Energetic Particle Composition Measurements at High
Heliographic Latitudes Around the Solar Activity Maximum
M. Y. Hofer*, R. G. Marsden*, T. R. Sanderson* and C. Tranquille*
* Space Science Dept. ofESA, ESTEC, P.O. Box 299,2200 AG Noordwijk, The Netherlands.
Abstract. From 1999 until the end of 2000 the Ulysses spacecraft was traveling back towards the southern solar
polar region from about 5 AU to less than 2.5 AU. In this time period the solar activity was close to the maximum.
The results of a preliminary analysis of energetic particle composition data covering the energy range 4-20 MeV/n
recorded by the COSPIN/LET instrument are shown. We attempt to use the composition signatures to identify
the sources of energetic particles observed at high latitudes.
INTRODUCTION
DESCRIPTION OF THE DATA
Following aphelion passage in 1998 the Ulysses spacecraft began its second climb to high southern heliolatitudes, reaching its maximum latitude of 80.2 ° in November 2000. During this time period the spacecraft's heliocentric distance decreased from about 5 AU to less than
2.5 AU. In contrast to the first south polar passage in
1994 (e.g. [1], [2]), which took place as the level of solar activity was approaching the minimum of cycle 22,
the recent high-latitude observations correspond to nearmaximum activity conditions with large transient phenomena. In 1994, the 1-10 MeV/n particle fluxes measured at Ulysses consisted mainly of corotating interaction region (CIR)-related events and of the anomalous
cosmic ray (ACR) [3] component, what was obtained
based on the elemental abundance ratios. A decrease
of the proton to alpha ratio towards around 10 is often
used for the identification of particles accelerated at CIRs
(e.g. [4], [5]). At the beginning of solar energetic particle
(SEP) events the 3He/4He and the Fe/O ratios are often
found to increase as a result from resonant wave-particle
interaction in the flare plasma (see [6], [7]).
In this preliminary analysis the low energy fluxes
recorded in 1999 and 2000 by the COSPIN/LET instrument on the Ulysses spacecraft are plotted in order to
select two major events for a detailed analysis. For these
selected time intervals the elemental abundance ratios are
calculated to gain insight into the possible origin of the
particles and the likely acceleration history. In particular,
we are interested in identifying time intervals dominated
by SEP events or CIR passages.
The low energy particle data used in this study is from
the Low Energy Telescope (LET), one of the five telescopes in the Cosmic Ray and Solar Particle Investigation (COSPIN) onboard the Ulysses spacecraft [8]. The
LET instrument records the fluxes and the composition
of SEPs and of low energy cosmic ray nuclei from hydrogen up to iron over a range of energies from ~ 1 MeV/n
to 75 MeV/n (For details see [8], [3]).
RESULTS
In Figure 1 the proton and alpha intensities (upper panel)
recorded by COSPIN/LET on Ulysses and the proton/alpha ratio in the in the ~1 MeV/n range (proton:
1.2-3.0 MeV; alpha: 1.5-5.0 MeV/n) are plotted for the
years 1999 and 2000. In addition, a panel showing the
latitude of the spacecraft's trajectory (for the distance
see [3]) is added to the figure. In 1999 and 2000 the
spacecraft is traveling for the second time towards the
highest southern heliolatitude of the trajectory reached
in November 1999. The horizontal bars in the first panel
in June 1999 and in July 2000 mark the time intervals
selected for the current analysis.
The maximum proton intensity (1.2-3.0 MeV) in 1999
is about 60 protons/cm2/s/sr/MeV at the beginning of
July. In 2000 there are more than five events observed
with maximum intensity above this level. Furthermore,
the minimum value in the proton intensity recorded at the
end of September in 1999 is never reached in 2000. At
the end of 2000 the spacecraft is located above the southern solar pole where major particle events are observed.
CP598, Solar and Galactic Composition, edited by R. F. Wimmer-Schweingruber
© 2001 American Institute of Physics 0-7354-0042-3/017$ 18.00
189
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Heholatitude of the Ulysses spacecraft
J F M A M J J A S O N D J F M A M J J A S O N D
1999
2000
FIGURE 1. The proton (1.2-3.0 MeV) and the alpha (8.4-19 MeV/n) intensities, and the proton( 1.2-3.0 MeV) / alpha(1.0-5.0
MeV/n) ratios recorded by COSPIN/LET on the Ulysses. The heliolatitudes of the trajectory of the Ulysses spacecraft are plotted
in the third panel. The horizontal bars in June 1999 and in July 2000 mark the selected time intervals.
