126 
Downloaded 05 Sep 2008 to 131.215.225.9. Redistribution subject to AIP license or copyright; see http://proceedings.aip.org/proceedings/cpcr.jsp
10̂  
> 
0 
° 10" 
I 
CO 
J2-
O 
,? 10-S 
10-' 
1 1 M Mil 1 1 
• 3 events 
"W 8 events 
r A 26 events 
; 
• 
SW 
/ 
-'/ / -'/ / 
// / 
^/ ^ 
/ / / 
'//^ 1 
M MM 
/ 
• 
/ 
1 1 M MM 1 yy 
^ / / 
M^^ '/L'^^ k. 
k 
1 
V 
(a) 
10"' 10"' 10" lO'̂  10""" 10"' lO'' 
Fe/0 (0.11-0.14 MeV/n.) Fe/0 (3.3-10 MeV/n.) 
FIGURE 2: (a) Fe/O ratio measured at 0.11-0.14 MeV/nuc versus that measured at 3.3-10 MeV/nuc 
for 37 large SEP events, (b) Fe/O ratio measured at 3.3-10 MeV/nuc versus that measured at 12-60 
MeV/nuc for 36 SEP events. Yellow bands represent error limits for the Fe/O ratio measured in the slow 
solar wind (taken from [12]). 
the hnear fits obtained independently by fitting the data for IP shocks (blue) and He-
rich events (green) are well within the respective la error limits of the fit parameters 
for the three types of events. 
Figure 3 shows the following: (1) Enhancements in S/0 and Ca/0 are accompanied 
by simultaneous enhancements in the Fe/C ratio. (2) The fits to the data provide an 
excellent representation of the event-to-event variations and the enhancement pattern 
of the heavy ion abundances in large SEP events. (3) For each element, the power-law 
dependence in large SEP events provides remarkably good fits to the corresponding 
event-to-event variations seen in IP shock events and in He-rich SEP events. 
DISCUSSION 
Our survey of the -0.1-60 MeV/nuc heavy ion abundances in 64 LSEP events of 
cycle 23 shows the following: 
1. ^He is enhanced over the corona or the solar wind values in 46% of the events. 
2. The Fe/O ratio decreases with increasing energy in -92% of the events up to -10 
MeV/nuc and in -64% of the events up to -60 MeV/nuc. 
3. Event-to-event variations in the heavy ion abundances in LSEP events are 
organized according to the M/Q ratio and are remarkably similar to those seen in 
^He-rich SEP events and IP shock events. 
127 
Downloaded 05 Sep 2008 to 131.215.225.9. Redistribution subject to AIP license or copyright; see http://proceedings.aip.org/proceedings/cpcr.jsp
10' 
10-^ 
10"' 
: -^ isirs 
A IP shocks 
. • ^lie-rich SEPs 
=̂  J 
4jr 
1 1 
n 1 : 
N = 64; r = 0.95 
m = 0.46 ± 0.01' 
c= 0.06+0.001 
.1 1 . . . 
10" 
10" 
O 
10"̂  
10-
a LSWs 
A IP shocks 
• ^He-rich SEPs 
I 11 Mil 1—r 
(b) 
0.01 0.1 1 
Fe/C 
10 0.01 
r ^ -
N = 64;r = 0.95 • 
m = 0.64 ±0.019" 
c = 0.02 ±0.0004 
I I L 
10 
FIGURE 3: (a) S/O and (b) Ca/O plotted versus Fe/C ratio at 0.38 MeV/nuc for three different types of 
events. Blue triangles = IP shock events of [20]; red asterisks = large SEP events in this survey; and 
green squares = 'He-rich SEP events of [21]. Colored arrows along the axes represent the average 
values of the respective populations. Dashed black line represents the linear fit to the large SEP data. 
The quantities N and r represent the number of SEP events and the correlation coefficient. The 
quantities m and c represent the slopes and intercepts of the linear fits (taken from [10]). 
