PROCEEDINGS OF THE 31st ICRC, ŁÓDŹ 2009
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Observation of high ionic charge states of iron at suprathermal
energies
B. Klecker∗ , E. Möbius† , M. A. Popecki† , H. Kucharek† , A. B. Galvin† , M. Hilchenbach‡ ,
R. F. Wimmer-Schweingruber§ , and L. Berger§
∗ Max-Planck-Institut
für extraterrestrische Physik, Garching, Germany
Science Center, University of New Hampshire, Durham, NH, USA
‡ Max-Planck-Institut für Sonnensystemforschung, Katlenburg-Lindau, Germany
§ University of Kiel, Kiel, Germany
† Space
Abstract. The ionic charge states of heavy ions
show a large variability with energy, in particular
for Fe. In Fe-rich solar enegetic particle (SEP) events
the mean ionic charge of Fe (Qmean ) as observed
in interplanetary space ranges from ∼ 11 to 15 at
suprathermal energies (<100 keV/amu) to ∼16 to
20 at ∼0.5 MeV/amu. In large, interplanetary shock
related events Qmean of Fe ranges from ∼10 at
energies <0.5 MeV/amu to ∼20 at energies > 20
MeV/amu. However, high ionic charge states of Fe as
often observed in ICME-related solar wind were not
reported at suprathermal energies. In this paper we
investigate whether, and for which conditions, high
charge states of iron are observed in the suprathermal energy range. For our systematic search we use
one day averages of the mean ionic charge of iron,
obtained with the STOF sensor onboard SOHO in
the energy range 10 to 100 keV/amu. During the
time period 2001 to 2004 we find several cases with
average charge states of Fe in the range 12 -15 that
are not related to 3 He- or Fe-rich events. The ICMErelated solar wind shows mean charge states up to 15
during these time periods, and the mean ionic charge
of Fe at suprathermal energies is compatible with
the concomitant solar wind charge states, suggesting
local acceleration. Although high ionic charge states
of Fe are frequently observed in ICME-related solar
wind, the observation at suprathermal energies is
rather infrequent, with a few cases per year. We
investigated the time period between Day 85 and 95,
2001 in more detail and found that during this time
period the observation of high Fe ionic charge at
suprathermal energies is correlated with high solar
wind density and an interplanetary shock overtaking
a preceding ICME with high ionic charge of Fe in
the solar wind.
Keywords: suprathermal ionic charge, solar wind
ionic charge, shock acceleration
I. I NTRODUCTION
Measurements with advanced instrumentation on the
SAMPEX, SOHO and ACE spacecraft show a large
variability of the ionic charge of heavy ions with energy,
in particular for Fe. The ionic charge states for events
with low particle intensity and large enrichments of
heavy ions (usually called impulsive events) show a
characteristic increase of the mean ionic charge of Fe
from ∼ 11 - 15 at < 100 keV/amu to ∼16 - 20 at
∼ 0.5 MeV/amu ( [1], [2], [3]), consistent with charge
stripping in a dense environment, low in the corona
(e.g. [4], [5], [6], and references therein). In events with
large particle intensities, related to interplanetary and / or
coronal shocks (gradual events), the mean ionic charge
at suprathermal energies is usually compatible with the
charge states of slow and fast solar wind (e.g. Qmean ∼
9-11 for Fe, [7], [8], [9]), with an increase to Qmean ∼20
at energies of 10s of MeV/amu (e.g. [10], [11], [12], and
references therein).
So far reports on high charge states of Fe at energies
<100 keV/amu are sparse. However, a high mean ionic
charge of Fe in the solar wind, with Qmean >12 and
a significant fraction of charge states with Q>15, is
often observed in ICME-related solar wind (e.g. [13],
[14]) and, in fact, has been used as reliable tracer of
ICMEs (e.g. [15], [16], and references therein). In this
paper we perform a systematic search for high Fe charge
states at suprathermal energies. Because the CME rate is
varying by a factor of ∼10 between solar maximum and
minimum (e.g. [17], [18]), we concentrate our systematic
search for high Fe ionic charge at suprathermal energies
in this paper on the years 2001-2004, i.e. a time period
covering much of the maximum of solar cycle 23.
