Charge-to-mass fractionation during injection and
acceleration of suprathermal particles associated with the
Bastille Day event: SOHO/CELIAS/HSTOF data
K. Bamert , R. F. Wimmer-Schweingruber , R. Kallenbach†, M. Hilchenbach and
B. Klecker‡
Physikalisches Institut, University of Bern, Sidlerstrasse 5, CH-3012 Bern, Switzerland
†
International
Space Science Institute, Hallerstrasse 6, CH-3012 Bern, Switzerland
Max-Planck-Institut für Aeronomie, D-37189 Katlenburg-Lindau, Germany
‡
Max-Planck-Institut für Extraterrestrische Physik, D-85740 Garching, Germany
Abstract. We present SOHO/CELIAS/HSTOF data on suprathermal H, He, CNO, and Fe ions in the energy
range 0.035–2 MeV/amu associated with the Bastille Day coronal mass ejection event, July 14–16, 2000.
We observe a complicated evolution of the spectra in the plasma upstream from the strong interplanetary
shock on July 15, in the downstream turbulent compression region, and in the magnetic cloud following the
compression region. The spectra of suprathermal H, CNO, and Fe ions from the solar wind source fit a scheme
of ordering by their charge-to-mass (Q/A) ratio. This scheme suggests that the ions are stochastically accelerated
in the turbulence region downstream from the shock and then injected into first-order Fermi acceleration. The
suprathermal He flux consists of solar wind 4 He2 ions matching the Q/A-ordering scheme, and an additional
component.
INTRODUCTION
Suprathermal particles in the solar wind plasma are the
source population for particles accelerated to high energies by CME-driven shocks in gradual solar energetic
events. The STOF sensor offers one of the first opportunities to verify the evolution of ions in the energy range
between 35 keV/amu and a few MeV/amu. In this energy
range, the injection into first-order Fermi acceleration is
believed to occur.
Here, we evaluate spectra of suprathermal ions associated with the strongest interplanetary shock of the
Bastille Day event. The spatial evolution of the energy
spectra in the plasma upstream from the shock, in the
strong downstream turbulence, and in the following magnetic cloud is analyzed.
ment, and the detection of the residual energy in a solid
state device to determine the mass, charge, and energy of
an ion. A detailed description of the CELIAS sensors is
given in Ref. [1].
The data analysis is based on two sets of data: (1) The
Pulse Height Analysis (PHA) words indicate the time of
flight, the energy stored in the solid-state stop detector,
the position on the stop detector and on the microchannel plates creating the start signal, the gain flag of the
solid-state detector, and the energy per charge of an ion
when entering the instrument. If the particle fluxes are
high (as they are for July 14–16, 2000) only a subset of
PHA words is transmitted. (2) The rates are accumulated
on board after a classification and selection scheme. The
rates have better statistics, whereas the PHA words contain more information about a single event. We have verified that both data sets deliver consistent results.
DATA ANALYSIS
OBSERVATIONS
The CELIAS/HSTOF (Highly Suprathermal Time Of
Flight) mass spectrometer on board SOHO measures the
elemental and charge composition of ions with suprathermal energies in the range 35–2000 keV/amu. The sensor
applies electrostatic deflection, time-of-flight measure-
On July 14, 2000 (DOY 196, Bastille Day) at 1024 UT
an X-class flare, one of the brightest ever seen by SOHO,
was detected by SOHO/EIT. The flare was accompagnied by a very strong energetic proton storm. At 1054 UT
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
668
The Bastille Day Event
@ Sun:
X-flare & CME
IPS1
(July 14-16, 2000)
IPS2
MC1
MC2
SOHO/CELIAS/HSTOF (He rates)
SOHO/CELIAS/PM
2h
ACE/MAG
1
MC1
1
2
3 4 5
6
7
8
MC2
Shock & mag. turbulence
2
4
3
5
6
7
8
H
He
CNO
?
He
Fe
Fe
CNO
He
?
He
H
Fe
CNO
He
?