Two-stage Acceleration in Large SEP Events 
Rigidity-dependent shock acceleration processes tend to deplete the abundances of 
heavier ions when compared with the lighter ions (e.g., [20, 22-24]). Fig. 2 shows that 
the Fe/0 ratio decreases with increasing energy in the majority of the LSEP events in 
our survey. Such systematic energy dependence of the SEP Fe/0 ratios were also 
reported by a number of previous studies [9, 25-27]. Likewise, the Fe/0 ratios in IP 
shock events at ACE also exhibited similar properties [20, 24], and are consistent with 
expectations from shock acceleration models [28-29] where ions with higher M/Q 
ratios are accelerated less efficiently than those with lower M/g ratios. 
However, the ^He/'He ratio (Fig. lb) and the heavy ion abundances in large SEP 
events (Fig. 2a) are significantly enhanced relative to the slow solar wind or the 
ambient coronal abundances [10, 15-16]. Since ^He has a smaller M/g ratio than ''He 
while ions such as Fe have a larger M/Q ratio than O, such large simultaneous 
enrichments are difficult to reconcile with rigidity-dependent shock acceleration of 
solar wind material. This imphes that the dominant seed population for the majority of 
the large SEP events could not have originated either from the thermal or suprathermal 
solar wind or from the ambient corona [27, 30]. In fact. Figs. 2 and 3 point to the 
occurrence of two distinct M/Q dependent fractionation processes in the same large 
SEP event. The first process results in producing M/g-dependent enhancements, 
similar to those seen in the ^He-rich SEP events, while the second process—probably 
due to shock acceleration—causes the Fe/0 ratio to decrease with increasing energy. 
Figure 2b also shows that for -36% of the events there is a decrease in Fe/0 around 
~3-I0 MeV/nuc followed by an increase at the higher energies. This behavior is 
puzzling and could be due to a separate mechanism operating at these high energies 
e.g., a direct Fe-rich acceleration event near the Sun [31, 32], or perhaps the re-
128 
Downloaded 05 Sep 2008 to 131.215.225.9. Redistribution subject to AIP license or copyright; see http://proceedings.aip.org/proceedings/cpcr.jsp
acceleration of a flare seed population with Fe/0 ratios that increase with energy [26]. 
No doubt other possibilities exist, and we beheve that there is insufficient data to 
resolve the issue at this time. 
Using the fact that the heavy ion abundances in CME-driven IP shocks at 1 AU 
[20] are depleted systematically according to the ion's M/Q ratio when compared with 
the ambient suprathermal population measured prior to the arrival of the shocks, [10] 
suggested that the average abundances for the "source" population for LSEPs may be 
even more abundant in heavier ions than the observed abundances in LSEPs and could 
resemble that seen in impulsive SEPs. These new results indicate that current LSEP 
models must include the routine production of a dynamic suprathermal or pre-
accelerated seed population as a critical pre-cursor to CME shock acceleration. 
ACKNOWLEDGEMENTS 
Work at SwRI is partially supported by NSF grants ATM-0550960 and ATM-
0551127 and NASA grants: NNG05GM88G, NNG05GQ94G, NNX07AC12G, 
NNX07AG85G, NNX07AP69G, and NNX08AK87G. We thank the following 
organizations for permission to pubhsh copyrighted material: Elsevier (Fig. 2, from 
[12]); and American Astronomical Society (Fig. 1 from [15] and Fig. 3 from 10]). 
