Implications for Source Populations of Energetic Ions in CoRotating Interaction Regions from Ionic Charge States
D. Morris,1 E. Mobius,1 M. A. Lee,1 M. A. Popecki,1 B. Klecker,2L. M. Kistler,1
A. B. Galvin,1
1
Space Science Center and Department of Physics, University of New Hampshire, Durham, NH, USA
2
Max-Planck-Institut fur extraterrestrische Physik, Garching, Germany
Abstract. The ionic charge states of He have been observed in several co-rotating interaction regions (CIR) in 1999 and
2000 with ACE SEPICA. For all CIRs under study the He+/He2+ ratio increases consistently from the start of the event
towards the end, while the absolute flux of the energetic ions usually reaches a maximum close to the beginning of the
event. With time, the spacecraft is magnetically connected to the compression region at increasing distance from the sun.
Therefore, the increasing He+/He2+ ratio can be interpreted as an increase in the relative importance of interstellar pickup
ions over the solar wind as a source for the energetic ions. In addition, the observed He+/He2+ ratio and its increase with
the inferred distance from the sun provide an estimate of the injection and acceleration efficiencies for these species,
which is found to be higher by about two orders of magnitude for pickup ions compared with the solar wind.
INTRODUCTION
A substantial contribution of He+ to interplanetary
energetic particle populations was first reported by
Hovestadt et al. (1). Admixture of cold solar material
was thought to be a possible source of these ions. Subsequently, the detection of interstellar pickup He+ (2)
in the inner heliosphere introduced another source of
accelerated He+ in interplanetary space.
Compression regions and associated shocks between adjacent fast and slow solar wind streams,
called co-rotating interaction regions (CIRs), have
long been known to accelerate particles efficiently.
They are an important source of energetic particles in
interplanetary space, generally during times of low
solar activity (review (3)). The composition of these
energetic ions is mostly similar to that of solar energetic particles and the solar wind. However, differences, notably for He and C, have been reported ((3)
and references therein). For a CIR at 4.5 AU Gloeckler
et al. (4) used Ulysses SWICS to identify interstellar
pickup He+ as the major contributor to He in CIRs for
energies up to 60 keV. At this distance Franz et al. (5)
provided indirect evidence with Ulysses EPAC that
He+ must also be the main component at 0.6 - 2 MeV/n
since He was found to be overabundant by a factor
>2.5. He+ was also observed as part of the suprather-
mal CIR population (up to 300 keV/Q) at 1 AU with
SOHO STOP (6) and with Wind STICS (7).
These observations led to the suggestion that
pickup ions may constitute an important source of suprathermal ions for further acceleration at interplanetary shocks (8). Based on the increased efficiency of
pickup ions, with which they are injected into the acceleration processes to higher energies, Gloeckler et al.
(8) have argued that inner source pickup ions may also
contribute substantially to the energetic particle population in CIRs. As a consequence a substantial contribution of singly charged ions would be expected in the
energetic heavy ion population of CIRs.
For a series of CIRs close to solar maximum in
1999 and 2000, Mobius et al. (10) have reported a substantial fraction (7 - 35%) of He+ at 0.25 - 0.8 MeV/n,
which they attributed to interstellar pickup ions. However, except for a 4.7% contribution of Ne+ to Ne,
most likely of interstellar origin, no significant singly
charged component in the charge distributions of other
heavy ions was found. In general the mean charge
states resemble those of CME related solar energetic
particle events and the solar wind. In this paper we
extend our work on the He population in CIRs. We
find a substantial variation of the He+/He2+ ratio as
time progresses from the beginning of each CIR,
which is observed consistently in all of six events.
