Ulysses Measurements of the Solar Cycle Variation
of ~2-40 MeV/n Ions in the Inner Heliosphere
C. Tranquille*, R.G. Marsden* and T.R. Sanderson*
*Space Science Department ofESA, ESTEC, Noordwijk, The Netherlands
Abstract. We present measurements of energetic (~2 to 40 MeV/n) ion spectra and abundance ratios using
the COSPIN/LET instrument flown onboard Ulysses. These measurements span almost a complete solar cycle in
time and cover an extensive region of the inner heliosphere (heliocentric distance from 1 to 5 AU and heliographic
latitude from the ecliptic to ±80°). We are able to characterise ions associated with solar energetic particle events
and those that belong to the anomalous cosmic ray (ACR) component. Abundance ratios measured in the distinct
particle regimes encountered throughout the Ulysses mission are collated and compared to similar measurements
made by other spacecraft. We investigate in detail the evolution of the ACR oxygen energy spectrum during solar
minimum and relate our observations to models of ACR transport.
INTRODUCTION
Ion intensities during periods of maximum solar activity are dominated by solar energetic particles. It is well
known that ACR and OCR ions are subject to solar modulation in the heliosphere, leading to larger intensities at
solar minimum than at solar maximum [8].
The origin of ACR ions has been shown to be the
neutral interstellar gas [9], which contains atoms having
a high first ionization potential, such as helium, nitrogen, oxygen, neon and argon. These atoms, having zero
electric charge, are able to penetrate into the inner heliosphere with relative ease. During their transport, they
become singly ionized by solar ultraviolet radiation or
by charge exchange with the solar wind. Once charged,
they are picked up by the solar wind and convected out
into the distant heliosphere [10], where they become energised by interactions with the termination shock [11].
The ions then diffuse inwards and experience the effects
of convection and drift. The direction of the gradient drift
is determined by the solar magnetic dipole polarity [12].
For the previous solar cycle (A>0), ACR ion propagation
was characterised by positive radial and latitudinal gradients, resulting from preferential drift inwards and downwards from the heliospheric poles to the equatorial regions [13,14]. During the current (since mid 2000) magnetic solar cycle (A<0), this configuration is reversed.
Low intensities of ACR protons and carbon have also
been measured in the intermediate heliosphere [15].
Measurements of the latitudinal gradient of ACR ions,
including oxygen, have been reported extensively for the
first orbit of Ulysses [16, 17, 18], when solar minimum
conditions prevailed. A modest latitudinal gradient of a
few percent per degree was found for ACR ion species
Ulysses, launched in October 1990, has provided continuous measurements of particles and fields in the inner
heliosphere for almost a complete solar cycle. The mission began during the declining phase of the last (#22)
solar cycle, with the in-ecliptic transit to Jupiter lasting
approximately 1.5 years. During this leg of the mission,
solar activity was high resulting in the most intense levels
of charged particle fluxes seen at Ulysses to date [1]. In
contrast, the first traversals of the southern [2] and northern solar poles [3] took place near solar minimum, with
Ulysses immersed in steady, fast solar wind [4] originating from well formed coronal holes located at high heliolatitudes on the sun. Currently, the spacecraft is climbing
to the northern polar regions of the sun for the second
time, but on this occasion during the active phase of the
new (#23) solar cycle [5]. When Ulysses returns to the
northern solar pole in late 2001, it will have accumulated
in situ measurements of the inner heliosphere to heliolatitudes of ±80°, over a complete eleven year solar cycle.
A major goal of the Ulysses mission is to measure
the latitudinal variation of charged particles in the inner
heliosphere, and to test models of their production and
transport. The three main constituents of energetic ions
in the inner heliosphere are 1) solar energetic particles
(SEP), 2) low energy particles accelerated in the interplanetary medium (for example by travelling shocks [6]
or by corotating interaction regions [7]) and 3) cosmic
rays (composed primarily of galactic cosmic rays (OCR)
and the anomalous cosmic ray (ACR) component).
CP598, Solar and Galactic Composition, edited by R. F. Wimmer-Schweingruber
© 2001 American Institute of Physics 0-7354-0042-3/017$ 18.00
195
over the relevant energy range. The latitudinal gradient
was found to be strongest at low latitudes and weakest at
high latitudes. This is in contrast to OCRs which show
small latitudinal gradients in the streamer belt (between
0° and 30°) [19]. The results for ACR ions are consistent
with an independent analysis made using measurements
from the SAMPEX, Pioneer and Voyager spacecraft [20].
A radial gradient of approximately 15%/AU, derived
from a previous study using IMP and Pioneer data out to
a distance of 15 AU [21], has been shown to be consistent
with Ulysses measurements [22].
The Ulysses results also point to an asymmetry between the level of ACR intensities measured for the two
hemispherical regions of the inner heliosphere [17], with
greater fluxes seen in the northern hemisphere. A similar
asymmetry was also found for galactic cosmic rays [23,
24]. A possible explanation for such an asymmetry may
be a southward displacement of the heliospheric current
sheet [25] resulting in a weaker magnetic field strength
above the sun's magnetic equator.