REFERENCES 
1. D.V. Reames, Space Sci. Rev. 90, 413 (1999) 
2. H.V. Cane, D.V. Reames, T.T. von Rosenvinge, Astrophys. J. 93, 9555 (1988) 
3. E.W. Oliver, AIP Conference Proceedings, vol. 528, 21 (2000) 
4. E.G. Stone, et al.. Space Sci. Rev. 86, 1 (1998a) 
5. G. Gloeckler et al., Astron. Astrophys. Suppl. Ser. 92, 267 (1992) 
6. J. Geiss, G. Gloeckler, R. von Steiger, Space Sci. Rev. 78, 43 (1996) 
7. J.E. Mazur et al., Geophys. Res. Lett. 26, 173 (1999) 
8. E. Mobius et al., Geophys. Res. Lett. 26, 145 (1999) 
9. C.M.S. Cohen et al., J. Geophys. Res. 110, A09S16 (2005) 
10. M.l. Desai et al., Astrophys. J. 649, 470 (2006) 
11. B. Klecker et al.. Space Sci. Rev. 124(1-4), 289 (2006) 
12. M.l. Desai et al.. Space Sci. Rev. 649, 470 (2007) 
13. G.M. Mason et al.. Space Sci. Rev. 86, 409 (1998) 
14. E.G. Stone et al.. Space Sci. Rev. 86, 285-356 (1998b) 
15. G.M. Mason, J.E. Mazur, J.R. Dwyer, Astrophys. J. Lett. 525, L133 (1999) 
16. C.M.S. Cohen et al., Geophys. Res. Lett. 26, 2697 (1999) 
17. M. E. Wiedenbeck et al., AIP, CP, 528, 107, (2000) 
18. D.V. Reames, C.K. Ng, Astrophys. J. 610, 510 (2004) 
19. R.A. Mewaldtetal., Space Sci. Rev., doi:10.1007/sll214-007-9187-l (2007) 
20. M.l. Desai et al., Astrophys. J. 558, 1149 (2003) 
21. G.M. Mason et al., Astrophys. J. 606, 555 (2004) 
22. B. Klecker et al., Astrophys. J. 251, 391 (1981) 
23. H.V. Cane, D.V. Reames, T.T. von Rosenvinge, Astrophys. J. 373, 675 (1991) 
24. M.l. Desai etal., Astrophys. J. 611, 1156 (2004) 
25. J.E. Mazur et al., Astrophys. J. 401, 398 (1992) 
26. A. J. Tylka et al., Astrophys. J. 625, 474 (2005) 
27. R.A. Mewaldt, C.M.S. Cohen, G.M. Mason, in Geophys. Monograph Series, vol. 165, 115, (2006) 
28. M.A. Lee, Astrophys. J. 158, 38 (2005) 
129 
Downloaded 05 Sep 2008 to 131.215.225.9. Redistribution subject to AIP license or copyright; see http://proceedings.aip.org/proceedings/cpcr.jsp
29. G. Li, et al., J. Geophys. Res., 110, A06104 (2005) 
30. R.A. Mewaldt et al.. Adv. Space Res. 30, 79 (2002) 
31. H. V. Cane, et al., Geophys. Res. Lett. 30, Issue 12, SEP 5-1, (2003) 
32. H. V. Cane, et al., J. of Geophys. Res. I l l , Issue A6, doi: 10.1029/2005JA011071, (2006) 
130 
Downloaded 05 Sep 2008 to 131.215.225.9. Redistribution subject to AIP license or copyright; see http://proceedings.aip.org/proceedings/cpcr.jsp


Seed populations for large solar particle events of cycle 23