II. I NSTRUMENTATION AND DATA A NALYSIS
The study reported here was carried out with the
Suprathermal Time-Of-Flight (STOF) sensor of the
CELIAS experiment onboard SOHO [19] and using data
from the Solar Wind Ion Composition Spectrometer
(SWICS) onboard the Advanced Composition Explorer
(ACE) spacecraft [20]. SOHO and ACE were launched
into a halo orbit around the Lagrangian point L1 between
the Earth and the Sun in December 1995 and August
1997, respectively. STOF provides ionic charge distributions of suprathermal ions in the energy range ∼ 10 - 160
keV/amu (for Fe) and SWICS provides solar wind heavy
ion charge distributions up to ∼ 86 keV/e. For a systematic search of high ionic charge states of Fe at suprathermal energies we used daily averages of the mean charge
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KLECKER et al. IRON CHARGE STATES AT SUPRATHERMAL ENERGIES
Fig. 1: Distribution of daily averages (years 2001 to
2004) of the mean ionic charge of Fe in the energy range
10-100 keV/amu.
Fig. 3: Daily averages of the mean ionic charge of Fe
observed in 2001 with SWICS / ACE in the solar wind
(line) and with STOF / SOHO at suprathermal energies.
(diamonds)
Fig. 2: Correlation of daily averages of the mean ionic
charge of Fe at suprathermal energies, observed with
STOF / SOHO, with the mean ionic charge in the solar
wind as observed with SWICS / ACE.
Fig. 4: Solar wind density and speed (SWEPAM/ACE),
start (solid lines) and end times (dashed lines) of ICMEs,
and arrival times of interplanetary shocks (dotted lines);
for details see the text.
of Fe as observed with STOF and daily averages of
solar wind iron charge states from SWICS/ACE. The
bulk solar wind parameters proton density and speed as
measured by the Proton Monitor (PM) onboard SOHO
[21] and the SWEPAM sensor onboard ACE [22] were
used to characterize the solar wind environment at
SOHO and ACE. After identifying time periods with
high charge states we used higher time resolution, dependent on the available counting statistics, for a more
detailed analysis. For a comparison with the times of
interplanetary shocks and interplanetary coronal mass
ejections (ICMEs) we used the SOHO and ACE shock
lists at http://umtof.umd.edu/pm/FIGS.HTML and http://
www.ssg.sr.unh.edu/mag/ace/ACElists/obs list.html, respectively, and the list of ICMEs published by [23].
III. R ESULTS
Figure 1 shows the distribution of daily averages of
the mean ionic charge of Fe in the energy range 10 to
Fig. 5: Average ionic charge of Fe ions in the solar wind
and at suprathermal energies (for details see the text).
PROCEEDINGS OF THE 31st ICRC, ŁÓDŹ 2009
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Fig. 6: Ionic charge distribution of Fe ions on day 90, 2001, 00:00-02:00 in the solar wind
P (left panel) and at
suprathermal energies (right panel).The abundances of charge states Qi are normalized to i (Qi ) = 1.
Fig. 7: Differential intensity of solar wind and suprathermal Fe ions on day 90, 2001, 00:00 - 02:00.
100 keV/amu for the years 2001 to 2004. Days with
insufficient counting statistics (< 10 cts/day) and with
large instrumental background due to high fluxes of energetic particles in interplanetary space have been removed
from the data set. These selection criteria resulted in a
data set with 202 one day averages. The distribution of
the daily averages of Fe charge states (Fig. 1) shows a
large peak at Q∼9-12, with 29 days with Qmean > 12.
In Fig. 2 we correlate the mean ionic charge of Fe in
the solar wind with the mean ionic charge at 10 - 100
keV/amu, using 1 day averages obtained for the same
time period. Figure 2 shows that the average ionic charge
of Fe at suprathermal energies is generally correlated
with the average Fe ionic charge in the solar wind,
although with considerable scatter.
In Fig. 3 we plot for the year 2001 the mean ionic
charge of Fe in the suprathermal energy range and daily
averages of solar wind Fe charge states as a function of
time. Figure 3 shows that days with a high mean charge
of Fe at suprathermal energies generally coincide with
high solar wind charge states as observed with SWICS /
ACE. We select the time period that shows in 2001 the
largest Fe charge states at suprathermal energies (day 85
to 95) for further more detailed analysis.