He
H
FIGURE 1. Overview of the Bastille Day event. We show from top to bottom: The He intensities in three energy ranges
derived from rates data; the proton density from proton monitor (PM) data of SOHO/CELIAS (Carrington Rotation web site
http://umtof.umd.edu/pm/crn/); the magnetic field strength from Level 2 data of the MAG experiment on board ACE
(http://www.srl.caltech.edu/ACE/ASC/level2/lvl2DATA_MAG.html);
the values for y (see Eq. 2) used to fit
the spectra (open symbols: H, He , CNO, and Fe, closed symbols: He? ); the spectra of H, He, CNO and Fe (2-hour averages)
during the passage of the first magnetic cloud (MC1, 1+2), during the passage of the strong interplanetary shock (IPS2), during the
following strong magnetic
turbulence (3–5), and during the passage of the second magnetic cloud (MC2, 6–8); and the maxima of
the function f2 for H, He , CNO and Fe, and in addition for He? , vs. E/m and rigidity, respectively. The two interplanetary shocks
are marked by dashed lines. The boundaries of the magnetic clouds are indicated by dash-dotted lines. In addition, the onsets of the
X-flare and the CME as observed by SOHO/EIT and SOHO/LASCO, respectively, are marked by bold dashed lines.
669
the onset of an Earth-directed halo coronal mass ejection
(CME) was observed by SOHO/LASCO. The CME left
the Sun at a speed of 1775 km/sec. The associated solar energetic particle event was one of the strongest in
this cycle. The interplanetary shock driven by the CME
arrived at SOHO, which is in a halo orbit around the first
Lagrangian point L1, about 28 hours later. The events
during the three-day period from DOY 196 until DOY
199 are marked in Figure 1, which presents the data of
HSTOF and other sensors on board SOHO and ACE.
The intensity of suprathermal He is lower before the
shock passage, and higher in the magnetic turbulence after the shock. This indicates a strong locally generated
suprathermal particle population. In the second magnetic
cloud the fluxes drop successively to rise again after its
passage. The spatial variations of the suprathermal He
fluxes in the different energy ranges look qualitatively
similar. In the first magnetic cloud and during the shock
passage, particles with higher energies, i. e. in the ranges
0.5–0.75 and 0.75–1.0 MeV/amu, are as abundant as particles in the lowest energy range (0.25–0.5 MeV/amu).
However, after the passage of the shock, in the magnetic
turbulence and during the passage of the second magnetic cloud, the flux of low-energy particles is higher.
The spectra derived from the HSTOF data are fitted
by a sum of two semi-empirical distribution functions,
f f1 f2 . The function f 1 describes the ions resulting
from first-order Fermi acceleration with spectral index
γ1 [2]:
f 1 n1
γ1
E
m exp
E 1
m c1
δ1
A
Q
m Q
(1)
d1
E A where E m is the energy of the particle in keV/amu.
The spectral index γ1 is about 2.4. The variables c1 and
δ1 may be related to the rigidity-dependent escape from
first-order Fermi acceleration. The second argument in
the exponential term denotes the propagation of ions in
the upstream magnetic cloud. The variable d1 depends
on the distance to the shock. It presumably results from
a spatial integral over the plasma bulk speed divided by
the diffusion coefficient [2].
The function f 2 describes the suprathermal population
generated in the strong magnetic turbulence:
f2
n2 γ2
E
m
exp
exp E
m
E 1
m c2
β2
y
A
Q
Q
A
δ2
α2
amplitude of the magnetic fluctuations and their spectral distribution [3]. The same applies for the parameters β2 and α2 . The parameter y changes with distance
from the turbulence region. It may in part represent the
plasma conditions in the turbulence region, but also describes diffusion away from the turbulence region. The
exponents β2 and α2 also seem to be valid to describe
the spatial diffusion in the plasma.
The suprathermal ion spectra (differential flux) described by Eq. 2 have a maximum between 60 keV/amu
and 1 MeV/amu for H, He , CNO and Fe (Fig. 1).
