Using the Solar Isotope Spectrometer (SIS) on the Advanced Composition Explorer (ACE), we have studied the properties of a selection of small ^3He-rich solar energetic particle (SEP) events with heavy ion enhancements in the energy range ~11–22 MeV/nucleon. These events contain significantly increased ^3He/^4He ratios over the solar wind value of 0.0004 in the energy range ~4.5–5.5 MeV/nucleon. In order to characterize the events, the following features have been investigated. First, the heavy element content has been measured and compared to that found in past studies of impulsive SEP events. Next, the simultaneous 38–53 keV electron flux, measured with the Electron, Proton, and Alpha Monitor (EPAM) on ACE, has been examined for possible activity near the ^3He-rich event onset times. Finally a list of measured solar X-ray flares, with corresponding H-alpha flares where possible, has been scrutinized for potential correlations with these events. The results show an apparent correlation between event onset and increased electron flux, and a possible association with X-ray flares

Christian, E. R.

Cohen, C. M. S.

Cummings, A. C.

Leske, R. A.

Mewaldt, R. A.

Slocum, P. L.

Stone, E. C.

von Rosenvinge, T. T.

Wiedenbeck, M. E.

English

Caltech Authors - Main

Measurements of Heavy Elements in 3He-rich SEP Events
P.L. Slocum1, M.E. Wiedenbeck1, C.M.S. Cohen3, E.R. Christian2, A.C. Cummings3,
R.A. Leske3, R.A. Mewaldt3, E.G. Stone3, and T.T. von Rosenvinge2
1
 Jet Propulsion Laboratory, Pasadena, CA 91109
2NASA/Goddard Space Flight Center, Greenbelt, MD 20771
^California Institute of Technology, Pasadena, CA 91125
Abstract. Using the Solar Isotope Spectrometer (SIS) on the Advanced Composition Explorer (ACE), we have studied
the properties of a selection of small 3 He-rich solar energetic particle (SEP) events with heavy ion enhancements in the
energy range ~11-22 MeV/nucleon. These events contain significantly increased 3He/4He ratios over the solar wind
value of 0.0004 in the energy range ~4.5-5.5 MeV/nucleon. In order to characterize the events, the following features
have been investigated. First, the heavy element content has been measured and compared to that found in past studies
of impulsive SEP events. Next, the simultaneous 38-53 keV electron flux, measured with the Electron, Proton, and
Alpha Monitor (EPAM) on ACE, has been examined for possible activity near the 3He-rich event onset times. Finally a
list of measured solar X-ray flares, with corresponding H-alpha flares where possible, has been scrutinized for potential
correlations with these events. The results show an apparent correlation between event onset and increased electron flux,
and a possible association with X-ray flares.
INTRODUCTION
The heavy element content of 3 He-rich SEP events
may provide insight into the nature and origin of this
type of occurrence. 3 He-rich SEP events were first ob-
served in 1970 by Hsieh and Simpson (1), and their re-
lated heavy element enhancements were first noted by
Hurford et al. (2). Since 1970, enhancements of 3He
have come to be associated with "impulsive" SEP events,
one of the two main classifications of SEP events (impul-
sive and gradual). Impulsive events, which usually have
a duration on the order of several hours, typically contain
3
 He/4He ratios which are 2-3 orders of magnitude greater
than that of the solar wind. In addition, the Fe content
of impulsive events is normally ~10 times that of the so-
lar wind (3, 4, 5), while the Ne, Mg, and Si abundances
are enhanced by a factor of ~3 (4, 5). Impulsive events
have been associated with ~keV electron emission from
the Sun (6), as well as with X-ray flares.
Gradual events, on the other hand, have been thought
to contain material which reflects closely the composition
of the solar wind and corona. However, recent measure-
ments of the composition of gradual events by Mason et
al. and Cohen et al. (3, 7) have revealed deviations from
coronal abundances toward the impulsive event character-
istics described above. Because of these measurements,
one can no longer assume that gradual events do not have
3He enhancements. This complicates the distinction be-
tween impulsive and gradual events. The material in