Seed Populations for Large Solar Particle 
Events Of Cycle 23 
M. I. Desaf, G. M. Mason^ R. E. Gold^ S. M. Krlmlgls^ C. M. S. 
Cohen", R. A. Mewaldf, J. R Dwyer'', and J. E. Mazur ' 
'^Southwest Research Institute, 6220 Culebra Road, San Antonio, TX 78238 
.Johns Hopkins University/Applied Physics Laboratory, Laurel, MD 20723 
'California Institute of Technology, Pasadena, CA 91125 
Florida Institute of Technology, Melbourne, FL 32901 
e 
The Aerospace Corporation, Chantilly, VA 20151 
Abstract . Using high-resolution mass spectrometers on board the Advanced Composition 
Explorer (ACE), we surveyed the event-averaged -0.1-60 MeV/nuc heavy ion elemental 
composition in 64 large solar energetic particle (LSEP) events of cycle 23. Our results show the 
following: (1) The rare isotope 'He is greatly enhanced over the corona or the solar wind values 
in 46% of the events. (2) The Fe/O ratio decreases with increasing energy up to ~10 MeV/nuc in 
-92% of the events and up to ~60 MeV/nuc in -64% of the events. (3) Heavy ion abundances 
from C-Fe exhibit systematic M/g-dependent enhancements that are remarkably similar to those 
seen in 'He-rich SEP events and CME-driven interplanetary (IP) shock events. Taken together, 
these results confirm the role of shocks in energizing particles up to -60 MeV/nuc in the 
majority of large SEP events of cycle 23, but also show that the seed population is not 
dominated by ions originating from the ambient corona or the thermal solar wind, as previously 
believed. Rather, it appears that the source material for CME-associated large SEP events 
originates predominantly from a suprathermal population with a heavy ion enrichment pattern 
that is organized according to the ion's mass-per-charge ratio. These new results indicate that 
current LSEP models must include the routine production of this dynamic suprathermal seed 
population as a critical pre-cursor to the CME shock acceleration process. 
Keywords: Solar Energetic Particles, — Particle Acceleration - Sun — Solar Cycle 
PACS: 96.50.-e; 96.50.Pw; 96.50.sb 
INTRODUCTION 
Through much of the 1970's - 1990's, the near-Earth observations of energetic 
particle events from the Sun (SEPs) were grouped into two classes - impulsive and 
gradual. In this two-class picture the gradual events occurred as a result of diffusive 
acceleration of ambient coronal or solar wind material at CME-driven coronal and 
interplanetary (IP) shocks, while the impulsive events were attributed to stochastic 
acceleration of coronal material heated up to ~I0 MK during magnetic reconnection in 
solar flares (see review [I]). The gradual or CME shock-accelerated events lasted 
several days and had larger fluences, while the impulsive or flare-accelerated events 
lasted a few hours and had smaller fluences. Impulsive events were observed when the 
observer was magnetically connected to the flare site, while ions associated with large-
CP1039, Particle Acceleration and Transport in the Heliosphere and Beyond— 7^^ Annual Astrophysics Conference 
edited by G. Li, Q. Hu, O. Verkhoglyadova, G. P. ZanJi, R. P. Lin, and J. Lulimann 
O 2008 American Institute of Pliysics 978-0-7354-0566-0/08/$23.00 
124 
Downloaded 05 Sep 2008 to 131.215.225.9. Redistribution subject to AIP license or copyright; see http://proceedings.aip.org/proceedings/cpcr.jsp
scale CME-driven shocks populate magnetic field lines over a broad range of 
longitudes [2]. The impulsive events were electron-rich and associated with Type III 
radio bursts. These events also had ^He/''He ratios enhanced between factors of 10^-
lO"* and Fe/0 ratios enhanced by up to a factor of 10 over the corresponding solar 
wind values, and had Fe with ionization states up to -20. In contrast, the gradual 
events were proton-rich, were associated with Type II bursts, had average Fe/0 ratios 
of -O.I with Fe ionization states of -14, and were assumed to have ^He/'He ratios 
similar to the solar wind value [I, 3]. 
Since these earlier studies, instruments with greater sensitivity and resolution on 
board the Advanced Composition Explorer (ACE) [4] have provided major 
observational advances in terms of measuring (I) the solar wind ion composition and 
its variations [5-6] and (2) the energy-dependence and event-to-event variability of the 
ionic charge states, elemental, and isotopic composition in SEP events over a broad 
energy range [7-12]. These new observations have made it possible to re-examine the 
origin of the seed populations and probe details of the acceleration and transport 
mechanisms for individual large SEP events. In this paper, we discuss key 
observations that have spawned a re-evaluation of a key aspect of large SEP events -
that of the origin of the seed populations. 