CP598, Solar and Galactic Composition, edited by R. F. Wimmer-Schweingruber
© 2001 American Institute of Physics 0-7354-0042-3/017$ 18.00
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INSTRUMENT AND OBSERVATIONS
ACE was launched on August 25, 1997, and injected into a halo orbit around the Lagrangian point LI
on December 17, 1997 (11). Among a complement of
high-resolution spectrometers to measure the composition of solar and local interstellar matter, as well as
galactic cosmic rays, SEPICA provides the ionic
charge state distribution of energetic particles.
To simultaneously determine the energy E, nuclear
charge Z and ionic charge Q of incoming particles
SEPICA combines electrostatic deflection in a collimator-analyzer assembly with an energy loss versus
residual energy particle telescope. Z and E are determined in the latter, while Q is derived from the electrostatic deflection. A complete description of the
SEPICA instrument and its data system may be found
elsewhere (12). After problems with its pressure control valves for the proportional counters SEPICA has
been operating for most of the time between early
1998 and 2000 with one of its two high geometric factor sensor units. Its ionic charge resolution at an energy of 1 MeV/Q is approximately AQ/Q = 0.3 and its
geometric factor is 0.09 cm2sr.
FIGURE 1: Variation of the He+/He2+ ratio over the course
of the CIR on DOY 338 - 342,1999.
Table 1:
CIR
1
2
3
4
5
6
Dates
284 - 288 1999
311-3141999
338 - 342 1999
364 1999 - 004 2000
026 - 032 2000
054 - 058 2000
F - F' Boundary
284 00 UT 1999
3 12 17 UT 1999
338 17 UT 1999
001 00 UT 2000
028 12 UT 2000
055 1430 UT 2000
DISCUSSION AND CONCLUSIONS
With ACE SEPICA we have observed a substantial
increase in the He+/He2+ ratio with time from the beginning of the CIR. This observation appears consistently in all six recurrences of the same CIR in 1999
through 2000. While He2+ represents the solar wind
source, He+ originates from interstellar pickup ions. As
is shown in Fig. 2, a spacecraft at 1 AU is magnetically connected to the CIR at increasing distances r
from the sun as time progresses from the start of the
event. The integral pickup ion density «PU varies as 1/r
because of continuous production.
Close to solar maximum a CIR caused by a low
latitude coronal hole was observed with ACE. This
CIR was tracked over six consecutive solar rotations in
late 1999 and early 2000. First noticed during its appearance from DOY 337 to 340, 1999, was a distinct
increase in the He+/He2+ ratio over time, as shown in
Fig. 1. The ratio has been taken over the energy range
0.4 - 0.8 MeV/n. The horizontal bars indicate the time
interval, over which the ratio has been accumulated,
and the vertical bars reflect the statistical error of the
ratio. Roughly 1/2 day has been chosen as accumulation interval, but it usually is extended towards the end
of the CIR, when the fluxes decrease and thus reduce
the counting statistics. The vertical dashed line indicates the time when the spacecraft crossed the boundary between the fast and the fast compressed solar
wind, later denoted F and F', respectively, in accordance with Chotoo et al. (7). The time periods of He
observations for each recurrence are compiled in Table
1 along with the times of the traversal of the F - F'
boundary, which evolves into a fast shock at larger
distances from the sun.
ftpT,*,
• r2
(1)
/%eo is the interstellar He density, \\ the ionization rate
at r0 = 1 AU, and Vsw the solar wind speed. For simplicity we assume that nHe does not vary significantly
with r, which is appropriate for He at r > 1 AU. The
solar wind density decreases as
(2)
because of flux conservation. Therefore, the ratio of
He+ pickup ions to solar wind He2+ increases linearly
with r.
A substantial increase of the He+/He2+ ratio with
time from values at the beginning of the CIR roughly
between 0.1 and 0.2 to values at the end between about
0.5 and 1 is observed for all six recurrences. The ratio
is between 0.2 and 0.3 at the crossing of the F - F'
boundary, similar to the event averages (10).
(3)
Most relevant for the production of energetic particles
in a CIR at large distances, is the fast shock that travels into the fast solar wind, i.e. the F - F' boundary.