The position of this maximum on the energy-per-mass
scale and on the rigidity scale varies spatially and for
ion species with different mass-per-charge ratio A Q. For
ions with small A Q the maxima are at higher energies
per mass than for ions with large A Q. The energy per
mass of maximum differential flux is higher in the downstream turbulence region than in the downstream magnetic cloud. Before the shock passage the dispersion in
A Q becomes larger with increasing distance from the
shock. Presumably, the ion species are fractionated due
to propagation effects which depend on the charge-tomass ratio of the ions. We also observe this behavior during the April 4–7, 2000 CME event which served as a
reference. For future purpose, we note that the parameter β2 1, and c2 and y have been determined to be
500 keV/amu and 250 keV/amu, respectively, in the
downstream turbulence region.
The component He? in the spectra represents a special
species, which is not identified unambiguously, yet. The
distribution functions of He? measured by HSTOF do
not fit the A Q ordering scheme derived from the other
species, neither under the assumption that these ions are
4 He , nor under the assumption that they are 4 He or
3 He ions. The mass spectra and studies on other CME
events suggest that He? is 3 He from an impulsive flare.
MODEL
The empirical functions used to fit the spectra match a
model [3] applying theoretical considerations of Refs. [2,
4, 5]. It combines the mechanisms of stochastic acceleration in the turbulence near interplanetary shocks, firstorder Fermi acceleration, and stationary diffusion in the
field irregularities of magnetic clouds. The data suggest
that the model may be valid, if the spectral index of the
magnetic field irregularities is about n 1.
(2)
The parameters c2 , δ2 , and γ2 describe spectra resulting
from stochastic acceleration and are presumably related
to the plasma conditions in the downstream turbulence
and in the downstream magnetic cloud, e.g. the relative
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DISCUSSION
The observations on ions in the energy range 0 035 and
2 MeV/amu associated with the Bastille Day event
theoretical considerations suggest that the dominant
suprathermal particle population is the one generated by
stochastic acceleration in the turbulent compression region between the strong interplanetary shock and the
magnetic cloud downstream from the shock. The evolution of spectra during the event indicates that 1) most
ions of H, He, CNO, and Fe have solar wind charge states
and are stochastically accelerated in the downstream turbulence region. If this turbulence is one-dimensional
Alfvénic turbulence, its spectral index is n 1. 2) This
seed population is injected into first-order Fermi acceleration at the shock.
Upstream from the shock, the ions resulting from
the first-order Fermi process dominate. They propagate
through the upstream magnetic cloud with magnetic field
irregularities that may have a spectral index nMC1 1,
pending an analysis of magnetic field data and pending
an evaluation of other models on spatial and momentum
diffusion of suprathermal ions in the solar wind plasma.
Downstream from the shock, mainly the suprathermal
population resulting from stochastic acceleration is observed. These ions diffuse from the turbulence region
into the downstream magnetic cloud. The spectra near
this cloud indicate that the suprathermal ions of CNO and
Fe have somewhat higher charge than usually observed
in the solar wind. This may be due to contributions from
the source of bulk ions confined in the downstream magnetic cloud [6].
The He spectra show a signature in addition to the
signatures of solar wind ions. Possibly, this signature is
due to suprathermal 3 He ions from an impulsive flare.
ACKNOWLEDGMENTS
The STOF sensor onboard the SOlar and Heliospheric
Observatory (SOHO), a joint project between ESA and
NASA, is part of the Charge, ELement, and Isotope
Analysis System (CELIAS), which is a joint effort of
five hardware institutions under the direction of the MaxPlanck-Institut für extraterrestrische Physik (pre-launch)
and the University of Bern (UoB; post-launch). The
Max-Planck-Institut für extraterrestrische Physik is the
prime hardware institution for (H)STOF. The University
of Bern provided the entrance system. The data processing unit (DPU) was provided by the Technical University
of Braunschweig.
This work was supported by the Swiss National Science Foundation.
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