OBSERVATIONS 
We use data from the Ultra-Low-Energy Isotope Spectrometer (ULEIS, [13]) and 
the Solar Isotope Spectrometer (SIS, [14]) on board ACE. We selected 64 large SEP 
(LSEP) events from NOAA's list of 85 solar proton events (SPEs) that affected 
Earth's environment between November 1997 and January 2005. For details of the 
event selection and their sampling intervals see [10]. 
He Enhancements 
Figure la shows the 0.5-2.0 MeV/nuc ^He and ''He time-intensity profiles in a large 
SEP event that occurred on June 4, 1999 (from [15]). The temporal profiles of the two 
species are remarkably similar, which indicates that they probably have the same 
acceleration and transport history. In this event, the ^He is enriched by a factor of 16 ± 
3 while the Fe/0 ratio (not shown) is enhanced by about a factor of 10 over the 
corresponding solar wind values. 
In this survey, we find that the 0.5-2.0 MeV/nuc ^He/''He ratio in 29 of the 64 
events (-46%) is enhanced between factors ^ - 1 5 0 over the corresponding solar wind 
value. Large enrichments in the ^He/''He ratio are also observed above -10 MeV/nuc 
[16-17]. Figure lb shows the - I MeV/nuc He mass histogram from several such 
events. Notice that the ^He is clearly resolved from ''He and the background. 
125 
Downloaded 05 Sep 2008 to 131.215.225.9. Redistribution subject to AIP license or copyright; see http://proceedings.aip.org/proceedings/cpcr.jsp
ACE / ULEIS 0.7 MeV/nucleon 
Mass (AMU) 
FIGURE 1: (a) Temporal profiles of-0.7 MeV/nuc ^He and "He ions in a large SEP event, (b) 0.5-2.0 
MeV/nuc He mass histogram obtained during several large SEP events. The right scale corresponds to 
the open histogram (taken from [15]). 
Energy-Dependence of Fe/O 
We compared (see [12]) the average LSEP abundances with the average slow solar 
wind values as a function of M/Q or First Ionization Potential (FIP), and found that the 
LSEP abundances at -0.4 MeV/nuc were poorly correlated with the solar wind values. 
In particular, we found that elements with similar mass-per-charge (M/Q) values like 
C, N, O, Ne, and Mg exhibited highly unsystematic enhancements and depletions 
relative to the corresponding solar wind values, with the C/0 ratio in particular being 
depleted by about a factor of 2. 
Figure 2 shows the energy-dependent behavior of Fe/0 in 37 of the 64 large SEP 
events in this survey. The Fe/0 ratios are 0.11-0.14 MeV/nuc from ULEIS [12]; 3.3-
10 MeV/nuc from LEMT [18]; and 12-60 MeV/nuc from SIS [12]. Note that the Fe/0 
ratio either decreases or remains constant with increasing energy in 34 of the 37 events 
(-92%) up to -10 MeV/nuc and in 26 of the 36 events (-64%) up to -60 MeV/nuc. 
However, an unexpected and puzzling feature of Fig. 2a is that the low-energy Fe/0 
ratio between 0.11-0.14 MeV/nuc in most of the LSEP events (55 of the 64 events; 
see [12]) is enhanced when compared with the average solar wind value. 
Mass-per-Charge Dependent Enhancements 
In order to examine whether the large enhancements and the event-to-event 
variability seen in the low-energy Fe/0 are systematically organized by the ion's M/Q 
ratio, we plot in Fig. 3: (a) the S/0 ratio and (b) the Ca/0 ratio versus the Fe/C ratio at 
-0.38 MeV/nuc measured in the 64 LSEP events in the survey. We chose to plot the 
event-averaged abundances of S, Ca, and Fe (i.e., low FIP elements) relative to C and 
O (i.e., high FIP elements) to minimize the so-called FIP fractionation effect seen in 
large SEP events [19]. 