Therefore, the time when the spacecraft crosses this
boundary is the starting point with r - 1 AU = 0. We
202
compute the radial distance from each point along the
spacecraft path at 1 AU to the connection of the field
line with the shock, assuming two Parker spirals that
meet. One of them is typical for the local fast solar
wind (dotted line through the S/C in Fig. 2) and the
other one applies to the mean of the fast and slow solar
wind speed (representative of the S' - F' interface in
Fig. 2). This simplified picture ignores the progressive
widening of the CIR with distance from the sun due to
the shock motion into the fast wind, but corrects for
the offset at 1 AU by starting at the F - F' boundary.
velocity space with 0 < V < 2VSW. Our values for the
enhanced efficiency of He+ relative to He2+ (150 - 300
at 1 AU) are consistent with those at 4.5 AU (4).
As can be seen from eq. (3) the source ratio of He+
and He2+ at 1 AU and its slope with distance (in AU)
from the sun should be approximately equal. As derived from the linear fit in Fig. 3, the slope of the ratio
is substantially lower than the ratio at 1 AU, as if the
interstellar source became less efficient with distance
r. It should also be noted that our projected ratio for
He+/He2+ at 4.5 AU is lower by a factor of >10 compared with Ulysses observations at 4.5 AU. This seems
to be compatible with the observation of a flatter
slope. However, even if the slope were the same as the
ACE SEPICA_____________CIR 1999-2000
-
1
0
1
r-1
+
2
3
4
5
[AU]
2+
FIGURE 3: He /He ratio versus inferred distance of connection to the CIR shock for events 2 - 6 in Table 1.
I
ratio at 1 AU, the projected He+/He2+ ratio would
reach ~1.5, still lower by a factor of 6 than the value of
-10 in Gloeckler et al. (4).
FIGURE 2: Schematic view of a CIR and apparent spacecraft path at 1 AU across the structure (adapted from (13)).
The He+/He2+ ratios for events 2 - 6 are shown as a
function of the deduced radial distance from the
spacecraft in Fig. 3. Although event 1 shows a similar
increase, it has been omitted from the compilation.
Contrary to all other events it has a second flux
maximum, which may originate from an interfering
solar event, and both ratios and fluxes are much lower.
The He+/He2+ ratio in Fig. 3 increases approximately
linearly with r with some scatter of the data points.
This behavior is expected, if the increase of the interstellar source over the solar wind with distance from
the sun is reflected in the energetic CIR population.
Using ftHe = 0.015 cm"3 (14), an ionization rate of
1.25 10"7 s"1 (15) for elevated solar activity, and VSw =
600 km/s, a He+/He2+ source ratio of ~10~3 is obtained.
This is substantially lower than the observed energetic
particle ratios of 0.15 - 0.3 at 1 AU. This difference
suggests strongly enhanced injection/acceleration efficiencies for interstellar pickup ions over the solar
wind. Only suprathermal ions are efficiently accelerated at shocks. This provides a significant advantage
for interstellar pickup ions, which populate a sphere in
203
There are several possible explanations for such a
behavior. Firstly, as pointed out the time-distance relation that we have used may be too simplistic, and a
more thorough approach is necessary. A relation for
the azimuthal shock motion and its connection to 1 AU
has recently been evaluated analytically (16). Based on
this relation the connection distances in Fig. 3 would
be reduced by -30%. Secondly, particle transport effects along and perpendicular to the magnetic field
may alter the ratios. He+ and He2+ have rigidities that
are different by a factor of two. Thus transport along
the field line from the CIR shock to the observer may
alter the original source ratio (17). This effect will be
tested in a separate investigation with different species
that originate from the solar wind source, but have
different rigidities because of their mass/charge ratios.
