A Technique For Comparing Solar Wind Structures
Observed By ACE And Ulysses
H. A. Elliott and D. J. McComas
Southwest Research Institute, San Antonio, TX
P. Riley
Science Applications International Corporation, San Diego, CA
Abstract. Comparison of solar wind observations from the ACE spacecraft, in the ecliptic plane at ~1 AU, and the
Ulysses spacecraft as it orbits over the Sun's poles, may provide valuable information about the latitudinal extent and
variation of solar wind structures in the heliosphere. While qualitative comparisons can be made using average
properties observed at these two locations, comparison of specific, individual structures requires a procedure to
determine if a given structure has been observed by both spacecraft. Before addressing the challenge of comparing such
structures at different latitudes, we seek to benckmark our procedure by comparing solar wind structures observed when
both spacecraft were near the ecliptic plane. In this paper we assess the use of a 1-D hydrodynamic code to propagate
ACE plasma measurements out to the distance of Ulysses and develop a procedure for compensating for the differing
longitudes of the ACE and Ulysses spacecraft. In addition to comparing the plasma parameters and their characteristic
profiles, we examine superthermal electron measurements and magnetic field polarity to determine if the same features
are encountered at both ACE and Ulysses. We find that when both spacecraft are near the ecliptic plane; they frequently
observe the same features.
INTRODUCTION
three-dimensional configuration of solar wind
structures in the heliosphere, particularly around solar
maximum. We focus on the in-ecliptic measurements
to determine if the same features can be observed at
two spacecraft with small latitude separations. The
results of this study demonstrate that such features can
be reliably identified, enabling us to extend this work
to larger latitude separations in future studies.
Coronagraph measurements have provided
information about the size and three-dimensional
shape of coronal structures within ~30 Rs of the Sun,
but further out in the solar wind, much less is known
about how such large-scale structures evolve. Over the
past forty years, most of what has been learned about
the heliosphere is based on in-ecliptic measurements.
However, the unique orbit of Ulysses, which passes
over the Sun's poles, provides in situ measurements as
a function of latitude. Ulysses has already greatly
enhanced our knowledge of the 3-D structure of the
inner heliosphere during solar minimum when the
large-scale structure varies slowly [e.g., McComas et
al., 2000, and references therein]. For example,
Ulysses solar minimum measurements showed a
remarkably simple solar wind structure with highspeed streams filling the high latitude regions when
there are large, persistent polar coronal holes. In
contrast, the near-maximum solar wind is much more
complex with highly variable flows arising from a
variety of coronal sources observed at all heliolatitudes
[McComas et al., 2002].
DATA AND MODEL
We use primarily ion and electron measurements
from the Advanced Composition Explorer (ACE) and
Ulysses spacecraft. These observations are taken from
the ACE Solar Wind Electron Proton Alpha Monitor
(SWEPAM) [McComas et al., 1998] and the Ulysses
Solar Wind Observations Over the Poles of the Sun
(SWOOPS) instruments [Bame et al., 1992]. We also
use supporting observations from the magnetometers
on Ulysses [Balogh et al., 1992] and ACE [Smith et
al., 1998].
To propagate data from 1 AU (ACE) out to
Ulysses, we use the Zeus Astrophysical single-fluid
magnetohydrodynamic model [Stone and Norman,
1992]. The model uses a Eulerian finite difference
The goal of this paper is to develop an analysis
technique that will provide more information about the
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
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scheme. Although the Zeus model can include
magnetic fields and radiation transport, in this study
they are neglected and only hydrodynamic effects are
examined. We propagate magnetic sector by defining
boundaries of given regions (such as a CME or highspeed stream) in the ACE data and then track those
boundaries by advancing them at each time step in the
simulation using the velocity and acceleration at that
grid point. The model solves a system of continuity,
momentum, and internal energy equations. We use a
polytropic index of 3/2 since most solar wind
measurements are generally consistent with this value
[Riley et al., 2001]. Although Zeus is a 2-D model,
only one dimension is used so that we can use single
point in situ measurements to drive the model. Using
the 1-D model allows us to focus on the dynamic
evolution of structures between 1 and 5.4 AU without
wind measurements from multiple spacecraft,
Richardson et al. [2001] found that correlation lengths
perpendicular to the flow are 70 RE for the solar wind
speed and 100 RE for the density.