gradual events is understood to have been accelerated out
of the interplanetary medium by shocks which are driven
by coronal mass ejections. Gradual events, unlike impul-
sive events, typically have a duration of days.
There are several observational differences between
impulsive and gradual SEP events with respect to their
composition and accompanying electromagnetic emis-
sions. By examining the heavy element content and co-
incident solar activity of small 3He-rich SEP events, it is
possible to gain information relevant to the classification
of these events and to the origin of the ejected material.
DATA ANALYSIS AND DISCUSSION
Using the Solar Isotope Spectrometer (SIS) (8), a
set of eight small 3 He-rich SEP events with heavy el-
ement enhancements which occurred between Septem-
ber 6, 1998 and July 12, 1999 have been selected. The
events have been defined by the following criteria: (1)
The 3He/4He ratio is >4% in the energy range ~4.5-
5.5 MeV/nucleon. (2) The heavy element (Z>10) flux in
the energy interval ~11-22 MeV/nucleon is greater than
that of the solar quiet time background. (3) Event onset
times are chosen to coincide with a rise in the 3He flux.
(4) Event stop times are defined by the return of the heavy
element fluxes to background levels, determined by in-
spection. All eight of the events are shown in Figure 1,
including the heavy element counts, 3He flux, coinci-
CP528, Acceleration and Transport of Energetic Particles Observed in the Heliosphere: ACE 2000 Symposium,
edited by Richard A. Mewaldt, et al.
© 2000 American Institute of Physics l-56396-951-3/00/$17.00
103
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Z < 9
3
Electron
CMX
He
Electron
CMX
Z < 9
Electron
CMX
Z < 9
3He
Electron
CMX
Z < 9
3He
Electron
CMX
Z < 9
Electron
CMX
Z < 9
Electron
CMX
Z < 9
3He
Electron
CMX
<x> o o"
HI I I II II II I IIIHIIIWIIHIIIIMIII1IIHIIIHII
..«..,«.
~<^^«).p....^.^^.....ft.^^^...y....:€lO^..
III! lit III I I I) III I II I I I H I
..................^.^ . . . . . . . . . . . . . . . . . . . . .
i i i i mi i mm i HUH i 11 HI i t HI i i iw i i
III H II II I III I I
^en r-
en in
*~ en
. O
LO n
cn iT>
cn .^
K S
>
o oQ
cn
en
cn o
-2 0 1 2
Time - Event Start Time (Days)
FIGURE 1. The eight events selected for this study. Each panel in the figure represents one event. The event start and stop times are
denoted by the two vertical bars. The date of the event onset is noted at the far right side of the panel. The top row of ticks represents
the arrival times of heavy nuclei with energy ~ 11-22 MeV/nucleon. The 3He trace plot represents the 3He flux measured using SIS
for energy ~4.5-5.5 MeV/nucleon, plotted on a log scale where the first bin on the vertical axis equals one decade. The 38-53 keV
electron flux, also shown on a log scale with a one decade indication, was measured using EPAM on ACE. The "CMX" plot represents
the incidence of solar X-ray flares which are of C-Class or greater, as measured with the GOES-8 satellite. The diamond symbols
denote C-Class flares, and circles M-Class. No X-Class flares were detected during these time periods. X-ray flares which could be
associated with an H-alpha flare from the western (eastern) hemisphere of the Sun are shown above (on) the dotted line, while those
which could not be associated with an H-alpha flare are plotted below the dotted line.
104
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dent X-ray flares (C, M, and X-Class) and the 38-53 keV
electron flux which has been provided by the Electron,
Proton, and Alpha Monitor (EPAM) on ACE. The list
of X-ray flares has been extracted from a GOES satel-
lite database (http://www.sel.noaa.gov/index.html) at the
NOAA Space Environment Center. Where possible, the
X-ray flares have been associated with an H-alpha flare
originating from a known location. The X-ray events in
Figure 1 are therefore separated according to east, west,
and unknown longitudinal origin on the Sun.
All eight of the events in Figure 1 have C or M-Class
X-ray flares < 2 hours before the onset time. If one
assumes that the X-rays and energetic nuclei associated
with a SEP event depart from the Sun simultaneously,
then the minimum difference in transit times for X-rays
