Relative Abundance Variations of Energetic He /He2 in
CME Related SEP Events
H. Kucharek, E. Möbius, W. Li, C. Farrugia, M. Popecki, A. Galvin, , B. Klecker†,
M. Hilchenbach and P. Bochsler‡
University of New Hampshire, Space Science Center, Durham, 03824 United States
Max-Planck-Institut für extraterrestrische Physik, P.O. Box 1603, Garching, 85740 Germany
Max-Planck-Institut für Aeronomie, Max-Planck-Str. 2, Katlenburg-Lindau, 37819 Germany
‡
University of Bern, Sidler Str. 5, Bern, 3012 Switzerland
†
Abstract. We have investigated several CME-related SEP events with unusually high abundance of He relative to He2 in
the energetic particle population which have been observed between 1998 and 2000 with ACE/SEPICA and SOHO/CELIAS.
Usually the abundance of He is below a few percent whereas at these times the He /He2 ratio can be closer to one. Possible
sources for He are interstellar pickup ions or cold plasma in CME’s. We have investigated in detail the temporal evolution
and the energy spectra of these events. We find that the maximum of the He /He2 ratio usually coincides with the arrival
of the shock or a solar wind structure. This is a strong indication for local acceleration of these ions. The He enhancement
does not seem to be associated with cold plasma within CME’s itself. Therefore, most probably interstellar pickup ions are
the source for the He enhancement. Furthermore, the He /He2 ratio appears to be consistently lower at higher energies.
Also, the observed temporal variability decreases with increasing energy. These two results seem to indicate two different
populations for He and He2 with different energy spectra.
SWICS for energies up to 60 keV. More recently, He was also found as part of the suprathermal CIR population at 1 AU up to 200 keV/Q with SOHO STOF
(Hilchenbach et al. 1999) and with Wind STICS (Chotoo
et al. 2000). These observations led to the suggestion that
pickup ions may constitute a generally important source
of suprathermal ions for further acceleration at interplanetary shocks (Gloeckler, 1999).
For a recent series of unusual CIR’s close to solar maximum in 1999 and 2000, Möbius et al. [2001a] have reported a substantial fraction (10 - 30%) of He in the
energy range 0.25 - 0.8 MeV/nuc, which they attributed
to interstellar pickup ions. For the same CIR events, Morris et al. [2001] have shown that the He /He2 ratio increases as time elapses during successive passage of the
CIR. On the other hand in a recent publication, Skoug
et al. [1999] pointed out a prolonged He enhancement
within a coronal mass ejection in the solar wind. They
indicated that the source for the He could be prominence material. Klecker et al. [2001] and Kucharek et al.
[2001] have reported substantial variation of He /He2
abundance ratio during the passage of a coronal mass
ejection. They investigated twelve hour averages of the
He /He2 abundance ratio of SOHO/CELIAS/STOF
and ACE/SEPICA data during CME events and they
found substantial variations ranging from 0.01 up to 0.8.
INTRODUCTION
One of the first charge state measurements of energetic
He during solar energetic particle events have been reported by Hovestadt et al. [1984]. Analyzing data from
the ULEZEQ instrument on board the ISEE 3 spacecraft
they observed a large variability of the He /He2 ratio
in the energy range of 0.41 - 1.05 MeV/nuc ranging between 0.1 and 1.0. They concluded that a possible source
for these ions could be an admixture of cold solar material. However, in solar energetic particle SEP events with
a high He abundance the charge state distributions for
heavy ions did not show an indication of lower charge
states. This seemed to speak against an admixture of cold
solar material during these events. The detection of interstellar He pickup ions by Möbius et al. [1985] suggested an other possible source for He . These particles
enter the heliosphere as interstellar neutral particles and
are then ionized by UV radiation and picked-up by the interplanetary field. These ions had been suggested already
earlier by Fisk et al. [1974] as the source for the anomalous component of cosmic rays. In a study by Gloecker
et al. [1994] these ions have been observed at corotating interaction regions (CIR) at a radial distance of 4.5
AU. They identified interstellar pickup He as the major
contributor of the suprathermal He in CIRs with Ulysses
CP679, Solar Wind Ten: Proceedings of the Tenth International Solar Wind Conference,
edited by M. Velli, R. Bruno, and F. Malara
© 2003 American Institute of Physics 0-7354-0148-9/03/$20.00
648
Furthermore Klecker et al. [2001] correlated daily averages of the He abundance with the solar wind parameters and found an general anti-correlation of the He abundance with the solar wind velocity and the solar
thermal velocity. In this paper we provide a survey of the
He /He2 abundance ratios in energetic particle events
during 1998 - 2000. Another major topic addressed in
this paper is the dependence of the temporal variability
of the energetic He on the energy. We extend the work
by Klecker et al. [2001] to higher energies to test for the
He /He2 correlation with the solar wind velocity. In addition we test for suggested sources by studying carefully
the time variation of the He /He2 abundance ratio during one series of CME events.