Figure 3 also compares the LSEP abundances with those seen in IP shock events of 
[20] and ^He-rich SEP events of [21]. The dashed line represents a linear fit to the 
LSEP data with slope m and intercept c. We remark that the slopes and intercepts of 
126 
Downloaded 05 Sep 2008 to 131.215.225.9. Redistribution subject to AIP license or copyright; see http://proceedings.aip.org/proceedings/cpcr.jsp
10^  
> 
0 
° 10" 
I 
CO 
J2-
O 
,? 10-S 
10-' 
1 1 M Mil 1 1 
• 3 events 
"W 8 events 
r A 26 events 
; 
• 
SW 
/ 
-'/ / 
-'/ / 
// / 
^/ ^ 
/ / / 
'//^ 1 
M MM 
/ 
• 
/ 
1 1 M MM 1 yy 
^ / / 
M^^ 
'/L'^^ k. 
k 
1 
V 
(a) 
10"' 10"' 10" lO'^  10""" 10"' lO'' 
Fe/0 (0.11-0.14 MeV/n.) Fe/0 (3.3-10 MeV/n.) 
FIGURE 2: (a) Fe/O ratio measured at 0.11-0.14 MeV/nuc versus that measured at 3.3-10 MeV/nuc 
for 37 large SEP events, (b) Fe/O ratio measured at 3.3-10 MeV/nuc versus that measured at 12-60 
MeV/nuc for 36 SEP events. Yellow bands represent error limits for the Fe/O ratio measured in the slow 
solar wind (taken from [12]). 
the hnear fits obtained independently by fitting the data for IP shocks (blue) and He-
rich events (green) are well within the respective la error limits of the fit parameters 
for the three types of events. 
Figure 3 shows the following: (1) Enhancements in S/0 and Ca/0 are accompanied 
by simultaneous enhancements in the Fe/C ratio. (2) The fits to the data provide an 
excellent representation of the event-to-event variations and the enhancement pattern 
of the heavy ion abundances in large SEP events. (3) For each element, the power-law 
dependence in large SEP events provides remarkably good fits to the corresponding 
event-to-event variations seen in IP shock events and in He-rich SEP events. 
DISCUSSION 
Our survey of the -0.1-60 MeV/nuc heavy ion abundances in 64 LSEP events of 
cycle 23 shows the following: 
1. ^He is enhanced over the corona or the solar wind values in 46% of the events. 
2. The Fe/O ratio decreases with increasing energy in -92% of the events up to -10 
MeV/nuc and in -64% of the events up to -60 MeV/nuc. 
3. Event-to-event variations in the heavy ion abundances in LSEP events are 
organized according to the M/Q ratio and are remarkably similar to those seen in 
^He-rich SEP events and IP shock events. 
127 
Downloaded 05 Sep 2008 to 131.215.225.9. Redistribution subject to AIP license or copyright; see http://proceedings.aip.org/proceedings/cpcr.jsp
10' 
10-^ 
10"' 
: -^ isirs 
A IP shocks 
. • ^lie-rich SEPs 
=^  J 
4jr 
1 1 
n 1 : 
N = 64; r = 0.95 
m = 0.46 ± 0.01' 
c= 0.06+0.001 
.1 1 . . . 
10" 
10" 
O 
10"^  
10-
a LSWs 
A IP shocks 
• ^He-rich SEPs 
I 11 Mil 1—r 
(b) 
0.01 0.1 1 
Fe/C 
10 0.01 
r ^ -
N = 64;r = 0.95 • 
m = 0.64 ±0.019" 
c = 0.02 ±0.0004 
I I L 
10 
FIGURE 3: (a) S/O and (b) Ca/O plotted versus Fe/C ratio at 0.38 MeV/nuc for three different types of 
events. Blue triangles = IP shock events of [20]; red asterisks = large SEP events in this survey; and 
green squares = 'He-rich SEP events of [21]. Colored arrows along the axes represent the average 
values of the respective populations. Dashed black line represents the linear fit to the large SEP data. 
