Characterization of SEP events at high heliographic latitudes
S. Dalla , A. Balogh , S. Krucker†, A. Posner , R. Müller-Mellin , J.D. Anglin‡ ,
M.Y. Hofer§ , R.G. Marsden§ , T.R. Sanderson§ , B. Heber¶, M. Zhang and R.B.
McKibben††
Blackett Laboratory, Imperial College, London, UK
Space Sciences Laboratory, University of California, Berkeley, USA
University of Kiel, Germany
‡
National Research Council of Canada, Ottawa, Canada
§
Research and Scientific Support Dept. of ESA, ESTEC, The Netherlands
¶
University of Osnabrück, Germany
Florida Institute of Technology, USA
††
University of Chicago, USA
†
Abstract. Between February 2000 and May 2002, the Ulysses spacecraft made the first ever measurements of solar energetic
particles (SEPs) at high heliographic latitudes. Nine large gradual SEP events were detected at latitudes greater than 45Æ , their
signatures being clearest at high particle energies, i.e. protons >30 MeV and electrons >0.1 MeV. In this paper we measure the
onset times of Ulysses high latitude events in several energy channels, and plot them versus inverse particle speed. We repeat
the procedure for near Earth observations by Wind and SOHO. Velocity dispersion is observed in all the events near Earth
and in most of them at Ulysses. The plots of onset times versus inverse speed allow to derive an experimental path length and
time of release from the solar atmosphere. We find that the derived path lengths at Ulysses are longer than the length of a
Parker spiral magnetic field line connecting it to the Sun, by a factor between 1.2–2.7. The time of particle release from the
Sun is typically between 100 and 200 mins later than the relase time derived from in-ecliptic measurements. Unlike near Earth
observations, Ulysses measurements are therefore not compatible with scatter-free propagation from the Sun to the spacecraft.
INTRODUCTION
Ulysses measurements have shown that solar energetic
particles (SEPs) can easily reach high latitudes [1]. Three
possible explanations for these observations are as follows: (a) the CME shocks accelerating the particles extended to high latitudes and crossed the interplanetary
magnetic field lines connecting to Ulysses; (b) the CME
shocks did not extend to high latitudes but significant
particle cross-field diffusion took place [2]; and (c) magnetic field lines connecting high latitudes with low latitude active regions existed in the solar corona, allowing
particles to reach high latitudes close to the Sun [3].
Onset time analysis can provide a measurement of the
distance travelled by particles prior to their arrival at a
spacecraft (path length), and of their release time from
the solar atmosphere. Experimental values of the path
length for high latitude SEP events could be used to
constrain models of particle and cosmic ray propagation.
In this paper we analyse onset times for 9 high latitude
events using data from the Ulysses COSPIN experiment.
We measure the onset times in several energy channels,
and plot them versus c v where v is the particle speed
and c the speed of light. This allows us to verify whether
dispersion is observed, and to calculate the apparent path
length travelled by the particles. We compare Ulysses
measurements with those by near Earth spacecraft.
DATA ANALYSIS
Figure 1 shows an overview plot of SEP measurements
by the Ulysses COSPIN/KET instrument between January 2000 and May 2002. During this time Ulysses travelled over the South Pole of the Sun, then moved northwards crossing the ecliptic and passing over the North
Pole. We considered times when the spacecraft latitude
was 45Æ (the non shaded parts of Figure 1). We selected
SEP events producing large count rates in the KET 38125 MeV proton channel and obtained a list of 9 events.
Details of the events, and of their associated Soft-X-Ray
flares, are given in Table 1. During the events, Ulysses’
distance from the Sun was between 1.63 and 3.28 AU,
and for most its latitude was between 70 Æ and 80Æ .
All 9 events in our list were observed near Earth. In
fact, a comparison of Ulysses with IMP8 profiles shows
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
656
TABLE 1. SEP events at high latitudes
event n year
date
doy SXR onset
1
2
3
4
5
6
7
8
9
2000
2000
2000
2001
2001
2001
2001
2001
2002
14 Jul
12 Sep
8 Nov
15 Aug
24 Sep
4 Nov
22 Nov
26 Dec
21 Apr
196
256
313
227
267
308
326
360
111
10:03
12:13
22:42
23:54
09:36
16:03
20:22
04:32
00:59
flare loc.
SXR class
RUlysses (AU)
Ulysses heliolat
N22 W07
S17 W09
N10 W77
S16 E23
N06 W18
S25 W67
N08 W54
S14 W84
X 5.7
M 1.0
M 7.4
X 2.6
X 1.8
M 3.8
M 7.1
X 1.5
3.17
2.80
2.41
1.63
1.90
2.18
2.31
2.54
3.28
-62.1
-70.9
-79.3
+63.1
+78.2
+77.6
+73.7
+66.7
+47.9
onset of associated CME (no SXR flare)
Ulysses/KET count rates (s -1)
1e+2
1e+1
1
2
4
3
5
6 7
9
8
1e+0
1e-1
1e-2
p 38-125 MeV
p125-180 MeV
(/100)
1e-3
1e-4
1e-5
2000.0
2000.2
2000.4
2000.6
2000.8
2001.0
2001.2
2001.4
2001.6
2001.8
2002.0
2002.2
FIGURE 1. Overview of Ulysses SEP high latitude measurements, for two COSPIN/KET proton channels. Times when the
spacecraft was at latitudes 45Æ are shaded in grey and were not considered in this study.
that at energies above 10 MeV almost all large events
near Earth produce significant increases at Ulysses [1].
Onset time analysis
For a particle of energy E, the time t s c of arrival at the
detecting spacecraft is given by:
ts
c
E tSun L v E proton dispersion compatible with path length of 1.2
AU, but proton release delayed by about 1 hour from
electron release. The second class had events with simultaneous release of protons and electrons (within uncertainties of 20 mins), but proton dispersion indicating a
path length of 2 AU.