1999
2000
He+/He2+
1998
FIGURE 1. shows the He /He2 ratio of ACE/SEPICA for
the years 1998 - 2000 (separated by dashed lines) in the energy
range of 250 - 800 keV/nuc.
INSTRUMENTS AND SPACECRAFT
For this investigation we used data from two different
sensors on board two different spacecraft, the Advanced
Composition Explorer (ACE) spacecraft and the SOlar
and Heliospheric Observatory (SOHO) spacecraft have
halo orbits around L1. The Suprathermal Time Of Flight
(STOF) sensor is part of the Charge ELement and Isotopic Analyzing System (CELIAS) instrument and is designed to study the composition of the solar wind and of
solar and interplanetary energetic particles on the SOHO
spacecraft. This sensor covers the energy range from 35
to 630 keV/Q by employing an electrostatic deflection
system. As a second sensor we used the Solar Energetic
Particle Ionic Charge Analyzer (SEPICA). This sensor is
the main instrument on board the ACE spacecraft to determine the ionic charge states of solar and interplanetary
energetic particles in the energy range from 0.2 MeV/nuc
to 5 MeV/charge.
the He /He2 abundance ratio for both data sets as a
function of the solar wind bulk speed for 1999 according to Klecker et al. [2001]. This data set indicate a dependence on solar wind speed. Higher solar wind bulk
speed show a lower He /He2 ratio. However, when we
determine the He /He2 ratios from twelve hour averages by using the ACE/SEPICA data in the same time
interval, we found no obvious correlation between solar
wind bulk speed and abundance ratio in the energy range
of 0.25 MeV/nuc - 0.8 MeV/nuc. The He /He2 ratio
seems to increase towards the suprathermal energies. In
addition, the ratio is highly variable over the course of
individual events, with a stronger variation at the lower
energies. This strong variability is under current investigation. The key questions at this point are, what are
the possible sources for He and what causes the observed variation. In the rest of the paper we will address
to the determination of the source of the He . In order to
identify the structures which are associated with He enhancements. We have investigated a CME event in high
time resolution which has a substantial enhancement of
He in the solar wind. The time span chosen contains a
sequence of events corresponding to the first very active
burst in the rising phase of solar cycle 23. Some of these
have been also investigated by other authors, such as Skoug et al. [1999] and Farrugia et al. [2002]. Figure 3 show
the He /He2 ratio of a ten day time period of 1998.
This is a very interesting episode. On DOY 120 (April,
30) a shock passes the spacecraft followed two days later
(May, 2) by a CME event, which contains a magnetic
cloud. Skoug et al. [1999] found in the ACE/SWEPAM
a prolonged He enhancement in the solar wind population at the same time that Farrugia et al. [2002] noted a
complete dropout of electron halo population. Skoug et
al. [1999] suggested that the source for the He could be
OBSERVATIONS
We have determined the He /He2 abundance ratio from
the pulse height data of ACE/SEPICA for the entire time
period 1998 - 2000, by employing a double gaussian fit
curve to the actual charge state distribution. The integration times for the charge state distribution were variable and chosen according the minimal statistics requirements of 3000 counts within a certain time interval. Figure 1 shows the result of the survey for the energetic
He /He2 abundance within the energy range of 250 800 keV/nuc ratio during 1998 - 2000. As one can see,
the ratio shows a significant variation between 0. and
0.8. During this time period many shocks associated with
CME’s, flares, and CIR’s at 1 AU have been reported.