In 1999 the smallest proton/alpha ratios are lower than
those in 2000, and the proton/alpha decreases are more
frequent than in 2000. Based on these parameters there
seems to exist a tendency for an increase of the activity
from the beginning of 1999 towards the end of 2000.
The high variability in the particle data is accompanied by frequent changes in the magnetic field, in the solar wind velocity (for details please consult the Ulysses
data archive: http://helio.estec.esa.nl/Ulysses/archive/)
and by a complex shape of the heliospheric current
sheet (HCS). During the entire time interval the HCS
is highly tilted resulting in low speed streams coming
from streamer belts and a well defined sector structure
observed even over the pole [9].
Two large events in June 1999 and July 2000 are selected for the further analysis as marked by the horizontal
bars in the first panel of Figure 1.
In Figure 2, the the oxygen intensity and the three day
averaged abundance ratios for helium, carbon, nitrogen,
neon and iron with respect to oxygen are shown for the
selected time intervals and energy ranges as indicated.
The ratios are represented in both panels of Figure 2 by
symbols and the corresponding errors by vertical lines.
In case of a single count, (which may cause an error
larger than the value), the lower part of the error bar is not
drawn. The black arrows at the bottom of the figure mark
the time of the shock occurrence as determined from
magnetic field and solar wind parameters ([10], [11]).
The upper panels in Figure 2 show the oxygen intensity and the elemental abundance ratios for the days 140210 in 1999. Only three significant Fe/O values are obtained for this event.
The lower panels of Figure 2 show the same results
for the selected (mainly gradual) event from day 180 until 250 in 2000 having less variations in the abundances
than the event in June 1999. From day 220 on in 2000
the particle flux decreased rapidly, and therefore no statistically significant abundances can be derived.
Generally, the abundance ratios of He/O evolve in a
mirrored way compared with those of the heavier ions
what is visible in both events.
In Table 1 the averaged values of the statistically
significant elemental abundances within a common
regime in the selected time interval for helium, carbon,
190
Ulysses: COSPIN/LET
COSPIN/LET
Ulysses:
10-3
Intensity [particles/cm2 s sr MeV]
+ Oxygen: 4.25-5.25 MeV/n
10-4
10-5
10-6
1000.00
He/O 4.25-6.75 MeV/n
C/O 4.25-6.75 MeV/n
N/O 4.25-6.75 MeV/n
Ne/O 5.5-7.5 MeV/n
Fe/O 5.5-7.5 MeV/n
Abundance Ratio
100.00
10.00
I f t l 'I 'f It ' f
1.00
0.10
I II
0.01
140
150
16O
160
17O
18O
170
180
Day of
of the
the Year
Year 1999
1999
Day
I I
19O
190
200
210
Ulysses: COSPIN/LET
COSPIN/LET
Ulysses:
10-2
Intensity [particles/cm2 s sr MeV]
+ Oxygen: 4.25-5.25 MeV/n
10-3
10-4
10-5
10-6
1000.00
He/O 4.25-6.75 MeV/n
C/O 4.25-6.75 MeV/n
N/O 4.25-6.75 MeV/n
Ne/O 5.5-7.5 MeV/n
Fe/O 5.5-7.5 MeV/n
Abundance Ratio
100.00
10.00
1.00
0.10
II
0.01
18O
180
19O
190
2OO
200
210
22O
220
Day210
of the Year
2OOO
Day of the Year 2000
23O
230
24O
240
25O
250
FIGURE 2.
2. Three
Three day
day averaged
averaged elemental
elemental abundances
abundances and
and corresponding
corresponding oxygen
FIGURE
oxygen intensity
intensity recorded
recorded during
during the
the selected
selected events
events
in
1999
and
2000.
The
values
are
represented
by
the
corresponding
symbols
and
the
errors
by
vertical
lines.
The
black
in 1999 and 2000. The values are represented by the corresponding symbols and the errors by vertical lines. The blackdownward
downward
arrows mark
mark the
the time
time of
of the
the shock
shock occurrence.
occurrence. Upper
Upper panels:
panels: Three
Three day
arrows
day averaged
averaged values
values of
of the
the abundances
abundances of
of helium,
helium, carbon,
carbon,
nitrogen,
neon,
and
iron
with
respect
to
oxygen
from
day
140
to
210
in
1999.
Lower
panels:
Three
day
averaged
nitrogen, neon, and iron with respect to oxygen from day 140 to 210 in 1999. Lower panels: Three day averaged values
values of
of the
the
abundances of
of helium,
helium, carbon,
carbon, nitrogen,
nitrogen, neon,
neon, and
and iron
iron with
with respect
respect to
abundances
to oxygen
oxygen from
from day
day 180
180 to
to 250
250 in
in 2000.
2000.