Instruments and onset time measurement
(1)
where tSun is the time of particle release at the Sun, L is
the path length and v is the particle speed. The quantities
tSun and L are assumed to be independent of energy in
eq.(1). If this is the case, onset times plotted versus 1 v
lie on a straigth line, whose slope gives the path length
and intercept with the y-axis the time of particle release.
In their analysis of Wind/3DP data Krucker et al. measured L and tSun for several medium-sized SEP events,
and found that they could be divided into 2 classes [4].
The first class consisted of events with both electron and
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We used energetic particle data from the Ulysses
COSPIN experiment and from Wind/3DP and
Soho/COSTEP near Earth. The COSPIN experiment
consists of several separate energetic particle detectors.
We measured electron onset times from the HET instrument (electrons 3–5 MeV). For proton measurements
we have used channels from the HET, ATs and HFT.
Most of the HET proton channels are contaminated by
electrons during the onset of SEP events. For some of
these channels we were able to subtract the electron contributions. For Ulysses, we used spin-averaged data. The
FIGURE 2. Plots of onset time versus cv near Earth (empty symbols) and at Ulysses (filled symbols). The theoretical length Lp
(in AU) of the Parker spiral to Ulysses and the angle α between the flare and Ulysses’ footpoint are given. The last plot gives an
indication of the dispersion curves that would be expected for 3 different path lengths and injection at t=0.
658
pitch angle of the particles responsible for the measured
onset might not be zero, as is implied by eq.(1). This
would result in the experimental path length being larger
than the actual path length by a factor 1 cos θ , with θ
the pitch angle of the first arriving particles. A non zero
pitch angle would lead to underestimating the release
time. Wind/3DP electron data at zero pitch angle and
Soho/COSTEP electron and proton data were used.
We determined onset times by following the procedure
outlined by Krucker et al. [5]. This involves subtraction
of the pre-event background and calculation of its standard deviation σ . The upper limit t upper to the onset time
is the time at which the particle count rate reaches 4σ .
The onset time tonset is the first time prior to tupper for
which particle counts are above zero. The error bar on
each value of the onset time is given by 2 (t upper -tonset ).
Onset times were determined from 10 or 20 min averaged data for Ulysses and from 64 s averages for Wind
data. For a channel measuring particles of energy in the
range between E min and Emax , the measured onset time
has been assigned to the speed v E max , within the assumption that the fastest particles arrive first. For the few
channels for which a more accurate estimate of the most
probable speed existed, its value has been used.
RESULTS AND DISCUSSION
Figure 2 shows plots of onset times versus c v for 7
high latitude events. Empty symbols are for near Earth
channels and filled symbols for Ulysses ones. In the
plots, t=0 is the time of onset of the associated SXR flare.
For each event we have calculated the nominal length
L p of the Parker spiral magnetic field line that would
connect Ulysses to the Sun, using the measured solar
wind speed. We have also calculated the angle α between
the location of the Ulysses footpoint and the solar flare
associated to the event.
Both at Ulysses and near Earth, reasonable agreement
in the onset times measured by different instruments
is seen. For events 1,3 and 8, it was not possible to
determine onset times at low energies at Ulysses. For
event 1, this is due to intensities not being at background
level prior to the flare, for 3 to data gaps and for 8 to
very likely contamination by higher energy protons in
the low energy channels. Plots of onset times are not
reproduced for events 7 and 9, the former having large
pre-event fluxes which make onsets unreliable, and the
latter because some of the data was not yet available.
We observe that Wind/3DP electron onsets are consistent with scatter-free propagation along a field line of
length 1.2 AU. For event 3, instrumental problems are
hiding the velocity dispersion. A first correction for the
instrumental effect shows that also this event is consis-
659
tent with scatter-free propagation.
The release time deduced from Ulysses onset times is
much later than the one found from near Earth observations. For most events, the Ulysses release time is approximately 100 to 200 mins after the Wind release time.
Exceptions are event 8, for which the delay in release is
60 mins, and event 3 for which it is 260 mins.
Path lengths deduced from the Ulysses plots are larger
than the nominal length of the Parker spiral to the spacecraft. Approximate fits to the plots in Figure 2 give a ratio
between L and LParker between 1.2 and 2.7.
SUMMARY
In 6/7 of the events, the release time deduced from
high latitude onsets is delayed by more than 100
minutes with respect to the release time deduced
from near Earth measurements.
• Path lengths derived from Ulysses measurements
are larger than the length of a Parker spiral through
the spacecraft, by factors between 1.2 and 2.7.
• We conclude that onset times at high latitudes are
not compatible with direct scatter-free propagation
along a magnetic field line.
• The large path lengths and late release times suggest that propagation to high latitudes requires scattering.
•
ACKNOWLEDGMENTS
We acknowledge use of the Ulysses Data System. S.D.
acknowledges support from PPARC through a PostDoctoral Fellowship. M.Y.H. thanks ESA for the current
research Fellowship.
REFERENCES
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the energy and charge dependence of the propagation of
solar energetic particles to the Sun’s south polar regions,
Proc. 27th ICRC, Hamburg, 3281 (2001); McKibben et al,
Ulysses COSPIN observations of cosmic rays and SEPs from
the South Pole to the North Pole of the Sun during solar
maximum, submitted to Annales Geophys. (2002)
2. Zhang M., et al., Ulysses observations of solar energetic
particles from the July 14, 2000 event at high heliographic
latitudes, Proc. 27th ICRC, Hamburg (2001)
3. Neugebauer M., et al, Sources of the solar wind at solar
activity maximum, J. Geophys. Res., in press (2002)
4. Krucker S., et al., Two classes of solar proton events
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