In this paper we investigate the energy spectra and
abundance ratios of ~2-40 MeV/n ions measured by the
COSPIN/LET instrument throughout the complete mission, and compare the values to previous measurements
made by other spacecraft. We examine in detail the profile of the ACR oxygen ion energy spectra, and infer
some properties of the transport of these ions in the inner
heliosphere.
the ability of the instrument to differentiate ion species
up to iron, with negligible shift in the detector response
throughout the ten year lifetime of the mission. Analysis
of the PHA charge histograms also confirms the expected
performance of the LET instrument.
OBSERVATIONS AND DISCUSSION
Figure 2 is a colour spectrogram showing 10 day averages of oxygen intensities measured in 40 energy channels (of equal logarithmic width) from 1 to 100 MeV/n,
throughout the complete Ulysses mission. Only channels
between ~4 MeV/n and ~40 MeV/n provide valid oxygen intensities, and this is a restriction of the LET design and geometry. Intensities within non-zero energy
bins have been interpolated with a third order polynomial
to smooth the spectrogram. In addition to the time axis,
a horizontal axis has been included to show the radial
and latitudinal motion of the spacecraft, and important
milestones, such as perihelion, aphelion and maximum
southern and northern heliographic latitude, have been
marked. The colour coding of the intensities has been
chosen to accentuate a selected portion of the full dynamic range of the instrument. This was done to enhance
details in the energy spectra of oxygen ions at solar minimum, at the expense of the high flux transient events seen
during the first two years of the mission and during the
Jupiter flyby.
The spectrogram clearly shows two distinct oxygen
populations. At solar maximum, immediately after the
launch of Ulysses, the oxygen population is characterised
by discrete, high intensity events. These are oxygen ions
of solar origin, which have been possibly accelerated
close to the sun in the corona or alternatively in interplanetary space by travelling shocks or by CIRs. As solar
activity decreases after the Jupiter flyby, the frequency of
the transient events becomes significantly reduced. With
the exception of a few isolated events during solar minimum, oxygen ions of solar origin do not return until after
1998, when the current solar activity cycle begins its active phase. The second population of oxygen ions seen in
Figure 2 is that belonging to the ACR component. This
is characterised in the spectrogram by a continuous distribution of intensity over the energy range of the LET
instrument, lasting from mid 1993 for a five year period coincident with the last solar minimum. It is evident
that ACR oxygen ions are subject to latitudinal gradients,
with peak intensities seen when Ulysses was at maximum heliolatitudes, as expected for the configuration of
the solar magnetic dipole for the previous magnetic cycle. There is also evidence for higher intensities in the
northern hemisphere, over the complete energy range of
the instrument. Radial gradients are also present in the
INSTRUMENTATION
The data used in this analysis were obtained from the
Low Energy Telescope (LET) of the Cosmic Ray and Solar Particle Investigation (COSPIN) flown onboard the
Ulysses spacecraft [26]. The LET instrument measures
the flux, energy spectra and ion composition of solar energetic particles and low energy cosmic ray nuclei in the
energy range of ~1 MeV/n to ~50 MeV/n. The instrument uses the standard dE/dx versus E technique to provide pulse height analysis (PHA) information allowing
the identification of the chemical species and energy of
individual particles. The telescope is also able to provide
counting rate information of protons, alpha particles and
groups of particle species in fixed energy channels. The
present study will focus on PHA data of ~2-40 MeV/n
ions.
Figure 1 is a pulse height matrix for the complete mission constructed from a combination of two of the LET
detectors (Dl and D2). The inset shows the matrix for
protons and alpha particles (having significantly greater
statistics) separately from the remaining ions in the main
diagram. The box-like boundary of the helium track is an
artefact of the PHA analysis software. This figure shows
196
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COSPIN/LET - to Dec. 2000
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100
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FIGURE 1.
300
500
400
600
D1+D2(MeV/n)
The Dl versus D1+D2 pulse height matrix for the COSPIN/LET instrument measured from launch to December 2000.
Ulysses: CQSPIN/LET
100
Oxygen
<10
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10 day averages
AB
cf
10
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Heliocentric Distance (AU)
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1990 1991
1992 1993
1994
1995 1996
Year
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FIGURE 2. Colour spectrogram showing 10 day averages of oxygen intensities measured throughout the complete Ulysses
mission. The SEP events (A, B and C) referred to in Figure 3 are marked.
197
data but are not so easily recognisable.