Figure 4 shows for day 85 to 95 (March 26 to April 5)
2001 an overview of the solar wind parameters (proton
density and speed) as measured with the SWEPAM
experiment onboard ACE, the arrival times of interplanetary shocks (SOHO and ACE online shock lists), and
beginning and end times of three ICMEs taken from
the ICME list compiled by [23]. Figure 5 shows the
mean ionic charge of iron in the solar wind (24 h time
resolution) and at suprathermal energies (6h resolution),
and the count rate in the suprathermal energy range (6h
time resolution) for the same time period. It is evident
from Fig. 5 that the large increase of the flux of Fe
at suprathermal energies on day 90 is correlated with a
large density increase in the solar wind and the passage
of an interplanetary shock on day 90 (Fig. 4), reported to
pass at 00:15 at SOHO and at 00:23 at ACE. Because of
the high solar wind density and high particle intensities
at suprathermal energies we are able to look into this
time period at high time resolution.
Figure 6 shows the ionic charge distribution of iron
ions in the solar wind (left panel) and at suprathermal
energies (right panel) for the time period 00:00 to 02:00
on day 90 (March 31) 2001, i.e. in a 2 hour interval
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KLECKER et al. IRON CHARGE STATES AT SUPRATHERMAL ENERGIES
around the passage of an interplanetary shock at SOHO
and ACE. The ionic charge distribution in the solar wind
(left panel) shows a broad distribution with a significant
fraction of ionic charge states in the range 8 - 12, as
usually observed in slow and fast solar wind (e.g. [7],
[8]). The peak at Q=16 is part of a large fraction of
Fe ionic charge states with Q≥16, one of the criteria
usually used for identifying ICME-related solar wind
(e.g. [14], [15], [16], and references therein). The ionic
charge distribution at suprathermal energies (Fig. 6, right
panel) shows a similar broad distribution, however, with
a somewhat larger fraction of ionic charge states with
Q>13. This appears to suggest a somewhat increased
injection and acceleration of Fe ions with the higher
charge states from the solar wind into the suprathermal
ion distribution.
Next we investigate the energy spectra of iron ions
from solar wind to suprathermal energies. Figure 7
shows at low energies the flux of the dominant charge
states 9 and 16, and at suprathermal energies the Fe flux,
integrated over all charge states. The spectra indicate a
smooth transition from the tail of the solar wind distribution up to suprathermal energies of ∼160 keV/amu,
with a spectral slope of γ∼2.8, assuming a power-law
differential intensity of j ∼E−γ at E>10 keV/amu.
IV. S UMMARY
A systematic search for high average charge states of
Fe in the suprathermal energy range 10 - 100 keV/amu
with the STOF sensor onboard SOHO showed for
29 of 202 daily averages (15%) during 2001 - 2004
Qmean >12. For 85% of the days analyzed the mean
ionic charge of Fe in the suprathermal energy range was
in the range of Qmean ∼ 9 to 12, as usually observed
at energies <0.5 MeV/amu at interplanetary shocks [9]
and fast and slow solar wind [8]. A high mean charge of
Fe at suprathermal energies is generally correlated with
high solar wind charge states as observed with SWICS
/ ACE (Fig. 2 and Fig. 3). A detailed analysis of the
March 31, 2001 time period (day 90) with high mean
Fe ionic charge shows at suprathermal energies a similar
broad ionic charge distribution of Fe as in the solar
wind, however, with a somewhat larger fraction of Q>13
charge states for Fe (∼69% at suprathermal energies vs
43% in the solar wind), suggestive of M/Q-dependent
acceleration. High Fe ionic charge states at suprathermal
energies are observed on March 31, 2001 during a
time period with unusually high solar wind density, a
strong near perpendicular interplanetary shock on day
90, 2001 (at 00:23, with MA ∼10, ACE online shock
list), and with several ICMEs. This suggests that the
rare observation of high Fe ionic charge at suprathermal
energies is due to the fact that, at a given sensitivity
level of the experiment, an observation is only possible
if sevaral conditions are met: (1) high solar wind density,
(2) high Fe charge states in the seed populations, (3) a
strong shock, resulting in energy spectra sufficiently hard
for the detection with STOF. Whether these conditions
are also met for the other time periods during 2001 to
2004, when high charge states of Fe are observed, is
presently under investigation.
V. ACKNOWLEDGEMENTS
We acknowledge the use of solar wind data
from the ACE/SWEPAM Team and the use of the
SOHO and ACE interplanetary shock lists at http://
umtof.umd.edu/pm/FIGS.HTML and http://www.ssg.sr.
unh.edu/mag/ace/ACElists/obs list.html, respectively.
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