and 5.5 MeV/nucleon 3He over 1 AU is ~1.2 hours.
However, such an assumption is not clearly justified and
could be a significant source of uncertainty in calculating
expected X-ray flare onset times. Furthermore, because
the eight events in this study are relatively small it is fre-
quently difficult to determine their onset times to within
< 1 hour. Due to these two sources of uncertainty, and
because of the abundance of X-ray flares shown in Fig-
ure 1, it is clearly difficult to deduce a unique association
between the X-ray emissions and the 3He-rich events.
Therefore it is possible, but not definite, that these eight
events are associated with X-ray flares.
Also shown in Figure 1 are the coincident electron
fluxes, measured using EPAM. All of the events show in-
creases in the electron flux within < 2 hours of the event
onset time. The average difference in transit times for
38-53 keV electrons and 5.5 MeV/nucleon 3He over a
distance of 1 AU is ~1.1 hours. However, for the rea-
sons stated above, the expected onset time of the electron
flux relative to the 3He flux is not well determined, and
should not be used as a rigid criterion for correlation. In-
stead the apparent association can be seen by inspecting
the electron fluxes at or very near the event onset times in
Figure 1.
To classify these eight events further, the heavy ele-
ment content has been investigated, as shown in Figure 2.
In the figure, 17 average abundance ratio enhancements
are plotted for the eight SEP events studied here, and are
compared with those found by Reames et al. (4) in a study
of 228 impulsive SEP events with the ISEE-3 spacecraft.
The abundance ratio enhancements are defined relative to
coronal values (9). From the figure it is clear that simi-
lar abundance enhancements are present in the two data
sets. In this comparison it should be noted that the energy
interval measured using ISEE-3 is 1.9-2.8 MeV/nucleon,
somewhat lower than the ~11-22 MeV/nuc in this study.
In addition, while there have been many 3He-rich events
measured by SIS, most do not have large enhancements
of heavy elements (Z>6) in the energy interval ~11-
.SIS
O I S E E 3
Fe Si Mg Ne 0 N
C C C C C C
\
SL Ne Fe Fe Fe
Mg Mg MgSi Ne MgSi NeMgSi
Ne
CNO
MgSi NeMgSi Fe
CNO CNO CNO
Ne
Si
FIGURE 2. Average heavy element abundance ratios, normal-
ized to coronal values, for the eight SEP events occurring be-
tween September 6, 1998 and July 12, 1999. Uncertainties in
the ratios are statistical in nature, and are shown with one stan-
dard deviation.
22 MeV/nucleon. Consequently the ISEE-3 impulsive
event sample is much larger than that of SIS.
The background contribution to the heavy element
fluxes has been estimated using selected days of solar
quiet time from three consecutive time periods. Be-
tween May 25, 1998 and November 13, 1998, 59 days
of quiet time have been selected. From November
14, 1998 through July 22, 1999 there were 133 days
of quiet time, and from July 23, 1999 through Octo-
ber 31, 1999 there were 60 days. Quiet times are de-
fined by imposing a threshold on the He and O fluxes
in the lowest three energy ranges of SIS. The thresh-
olds are chosen such that the average O flux is less
than 2x 10~5 particles/(cm2Sr sec MeV/nucleon) for the
energy interval 7.1-15.6 MeV/nucleon, and the aver-
age He flux is less than lx lO~ 4 particles/(cm2Sr sec
MeV/nucleon) for 3.4-7.3 MeV/nucleon. This definition
of solar quiet time is driven primarily by the He flux.
Also of interest in observing 3 He-rich SEP events is
the possibility of a correlation between 3He/4He ratios
and heavy element abundances. Such a correlation could
place important restrictions on acceleration models for
this type of event. Recent results by Ho et al. (10) report
a correlation between the Fe/C and 3He/4He ratios, using
data from the ULEIS instrument on ACE. Past investiga-
tions (11,4) have not found a correlation between heavy
105
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>
0)
CN
CM
10.0
1.0
0.1
t- 10.0
1.0
0.1
t-
0.01 0.10 1.00
3He/4He (4.5 - 5.5 MeV/nuc)
0.01 0.10 1.00
3He/4He (4.5 - 5.5 MeV/nuc)
FIGURE 3. Plot of Fe/Mg (left panel) and Fe/C (right panel) against the 3He/4He ratio, for the eight events in this study.