Figure 3 shows oxygen energy spectra that are representative of the two different particle regimes. The energy spectra of SEP oxygen ions (left panel) are significantly higher in intensity than their ACR counterparts at lower energies. The most intense spectrum in
this figure was measured during the March 1991 period
(plus symbol in the left panel). This interval provided the
highest fluxes of energetic (MeV) particles seen during
the Ulysses mission to date. The ACR spectra at maximum southern (triangles in the right panel) and northern
(squares) heliolatitudes, are generally harder than typical
SEP spectra at higher energies. The ACR oxygen spectra
also exhibit a broad peak seen in the lower end of the energy range measured by the instrument. The ACR spectrum for oxygen can be used to test models of cosmic ray
propagation [27] which predict a correlation between the
energy of the peak in the spectrum and the solar wind velocity. A study by Heber et al. [28] using COSPIN/LET
and EPAC data from Ulysses provided evidence for the
existence of such a correlation.
Figure 4 shows abundance ratios of helium, carbon,
nitrogen and neon to oxygen in the energy range of 48 MeV/n, averaged over 40 days. Vertical lines through
each point indicate the statistical error associated with
the measurements. The 4-8 MeV/n oxygen intensity is
also shown in this figure as a reference profile for the
abundance ratios. The abundance ratios of nitrogen and
neon to oxygen remain fairly constant throughout the
complete mission, unlike those for helium and carbon
which are very sensitive to solar activity and latitudinal
effects. The Ne/O ratio however does increase above
the constant level when transient fluxes are measured at
Ulysses. The abundance ratios of helium and carbon to
oxygen show distinct boundaries for the population of
ACR ions. The He/O is very sensitive to helium ions
of solar origin and to the increased levels seen during
the so-called fast latitude scan (when Ulysses passed
through the ecliptic plane from the south to the north
pole between 1994 and 1995). The C/O ratio also shows
an increase (albeit less pronounced) during this time
interval.
Table 1 summarises the abundance ratios seen for the
ions presented in Figure 4. Values are given for the ecliptic transfer (ECL) the first southern (SPP1) and northern
(NPP1) polar passes (both during solar minimum conditions) and for the second southern (SPP2) polar pass
(during solar maximum). Periods of SEP events are excluded from the average polar pass values, as are measurements taken during the fast latitude scan. The abundance ratios measured during the first pair of polar passes
are consistent with ACR values documented by Reames
[29], with the exception of the He/O ratio. A possible reason for the discrepancy in the He/O ratio is the mismatch
between the LET energy range of 4-8 MeV/n and that
198
TABLE 1. Average values of 4-8 MeV/n ion
abundance ratios measured by Ulysses during
different phases of the Ulysses mission (errors
are in brackets): ECL (launch to February 1992);
SPP1 (days 230-270, 1994); NPP1 (days 190230, 1995); and SPP2 (days 310-350, 2000).
Equivalent measurements of ACR ions (at comparable energies) as documented by Reames are
also given for comparison.
He/O
C/O
N/O
Ne/O
ECL
67.9
(6.0)
0.47
(0.07)
0.15
(0.04)
0.18
(0.04)
SPP1
1.69
(0.17)
0.02
(0.01)
0.13
(0.03)
0.10
(0.03)
NPP1
0.96
(0.08)
0.013
(0.006)
0.14
(0.02)
0.08
(0.02)
SPP2
33.2
(2.8)
0.26
(0.04)
0.12
(0.04)
0.11
(0.03)
ACR
5.0
(1.0)
<0.01
0.12
(0.01)
0.07
(0.01)
used by Reames, which could be significant depending
on the helium energy spectrum.
CONCLUSIONS
The COSPIN/LET instrument provides a coherent and
continuous data set of energetic ion intensities in the
inner heliosphere over almost a complete solar cycle,
allowing the long term behaviour of these particles to
be investigated. The data also represent a unique set of
measurements of the ion population at high heliographic
latitudes which have been used in previous studies to
confirm general features of charged particle production
and transport in the global heliosphere.
Ion abundance ratios and energy spectra have been
analysed in this study to characterise particles originating
from SEP events and from ACR populations as a function
of the solar activity cycle. The recent series of energetic
solar events seen since mid 2000 have provided significant heavy ion intensities at high latitudes which need to
be investigated in detail. As the current active phase in
the solar cycle comes to an end, the ACR component is
expected to reappear and will be measured by the LET
instrument.
ACKNOWLEDGMENTS
We acknowledge the use of the Ulysses Data System in
the preparation of this paper.
Ulysses: COSPIN/LET
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FIGURE 3. Oxygen ion energy spectra (10 day averages) measured by the COSPIN/LET instrument during SEP events (left
panel) and at maximum southern and northern (right panel) heliolatitudes. The continuous lines represent polynomial fits to the
spectra.
Ulysses: CQSPIN/LET
O He/0
A N/0
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X Ne/0 (/100)
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4-8 MeV/n Oxygen Intensity (/crnVs/sr/MeV/n)
1991
1992
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1995 1996
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1997
1998
1999
2000
2001
FIGURE 4. 40 day averages of abundance ratios of helium, carbon, nitrogen and neon (divided by 100) ions to oxygen, in the
energy range of 4-8 MeV/n. Also shown is the 4-8 MeV/n oxygen intensity profile.
199
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