The quantities N and r represent the number of SEP events and the correlation coefficient. The 
quantities m and c represent the slopes and intercepts of the linear fits (taken from [10]). 
Two-stage Acceleration in Large SEP Events 
Rigidity-dependent shock acceleration processes tend to deplete the abundances of 
heavier ions when compared with the lighter ions (e.g., [20, 22-24]). Fig. 2 shows that 
the Fe/0 ratio decreases with increasing energy in the majority of the LSEP events in 
our survey. Such systematic energy dependence of the SEP Fe/0 ratios were also 
reported by a number of previous studies [9, 25-27]. Likewise, the Fe/0 ratios in IP 
shock events at ACE also exhibited similar properties [20, 24], and are consistent with 
expectations from shock acceleration models [28-29] where ions with higher M/Q 
ratios are accelerated less efficiently than those with lower M/g ratios. 
However, the ^He/'He ratio (Fig. lb) and the heavy ion abundances in large SEP 
events (Fig. 2a) are significantly enhanced relative to the slow solar wind or the 
ambient coronal abundances [10, 15-16]. Since ^He has a smaller M/g ratio than ''He 
while ions such as Fe have a larger M/Q ratio than O, such large simultaneous 
enrichments are difficult to reconcile with rigidity-dependent shock acceleration of 
solar wind material. This imphes that the dominant seed population for the majority of 
the large SEP events could not have originated either from the thermal or suprathermal 
solar wind or from the ambient corona [27, 30]. In fact. Figs. 2 and 3 point to the 
occurrence of two distinct M/Q dependent fractionation processes in the same large 
SEP event. The first process results in producing M/g-dependent enhancements, 
similar to those seen in the ^He-rich SEP events, while the second process—probably 
due to shock acceleration—causes the Fe/0 ratio to decrease with increasing energy. 
Figure 2b also shows that for -36% of the events there is a decrease in Fe/0 around 
~3-I0 MeV/nuc followed by an increase at the higher energies. This behavior is 
puzzling and could be due to a separate mechanism operating at these high energies 
e.g., a direct Fe-rich acceleration event near the Sun [31, 32], or perhaps the re-
128 
Downloaded 05 Sep 2008 to 131.215.225.9. Redistribution subject to AIP license or copyright; see http://proceedings.aip.org/proceedings/cpcr.jsp
acceleration of a flare seed population with Fe/0 ratios that increase with energy [26]. 
No doubt other possibilities exist, and we beheve that there is insufficient data to 
resolve the issue at this time. 
Using the fact that the heavy ion abundances in CME-driven IP shocks at 1 AU 
[20] are depleted systematically according to the ion's M/Q ratio when compared with 
the ambient suprathermal population measured prior to the arrival of the shocks, [10] 
suggested that the average abundances for the "source" population for LSEPs may be 
even more abundant in heavier ions than the observed abundances in LSEPs and could 
resemble that seen in impulsive SEPs. These new results indicate that current LSEP 
models must include the routine production of a dynamic suprathermal or pre-
accelerated seed population as a critical pre-cursor to CME shock acceleration. 
ACKNOWLEDGEMENTS 
Work at SwRI is partially supported by NSF grants ATM-0550960 and ATM-
0551127 and NASA grants: NNG05GM88G, NNG05GQ94G, NNX07AC12G, 
NNX07AG85G, NNX07AP69G, and NNX08AK87G. We thank the following 
organizations for permission to pubhsh copyrighted material: Elsevier (Fig. 2, from 
[12]); and American Astronomical Society (Fig. 1 from [15] and Fig. 3 from 10]). 
