Composition Measurements over the Solar Poles Close to
Solar Maximum - Ulysses COSPIN/LET Observations.
M.Y. Hofer, R.G. Marsden , T.R. Sanderson and C. Tranquille
Research and Scientific Support Dept. of ESA, ESTEC, Keplerlaan 1, 2201 AZ Noordwijk, The Netherlands.
Abstract. We present energetic particle composition measurements acquired on board the Ulysses spacecraft in 2000 and
2001 during its recent south and north polar passages. In an earlier study using data only from the south polar pass (Hofer et
al., 2002), we found that the high-latitude composition data reflected the generally high level of solar activity present during
that period. The observed particle populations during the south and north polar passages comprised predominantly of solar
energetic particles (SEP) accelerated in association with CMEs, rather than particles related to SIR or CIRs. In this work,
we compare the energetic particle composition signatures in the two helio-hemispheres, and find that the latest data from the
north polar pass show the same transient-dominated signature as in the south.
INTRODUCTION
Following aphelion passage in 1998 the Ulysses spacecraft began the second climb to high southern heliographic latitudes, reaching its maximum latitude of 80 2 Æ
at a solar distance of 2.27 AU on November 27, 2000
(DOY 332). On October 13, 2001 (DOY 286) the spacecraft was at the maximum northern heliographic latitude
of 80 2Æ. The south polar pass lasted from September 6 in
2000 (DOY 250) to January 16 in 2001 (DOY 16) and the
north polar pass from August 31 (DOY 243) to December 10 in 2001 (DOY 344), i.e. during which the spacecraft was above 70 Æ heliolatitude. The second orbit of the
spacecraft Ulysses during the solar maximum mission is
shown in Figure 1.
A key question to be addressed by Ulysses during
the polar passes at solar maximum is the origin of energetic particle populations observed at high heliolatitudes. Composition analysis can provide useful clues
in this regard, allowing a clear distinction between particles accelerated at transients associated with coronal
mass ejections (CMEs) and at corotating interaction regions (CIRs) or stream interaction regions (SIRs). Hofer
et al. [2002], in a study of particles increases recorded
at high latitudes by the COSPIN/LET instrument on
board Ulysses during the south polar pass in 2000, found
that the elemental particle composition in these events
was consistent with coronal abundances. This suggests a
transient-related, solar energetic particle (SEP) origin for
the bulk of the particles, perhaps not an unexpected result
given the near-maximum solar activity conditions.
In the present work, we extend this analysis done for
the data recorded above the southern polar region to the
subsequent time period between the two polar passes and
the entire northern polar passage in 2001 as shown in
Figure 1, and compare the characteristics of the particle
populations in the two helio-hemispheres.
DESCRIPTION OF THE DATA
The particle data used in this study are from the Low Energy Telescope (LET) on board the Ulysses spacecraft.
LET is one of the five telescopes in the Cosmic Ray
and Solar Particle Investigation (COSPIN) recording the
fluxes and the composition of solar energetic particles
and of low energy cosmic ray nuclei from hydrogen up
to iron over a range of energies from 1 MeV/n to 50
MeV/n using solid state detectors [Simpson et al., 1992].
For the current particle composition analysis three-dayaveraged values in the low MeV/n energy range are used.
The arrival times of interplanetary shocks at the
Ulysses spacecraft are provided by R.J. Forsyth (private
communication).
PROTON AND ALPHA INTENSITY
In order to provide context for the energetic particle
composition measurements, we show in Figure 2, the
hourly proton(1 2 3 0 MeV)/alpha(1 0 5 0 MeV/n)
ratios and the same alpha intensity for the years 2000 and
2001. The time periods of the south and the north polar
pass are marked with black horizontal lines in Figure 2
and are also shown in Figure 1.
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
183
Ulysses
North Polar Pass
Sep-Dec 2001
Second Solar Orbit
2003
2002
2004
(1998)
Earth Orbit
Aphelion
June 2004
Sun
Perihelion
May 2001
1999
Jupiter
2001
2000
South Polar Pass
Sep 2000-Jan 2001
Ulysses position on 01.12.2002
FIGURE 1. Second orbit of the spacecraft Ulysses during the solar maximum mission. The south and north polar passes are
indicated in the figure.