In addition, Dwyer et al. (18) pointed out that transport
perpendicular to the interplanetary magnetic field is
important in CIRs. This may lead to mixing of populations from neighboring magnetic flux tubes and thus
from different radial distances. Such a mixing would
reduce the slope of the He+/He2+ ratio. Finally, it is
5. Franz, M., et al., Energetic particle abundances at CIR
shocks, Geophys. Res. Lett., 26,17 - 20,1999.
conceivable that Gloeckler et al. (4) have observed an
exceptionally high He+/He2+ ratio, as they have only
studied one CIR.
6. 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, ed., 1999, American Inst. Physics: New York,
pp. 605 - 608.
+
In summary, the substantial contribution of He in
the energetic He population and its increase with the
inferred distance from the sun in the CIRs under investigation suggest that interstellar He+ pickup ions are a
major contribution to these ions. Our result agrees with
previous observations that He+/He2+ ~ 0.25 in the suprathermal population of CIRs at 1 AU (6, 7) and
He+/He2+ > 1 in CIRs at 4.5 AU (4, 5). We confirm the
earlier observations at 1 AU, extend them to higher
energies, and demonstrate an increase of the He+/He2+
ratio with the inferred distance from the sun. As
Chotoo et al. (7) have shown, ions are already being
accelerated effectively at 1 AU in a CIR, even without
a developed shock. Therefore, the ions observed early
in the events are likely to represent particle populations accelerated much closer to the sun than those
observed with Ulysses. In any case the observed ratios
are higher by more than two orders of magnitude than
the ratio of interstellar pickup He+ to solar wind He2+
at 1 AU. This enormous enhancement can be attributed
to a strongly enhanced injection and acceleration efficiency for pickup ions over solar wind, as already argued by Gloeckler et al. (4). Only suprathermal ions
can be effectively injected into acceleration. Pickup
ions are essentially suprathermal in the frame of the
solar wind, while the solar wind itself is rather cold.
7. Chotoo, K., et al., The suprathermal seed population for
corotating interaction region ions at 1 AU deduced from
composition and spectra of H+, He++, and He+ observed
on Wind, J. Geophys. Res., 105,23107 - 32122,2000.
8. Gloeckler, G., Observation of injection and preacceleration processes in the slow solar wind, Space Sci.
Rev., 89,91 -104,1999.
9. Gloeckler, G., L. A. Fisk, J. Geiss, N. A. Schwadron, and
T. H. Zurbuchen, Elemental composition of the inner
source pickup ions, J. Geophys. Res. 105, 7459 - 7463,
2000.
10. Mobius, E., et al., Charge states of energetic (-0.5
MeV/n) ions in corotating interaction regions at 1 AU
and implications on source populations, Geophys. Res.
Lett, subm., 2001.
11. Stone, E. C, et al., The Advanced Composition Explorer,
Space Sci. Rev., 86,1 - 22,1998.
12. Mobius, E., et al., The Solar Energetic Particle Ionic
Charge Analyzer (SEPICA) and the Data Processing
Unit (S3DPU) for SWICS, SWIMS and SEPICA, Space
Sci. Rev., 86,449 - 495,1998.
13. Richardson, I. G., L. M. Barbier, D. V. Reames, and T.
T. von Rosenvinge, Co-rotating MeV/amu ion enhancements at <1 AU from 1978 to 1986, J. Geophys. Res., 98,
13 - 32,1993.
ACKNOWLEDGMENTS
The authors are grateful to many unnamed individuals at UNH, at the MPE, and at TUB for their enthusiastic contributions to the completion of the ACE
SEPICA instrument. The work was supported by
NASA under NAS5-32626 and NAG 5-6912.
14. Gloeckler, G., The abundance of atomic 1H, 4He and 3He
in the local interstellar cloud from pickup ion observations with SWICS on Ulysses, Space Sci. Rev., 78, 335 346,1996.
15. Rucinski, D., et al., lonization processes in the heliosphere - rates and methods of their determination, Space
Sci. Rev., 78, 73 - 84,1996.
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