191
values decrease. Reames et al. [16] interpret at least the
initial part as a sign of an evolving non-Kolmogorov
Alfven wave spectra.
Furthermore, the abundance values found in this study
for the year 2000 are within the errors equal to the
maximum values of in-ecliptic measurements recorded
by the Wind spacecraft [17].
A possible small ACR contribution might be present
in the data, but could not be distinguished during the
selected SEP time intervals.
nitrogen, neon and iron with respect to oxygen are
listed. The corresponding standard deviation is given in
parentheses. The time intervals of each regime in days
are given in the first column. The last row of the Table
contains SEP values from Mason and Sanderson [12]
derived in 1991, before Ulysses data were available.
TABLE 1. Averaged elemental abundance ratios during
the selected time intervals in 1999 and in 2000 recorded
by COSPIN/LET and the SEP reference values from [12].
(He/O, C/O, N/O: 4.25 - 6.75 MeV/n; Ne/O, Fe/O: 5.5-7.5
MeV/n)
He/O
C/O
N/O
Ne/O
Fe/O
151-170,
1999
32
(6)
0.25
(0.05)
0.06
(0.015)
0.14
(0.02)
0.22
(0.02)
194-209,
2000
48
(3.8)
0.39
(0.04)
0.12
(0.02)
0.2
(0.04)
0.17
(0.04)
SEP
[12]
55.2
(3)
0.48
(0.02)
0.13
(0.01)
0.15
(0.01)
0.16
(0.02)
SUMMARY AND CONCLUSIONS
The heliospheric conditions encountered by the Ulysses
spacecraft during its second passage over the south pole
are clearly different from those recorded during the first
southern solar passage in 1994. Throughout the recent
high-latitude pass, the solar wind is highly variable,
showing little or no recurrent structure, an no evidence
of the high speed flows from polar coronal holes seen
earlier.
Shocks related to stream interactions are also seen
in 1999 and 2000, but these did not have the recurrent
character associated with CIRs. Gosling et al. [13] refer
to these structures as stream interaction regions (SIR).
The contributions to the particle fluxes recorded by the
COSPIN/LET instrument from these shocks are not yet
fully determined.
Enhanced solar activity gave rise to a large number of
transient particle events, observed up to highest southern latitudes. The occurrence of major energetic particle events around solar activity maximum over the solar
poles is not unexpected based on the general understanding of the solar evolution. But, the corresponding observations shown here with even surprisingly large amplitudes and large time durations are made for the first time.
The abundances measured for the two mainly gradual
events discussed here are consistent with the particles
having a SEP origin and are equal to in-ecliptic values
[17].
We suggest that they are accelerated by CME driven
shocks. If the acceleration occurs at lower latitudes, the
particle have somehow to be brought up to the higher latitudes. The propagation mechanism that allowed the particles to reach Ulysses is not certain, but the rapid onset
suggest an important role for perpendicular diffusion.
A future detailed analysis based on this preliminary
identification of the SIR and SEP dominated time intervals might give an even deeper understanding of the ongoing acceleration processes for particles in the MeV energy range.
DISCUSSION
Marsden et al. [4] used a proton/alpha ratio around 10 to
identify particle fluxes associated with CIR passages. In
1999 there are about six and in 2000 two periods with
values around this level. Gosling et al. [13] refer to these
structures as stream interaction regions (SIR). Lario et al.
[14] found a tendency for a recurrent structure in the first
half of 1999. Nevertheless, based on the corresponding
shock occurrence there is little or no recurrent structure
present for the rest of the time interval and therefore no
CIR could be identified.
The Fe/O abundances for SEP and CIR given by Mason and Sanderson [12] are 0.16 ±0.02 and 0.097 ±
0.011, respectively, and even lower for the ACR component. The average of Fe/O values in both selected event
are above the 10 percent level, indicating a probable SEP
source. Cohen et al. [15] claim that most of the large SEP
events in 1999 and 2000 are rather not Fe-enriched with
respect to the coronal values, what is consistent with our
findings.
The first of the selected events in June 1999 has an
average Fe/O value only slightly above the SEP value
as visible in Table 1. The two shocks on the days 155
and 161 mark the boundary of a central regime as shown
in the upper panel Figure 2. The peak in the He/O ratio
around day 181 occurs in vicinity of three shocks on the
days 181,183, and 184.
In 2000, in particular, around day 200 two high Fe/O
numbers are derived which could be associated with the
shock on day 203. During the following 10 days the
192
ACKNOWLEDGMENTS
We acknowledge the use of the Ulysses Data System in
the preparation of this paper. We thank Marek Szumlas
for the help in the preparation of the figures.
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