REFERENCES 
1. D.V. Reames, Space Sci. Rev. 90, 413 (1999) 
2. H.V. Cane, D.V. Reames, T.T. von Rosenvinge, Astrophys. J. 93, 9555 (1988) 
3. E.W. Oliver, AIP Conference Proceedings, vol. 528, 21 (2000) 
4. E.G. Stone, et al.. Space Sci. Rev. 86, 1 (1998a) 
5. G. Gloeckler et al., Astron. Astrophys. Suppl. Ser. 92, 267 (1992) 
6. J. Geiss, G. Gloeckler, R. von Steiger, Space Sci. Rev. 78, 43 (1996) 
7. J.E. Mazur et al., Geophys. Res. Lett. 26, 173 (1999) 
8. E. Mobius et al., Geophys. Res. Lett. 26, 145 (1999) 
9. C.M.S. Cohen et al., J. Geophys. Res. 110, A09S16 (2005) 
10. M.l. Desai et al., Astrophys. J. 649, 470 (2006) 
11. B. Klecker et al.. Space Sci. Rev. 124(1-4), 289 (2006) 
12. M.l. Desai et al.. Space Sci. Rev. 649, 470 (2007) 
13. G.M. Mason et al.. Space Sci. Rev. 86, 409 (1998) 
14. E.G. Stone et al.. Space Sci. Rev. 86, 285-356 (1998b) 
15. G.M. Mason, J.E. Mazur, J.R. Dwyer, Astrophys. J. Lett. 525, L133 (1999) 
16. C.M.S. Cohen et al., Geophys. Res. Lett. 26, 2697 (1999) 
17. M. E. Wiedenbeck et al., AIP, CP, 528, 107, (2000) 
18. D.V. Reames, C.K. Ng, Astrophys. J. 610, 510 (2004) 
19. R.A. Mewaldtetal., Space Sci. Rev., doi:10.1007/sll214-007-9187-l (2007) 
20. M.l. Desai et al., Astrophys. J. 558, 1149 (2003) 
21. G.M. Mason et al., Astrophys. J. 606, 555 (2004) 
22. B. Klecker et al., Astrophys. J. 251, 391 (1981) 
23. H.V. Cane, D.V. Reames, T.T. von Rosenvinge, Astrophys. J. 373, 675 (1991) 
24. M.l. Desai etal., Astrophys. J. 611, 1156 (2004) 
25. J.E. Mazur et al., Astrophys. J. 401, 398 (1992) 
26. A. J. Tylka et al., Astrophys. J. 625, 474 (2005) 
27. R.A. Mewaldt, C.M.S. Cohen, G.M. Mason, in Geophys. Monograph Series, vol. 165, 115, (2006) 
28. M.A. Lee, Astrophys. J. 158, 38 (2005) 
129 
Downloaded 05 Sep 2008 to 131.215.225.9. Redistribution subject to AIP license or copyright; see http://proceedings.aip.org/proceedings/cpcr.jsp
29. G. Li, et al., J. Geophys. Res., 110, A06104 (2005) 
30. R.A. Mewaldt et al.. Adv. Space Res. 30, 79 (2002) 
31. H. V. Cane, et al., Geophys. Res. Lett. 30, Issue 12, SEP 5-1, (2003) 
32. H. V. Cane, et al., J. of Geophys. Res. I l l , Issue A6, doi: 10.1029/2005JA011071, (2006) 
130 
Downloaded 05 Sep 2008 to 131.215.225.9. Redistribution subject to AIP license or copyright; see http://proceedings.aip.org/proceedings/cpcr.jsp


of Geophys. Res. I l l , Issue A6, doi: 10.1029/2005JA011071,

Space Sci.

to 131.215.225.9. Redistribution subject to AIP license or copyright; see http://proceedings.aip.org/proceedings/cpcr.jsp

Tylka et al.,

https://authors.library.caltech.edu/14301/1/DESaipcp08.pdf

Seed populations for large solar particle events of cycle 23

Abstract

Similar works

Full text

Available Versions

Caltech Authors - Main

Caltech Authors - Main