Ulysses: COSPIN/LET
104
Ulysses: COSPIN/LET
104
102
Proton/Alpha: (ca. 1 MeV/n, x10)
Intensity [particle/cm2 s sr MeV]
Intensity [particle/cm2 s sr MeV]
Proton/Alpha: (ca. 1 MeV/n, 10x)
South Polar Pass
100
10-2
10-4
102
10-2
10-4
Alpha: 1.0-5.0 MeV/n (x 0.1)
Alpha: 1.0-5.0 MeV/n (x 0.1)
10-6
1
31
62
92
122
153 183 213 244
Day of the Year 2000
North Polar Pass
100
274
304
335
365
10-6
1
31
62
92
122
153 183 213 244
Day of the Year 2001
274
304
335
365
FIGURE 2. The hourly proton(1 2 3 0 MeV)/alpha(1 0 5 0 MeV/n) ratio and the alpha intensity (1 0 5 0 MeV/n) during
the years 2000 and 2001 recorded by COSPIN/LET on the Ulysses spacecraft. The south (250/2000-16/2001) and the north polar
pass (243/2001-344/2001) are marked with horizontal lines. The arrows at the bottom of the panels indicate the times of the shock
arrival at the spacecraft.
During the south polar pass, transient events occurred
in rapid succession, resulting in overlapping particle
events. Intensities remained above background levels
throughout the period.
The data from the north polar pass are similar, although the frequency of large transient MeV increases
is somewhat lower, and occasionally quiet-time intervals
are present. The decrease in frequency could be a direct result of the evolution of the solar cycle. The proton/alpha ratio is clearly more variable above the north
solar polar region.
184
COMPOSITION ANALYSIS
With the energetic particle composition data it is possible to distinguish between the particle population accelerated in transients associated with CMEs from those energized by interaction regions, i.e. SIR or CIR. These two
structures pick up and accelerate the material in different
ways and at different locations in the heliosphere.
In Table 1, nominal CIR and SEP elemental abundances with respect to oxygen for the selected elements
are listed as reported from Mason and Sanderson [1999].
The last column contains the ratios CIR/SEP and their errors. High HeO at around 2 9 and low FeO at around
0 6 could be used as indicators for a non-SEP situation,
i.e. SIR or CIR dominated regimes.
TABLE 1. Reference CIR and SEP elemental abundances with respect to Oxygen [Mason and Sanderson,
1999] and their ratios and errors for selected elements.
CIR
SEP
CIR/SEP
He/O
159 1
55.2 3
2.9 0.2
CO
N O
NeO
0.89 0.04
0.14 0.01
0.17 0.02
0.48 0.02
0.13 0.01
0.15 0.01
1.9 0.1
1.1 0.1
1.1 0.2
Fe/O
0.097 0.01
0.16 0.02
0.6 0.1
In Figure 3, the three-day-averaged elemental abundance ratios of helium, carbon, nitrogen, neon and
iron with respect to oxygen at approximately 5 MeV/n
recorded in the years 2000 and 2001 divided by the corresponding reference SEP values are shown as a function of time. The corresponding error bars take the uncertainty of the measured element and the error of the
oxygen value into account. The horizontal line is drawn
at the the level of 1.0 for reference of a nominal SEP
value of all elements. The 30 minutes proton intensity
(1.2-3.0 MeV, 10x) multiplied by a factor of 10 is plotted
for comparison in green.
In Figure 4, the three-day-averaged elemental abundance ratios of helium and iron with respect to oxygen at
approximately 5 MeV/n recorded in the years 2000 and
2001 divided by the corresponding reference SEP values
are shown. The horizontal lines are drawn at the the level
of 2.9 and 0.6 for comparison.
Regarding the maxima of the intensity peaks measured
during the north polar pass they are almost one order of
magnitude higher then those measured above the south
polar pass. Furthermore, the decay rates are slightly different to the southern polar ones.
In Figures 3 and 4, three enhanced HeO values close
to 2.9 and FeO close to 0.6 mark SIR or CIR dominated
regions. Values close to unity for all selected elements
identify particles having an SEP signature. As noted in
Hofer et al. [2002], the majority of the particle events
during the south polar pass have an SEP signature, i.e. in
Figure 3 the majority of values during the second solar
polar pass lay within the error bar close to a measuredto-SEP ratio of unity. Inspection of the right-hand panel
show that the same statement can be made with respect to
the north polar pass in 2001. During the northern passage
185
the HeO ratio never reaches 2.9. All the abundances
look very similar to those recorded above the southern
polar region.
During the time interval between the south polar pass
and the north polar pass at least three enhanced HeO,
i.e. according to the values in Table 1 SIR-like, values
are found. The three enhanced HeO values do not occur
with the characteristic time difference of 26 days which
would be expected for a corotating structure. Therefore,
a CIR dominated situation can be excluded. The first
and the last high HeO ratios are accompanied by rather
low FeO values which would also be expected for an
SIR. But consulting the corresponding error bars of these
values in Figure 2 it is only a slight indication for such a
compression region.