For the same time period SOHO/CELIAS/STOF data are
available. In Figure 2 we show twelve hour averages of
649
He+/He++
cloud
Shocks
21 – 70 keV/n
FIGURE 3. This figure shows a time period in the year 1998
which contains shocks (dashed lines) and a well investigated
CME/cloud event with a high He abundance on the solar wind
(shaded band).
shocks.
The two suggested sources for He , i.e. cold solar
atmospheric material, which is occasionally found in the
solar wind (Gosling et al. 1974, Schwenn et al. 1987,
Skoug et al. 2001), and interstellar pickup ions (Möbius
et al. 1985; Gloeckler, 1999), are indeed present locally.
Our case study also provides clues to distinguish between
the two sources.
At the energies in our study, we see the largest enhancement at the shock at DOY 120, which is well separated from the cloud. The shocks closer to the cloud
shows only a very moderate He /He2 enhancement.
In fact the smallest enhancement occurs at the shock
on DOY 123 which is presumably overtaking the cloud.
If cold solar material were the major source we would
expect the highest enhancement to take place at this
shock. Although, some contribution from cold solar material cannot be excluded, for the adjacent shocks, it is
more compelling that the majority comes from pickup
ions. This conclusion is also supported by the fact, that
He enhancements are ubiquitous in our survey, while
He rich clouds are very rare. These observations are
also consistent with theoretical considerations that compared to the solar wind alpha particle pickup ions seems
to be preferentially injected into an acceleration process. Scholer [1999] and Scholer and Kucharek [1999]
have demonstrated that pickup ions are indeed essentially
suprathermal ions in the frame of the solar wind, while
the solar wind itself is rather cold.
The observed substantial increase of the He /He2 ratio towards lower energies, and the variability during individual events and the correlation with solar wind speed
(Klecker et al. 2001), may provide further evidence for
the source. The strong dependence on solar wind speed
at lower energies could reflect a strong variation of the
250 – 800 keV/n
FIGURE 2. shows the He /He2 abundance ratio versus
solar wind speed for SOHO/CELIAS/STOF (top panel) and for
ACE/SEPICA (bottom panel)
prominence material. As we see from Figure 3 there are
three marked shocks outside the cloud. A significant enhancement of the energetic He /He2 ratio is seen at the
shock arrival at DOY 120. At the second shock, driven by
the CME, we also see an enhancement of He , although
less pronounced than at the previous shock. It should be
pointed out that at the cloud associated with the CME has
not arrived yet. In the cloud itself, where a large amount
of He is found in the solar wind, no significantly enhanced values of He /He2 ratio in the high energetic
population are observed. In fact the only, very moderate,
enhancement occurs in the later part of DOY 123 at the
shock presumably overtaking the cloud (Farrugia et al.
2002).
DISCUSSION
One of the key observation of our CME the case study
is that the enhancement of the He /He2 ratio peaks
at shocks. This association clearly indicates that the acceleration occurs locally at these interplanetary disturbances. It should be noted here that many of the enhancements in the survey (Fig. 1) which are associated with
650
REFERENCES
pickup densities with solar wind speed or, more likely,
is related to the variability of the thermal distribution of
solar wind He2 or pickup ions, injected into the acceleration process. At higher energies other processes may
play an important role. Due to dispersion and transport
effects the dominance of the locally injected ions may be
washed out, and energetic ions from other sources, i.e.
flare material and ions accelerated closer to the Sun may
be added. This would reduce the ratios and time variability and it would destroy any correlation with solar wind
parameters.
1.
2.
3.
4.
SUMMARY AND CONCLUSIONS
5.
In this paper we have presented a survey of energetic He /He2 abundance ratio for the 3 year period
1998-2000. For this investigation we used data from
ACE/SEPICA in the energy range of 250 - 800 keV/nuc
and SOHO/CELIAS/STOF data in the lower energy
range of 85 - 280 keV/nuc. The ratio in these two energy ranges is very variable. Ratios starting from 0.01
up to 1 have been observed. The He /He2 ratio appears to be consistently lower at higher energies. In addition, the temporal variability decreases with increasing
energy. We found significant enhancements at interplanetary shocks, shocks associated with CME’s. A detailed
investigation of the temporal evolution and the energy
spectra of one of these events have shown that the maximum of the He /He2 ratio usually coincides with the
arrival of the shock. This is a clear indication of local acceleration of these ions. Also, the He enhancement does
not seem to be associated with cold plasma in the CME.