element abundance ratios and 3He/4He ratio, but these
studies have been limited to 3He/4He ratios >0.1 and
have been at somewhat higher energies than the ULEIS
measurements. The results by Ho et al. included events
with 3He/4He ratios as low as 0.01. In a search for simi-
lar correlations using SIS, two heavy element ratios have
been plotted as a function of 3He/4He ratio. The left panel
of Figure 3 shows the Fe/Mg ratio plotted against the
3He/4He ratio, while the right panel depicts the Fe/C ratio
as a function of 3He/4He ratio. In the Fe/Mg plot, there
does not appear to be a correlation with the 3He/4He ratio.
Similarly, the Fe/C plot does not show a correlation with
the 3He/4He ratio, although this is difficult to determine
without extending the 3He/4He measurements lower than
the 4% limit possible with the SIS instrument in this en-
ergy range.
Measurement of the 3He/4He ratio is limited by sev-
eral contributing factors. As mentioned above, the SIS
instrument is not sensitive to ratios lower than 4±1% in
the energy range 4.5-5.5 MeV/nucleon, due to spillover
contamination to the 3He peak from the 4He peak. A cor-
rection for this contribution to the 3He/4He ratio has been
applied to the data shown in Figure 3. It should also be
noted that no uncertainty has been added to account for
contributions to the 4He from neighboring SEP events.
Therefore, the 3He/4He ratios in Figure 3 should be inter-
preted as lower limits where indicated.
SUMMARY
The heavy element content of these eight 3 He-rich
events has been examined. The enhancements of Fe, Mg,
Ne, and Si, which are standard indications as to the na-
ture of the material which is present, are consistent within
statistical and experimental limitations with the previous
study of 228 impulsive SEP events by Reames et al. (4)
There does appear to be an association between elec-
tron emission and the onset of these events, which sug-
gests that the events may be impulsive. This is also sup-
ported by the presence of X-ray flares at the event onsets,
although the frequent occurrence of small X-ray events
hinders any direct correlation. To combat this difficulty,
future work may include studies of heavy ion velocity
dispersion as a tool to extract the expected X-ray flare
onset times. The combination of heavy element content,
electron flux association and X-ray emission suggests that
these eight events may be impulsive SEP events as de-
fined by previous studies.
ACKNOWLEDGMENTS
This research was supported by NASA at the Califor-
nia Institute of Technology (under grant NAG5-6912), the
Jet Propulsion Laboratory, and the Goddard Space Flight
Center. We thank the EPAM science team for providing
the electron data used in the first figure.
REFERENCES
1. Hsieh, K.C., Simpson, J.A., Astrophys J. Letters, 162, L191-
L196 (1970).
2. Hurford, G.J. et al., Astrophys J. Letters, 201, L95-L97
(1975).
3. Mason, G.M. et al., Astrophys J. Letters 525, L133-L136
(1999).
4. Reames, D.V. et al., Astrophys J. Suppl 90, 649-667 (1994).
5. Slocum, P.L. et al., Proc. 26th Intern. Cosmic Ray Conf. (Salt
Lake City) 6, 107-110 (1999).
6. Reames, D.V. et al., Astrophys J. 292, 716-724 (1985).
7. Cohen, C.M.S. et al. "The Isotopic Composition of Solar
Energetic Particles" in ACE 2000 Symposium, edited by
R.A. Mewaldt et al., AIP Conference Proceedings this vol-
ume, New York: American Institute of Physics, 2000.
8. Stone, B.C. et al., Space Sci. Rev. 86, 357-408 (1998).
9. Reames, D.V., Space Sci. Rev. 85, 327-340 (1998).
10. Ho, G.C. et al. "Heavy Ions in 3He Enhanced Solar Ener-
getic Particle Events" in ACE 2000 Symposium, edited by
R.A. Mewaldt et al., AIP Conference Proceedings this vol-
ume, New York: American Institute of Physics, 2000.
11. Mason, G.M. et al., Astrophys J. 425, 843-848 (1994).
106
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Measurements of Heavy Elements in ^3He-rich SEP Events

Heavy Ions in 3He Enhanced Solar Energetic Particle Events&quot;

The Isotopic Composition of Solar Energetic Particles&quot;

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Measurements of Heavy Elements in ^3He-rich SEP Events

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