The intensity profile is a result of overlaping intensity
events. Comparing the three-day-average SIR-like values
with the proton intensity time profile no striking time
coincidence with a large increase in the intensity can be
found.
DISCUSSION AND CONCLUSIONS
The recent high-latitude observations correspond to nearmaximum activity conditions with large transient phenomena, e.g. Hofer et al. [2001], Marsden et al. [2001],
McKibben et al. [2001] and Sanderson et al. [2001].
The data acquired during the north polar pass show
a slightly reduced frequency in the number of 5
MeV/n particle events, and occasional returns to nearbackground levels, indicating a small change in the level
of activity with respect to the south solar pass. This is
supported by the fact that continuous high-speed solar
wind flow was measured at Ulysses during the north
polar pass, presumably originating in the newly-formed
northern polar coronal hole. Furthermore, the maxima
and the decay rates of the events were found to be slightly
different over the north polar region as over the south
pole. Based on other observations, e.g. magnetic field
measurements, the overall situation in the inner heliosphere turns out to be slightly different during the north
polar pass in 2001 than during the south polar pass beginning at the end of 2000.
During the time period between the south and north
polar passes, the majority of the MeV particles had
an SEP signature. There were a small number of instances of three-day-averaged SIR-like composition with
enhanced HeO ratios and lowered FeO ratios. The corresponding intensity time profiles reflect overlaping of
several events. We suggest that, during the same time period a few single compression regions causing the characteristic change in the composition data were embedded
in a large number of SEP populations.
Ulysses: COSPIN/LET
Ulysses: COSPIN/LET
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
Elemental Abundance Ratios / SEP Values
Elemental Abundance Ratios / SEP Values
1000.00
100.00
South Polar Pass
10.00
1.00
0.10
3 Day Averages
100.00
31
62
92
122
North Polar Pass
10.00
1.00
0.10
3 Day Averages
Protons: 1.2-3.0 MeV (x10, 30min)
0.01
1
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
Protons: 1.2-3.0 MeV (x10, 30min)
153 183 213 244
Day of the Year 2000
274
304
335
0.01
1
366
31
62
92
122
153 183 213 244 274
Day of the Year 2001
304
335
365
FIGURE 3. The three-day-averaged elemental abundance ratios for the energy ranges as indicated of helium, carbon, nitrogen,
neon, and iron with respect to oxygen divided by the corresponding reference SEP values for the years 2000 and 2001. The solid
horizontal line marks a nominal SEP value at unity. The proton intensity (1.2-3.0 MeV) is plotted for comparison.
Ulysses: COSPIN/LET
Ulysses: COSPIN/LET
1000.00
1000.00
Elemental Abundance Ratios / SEP Values
Elemental Abundance Ratios / SEP Values
He/O 4.25-6.75 MeV/n
Fe/O 5.5-7.5 MeV/n
100.00
South Polar Pass
10.00
1.00
0.10
3 Day Averages
He/O 4.25-6.75 MeV/n
Fe/O 5.5-7.5 MeV/n
100.00
1.00
0.10
3 Day Averages
Protons: 1.2-3.0 MeV (x10, 30min)
0.01
1
31
62
92
122
North Polar Pass
10.00
Protons: 1.2-3.0 MeV (x10, 30min)
153 183 213 244
Day of the Year 2000
274
304
335
366
0.01
1
31
62
92
122
153 183 213 244 274
Day of the Year 2001
304
335
365
FIGURE 4. The three-day-averaged elemental abundance ratios for the energy ranges as indicated of helium, and iron with
respect to oxygen divided by the corresponding reference SEP values for the years 2000 and 2001. The solid horizontal lines mark
the ratios of the HeO and FeO, SIR-like value at 2 9 and at 0 6. The proton intensity (1.2-3.0 MeV) is plotted for comparison.
We also find that the populations of energetic particles
at 5 MeV/n measured during the north polar passage,
are predominantly of SEP origin, confirming the results
obtained earlier during the south polar pass.
We conclude that the second south and north solar polar passage and the intermediate time interval are dominated by SEP events which are most probably associated
with the coronal mass ejection (CME) shock acceleration
phenomenon.
ACKNOWLEDGMENTS
We acknowledge the use of the Ulysses Data System in
the preparation of this paper. MYH thanks ESA for the
current research fellowship.
186
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