Therefore, the most likely source of the ions are interstellar pickup ions. Apart from CME events, as discussed in
detail here, where we found a substantial enhancement
of the He /He2 ratio there are different interplanetary
configurations within this survey which are able to produce a substantial enhancement of this ratio. However, a
detailed discussion of these events is deferred to a future
study.
6.
7.
8.
9.
10.
11.
12.
13.
14.
ACKNOWLEDGMENTS
15.
The work on the SEPICA instruments was supported
by NASA under Contract NAS5-32626 and the data
analysis under Grant NAG 5-6912 where as the work
on STOF/CELIAS was supported under DLR contracts
50OC89056 and 50OC96059, NASA Grant NAG 52754.
16.
17.
18.
651
Chotoo, K., et al.,The suprathermal seed population for
corotating interaction region ions at 1 AU deduced from
composition and spectra of H , He2 , and He observed
on Wind, J. Geophys. Res., 105, 23107 - 32122, 2000.
Farrugia, C. et al., Wind and ACE observations during
the great flow of May 1-4 1998: Relation to solar activity
and implications for the magnetosphere. J. Goephys.
Res., 107 (A9), 1240, doi:10.1029/2001JA000188,2002.
Fisk, L.A., B. Kozlowsky, and R. Ramaty, An
interpretation of the observed oxygen and nitrogen
enhancements in low-energy cosmic rays, Astrophys. J.,
190, L35 - L38, 1974
Gloeckler, G., Observation of injection and preacceleration processes in the slow solar wind, Space Sci.
Rev., 89, 91 - 104, 1999.
Gosling, J.T., Mass ejections from the Sun: A view from
Skylab, J. Geophys. Res., 79, 4581, 1974.
Hilchenbach, M., et al., Observation of suprathermal
helium at 1 AU: charge states in CIRs, in: Solar Wind
Nine, S.R. Habbal, R. Esser, J.V. Hollweg and P.A.
Isenberg, Editor, 1999.
Hovestadt, D., B. Klecker, G. Gloeckler, F. M. Ipavich,
and M. Scholer, Survey of He /He2 abundance ratios
in energetic particle events, ApJ., 282, L39 - L42, 1984.
Klecker, B., et al., On the variability of suprathermal
He at 1 AU., Proc. of the 27th. International cosmic ray
conference, Hamburg, 2001.
Kucharek, H., et al., Variable abundance of energetic
He in CME related SEP events, Proc. of the 27th.
International cosmic ray conference, Hamburg, 2001.
Möbius, E., et al., Survey of Ionic Charge States of Solar
Energetic Particle Events During the First Year of ACE,
in: Acceleration and Transport of Energetic Particles
Observed in the Heliosphere, R. A. Mewaldt et al. ed.,
AIP Conf. Proc., 528, 131 - 134, 2000.
Möbius, E., et al., Direct observation of He pick-up
ions of interstellar origin in the solar wind, Nature, 318,
426 - 429, 1995.
Möbius, E., et al., Charge states of energetic ions
obtained from a series of CIRs in 1999 & 2000 and
implications on source populations, Geophys. Res. Lett.,
subm., 2001a.
Möbius, E., et al., Variation of Energetic He , He2
and Heavy Ions Across Co-Rotating Interaction Regions,
Proc. of the 27th. ICRC, Hamburg, 2001b.
Morris, D., et al., Implications for source populations
of energetic ions in co-rotating interaction regions
from ionic charge states, in: ACE-SOHO Workshop
on Solar and Galactic Composition, ed. R. WimmerSchweingruber et al., AIP Conf. Proc., p. 201, 2001.
Scholer, M., Injection and acceleration processes in
corotating interaction regions: theoretical concepts,
Space, Sci. Rev., 89, 105, 1999.
Scholer, M., and H. Kucharek, Interaction of pickup ions
with quasi-parallel shocks, Geophys. Res. Lett., 26, 29,
1999.
Schwenn, R., The solar wind, In ESA Space Astronomie
sna Solar System Exploration, p. 131 - 141, 1987.
Skoug R., A prolonged He enhancement within a
coronal mass ejection in the solar wind, Geophys. Res.
Lett., 26, 2613, 1999.