We use ACE measurements as the inner boundary
conditions for each time step of the simulation. We
then compare the propagated ACE measurements with
the Ulysses measurements. Since the Ulysses
spacecraft position varies slowly, the data are split into
4 segments per year to calculate an average radial
position. For each segment we compare Ulysses
SWOOPS measurements to ACE mapped data at the
radial grid point that most closely matches the average
radial distance of Ulysses. We use the magnetic
polarity, magnitudes and profiles of the velocity,
density, and temperature to identify specific features.
The polarity is determined by comparing the magnetic
field direction to the Parker spiral direction determined
using the solar wind speed [Forsyth et al., 1996].
Since we are interested in the large-scale structure, we
use a 12-hour running mean to calculate the sector
structure. Electron distributions are used to identify
coronal mass ejections.
RESULTS
The top panel of Figure 1 displays a time series of
ACE SWEPAM solar wind speed measurements at 1
AU (top curve), the radial propagation (RP) of the
ACE data at intermediate grid steps (5 middle curves),
and a direct comparison between Ulysses SWOOPS
measurements and mapped ACE data (bottom curve).
High-speed streams become steeper with distance as
faster plasma overtakes slower plasma and shocks
form. Many shocks form between 1.5 and 2.5 AU,
and can easily be tracked out to 5 AU, and many highspeed structures are worn down as momentum and
energy are transferred to lower speed plasma. For
example, from 1998.24 to 1998.34 and from 1998.4 to
1998.47 the model predicts that the high-speed
features are worn down, consistent with Ulysses
measurements of a more uniform low speed solar wind
speed. The peak in the velocity between 1998.34 and
1998.4, occurs at a later time in the radial propagation
model curve than in the Ulysses data. For this large
feature between 1998.34-1998.4, ACE and Ulysses
electron measurements both show the presence of
counterstreaming electrons indicating a CME. Gosling
[1996] wrote a review paper on signatures of CMEs in
the heliosphere and finds that the presence of
counterstreaming electrons is perhaps the most reliable
signature of a CME although no single signature
works all the time [Gosling, 1996; Neugebauer and
Goldstein, 1997].
Figure 1. ACE solar wind speed data at 1 AU as a thick line,
and model speeds at given propagation distances as thin lines
(top panel). Subsequent panels compare Ulysses data and
model results for the LARP method (velocity second panel,
density third panel, temperature the forth panel and the
polarity of the magnetic field last panel).
making assumptions about the solar wind properties at
locations away from our measurements. Gosling et al.
[1995; 1998] showed that a 1-D model predicts many
key features of CMEs, which are also found in more
complex 2-D models of CMEs [Riley et al., 1997].
When Ulysses and WIND were in radial alignment, De
Keyser et al. [2000] found that using a 1-D model they
could often identify sector boundaries at both
spacecraft, and Crooker et al. [2001] found that the
current sheet is coherent on a global scale, but has a
highly variable local structure. By comparing solar
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The RP method does not take into account the
longitudinal separation of the two spacecraft, which
varies between 0° and 360° each year, as the Earth and
L1 revolve around the Sun. To compensate for this
effect (at least for co-rotating structures), we have "derotated" one set of observations with respect to the
other. We will refer to the combination of longitude
adjustment and radial propagation as (LARP). The
propagation time for the LARP method can be thought
of as the time required for a spiral at ACE to propagate
to Ulysses. To examine Ulysses observations at higher
latitudes, the LARP method may need to be refined to
include the latitude dependence of the spiral winding.
The second panel in Figure 1 overlays the speed
profiles using the LARP method. This was
accomplished by subtracting the inertial heliospheric
longitudes for the two spacecraft and converting this
difference into an effective time shift using the solar
rotation rate. To minimize the effects of temporal
evolution of the solar source, the data are rotated the
shortest way around (forward or backward in time) so
that comparisons are always made between data that is
separated by less than two weeks. Using the LARP
method causes the large feature in the Ulysses data at
1998.34 to align with the double peaked feature in the
mapped data.
observe precisely the same parcels of plasma. ACE
and Ulysses show similar sector structure throughout
this entire interval. From 1998.31 to 1998.42, the
mean mapped density and density variations agree
well with the data. The mapped mean temperature and
temperature profile agrees well with the measured
temperature over a longer time period from 1998.31 to
1998.5, although from 1998.42 to 1998.5 the
difference between the model and data average
temperature is greater than earlier. It appears that the
model mapped density and temperatures do not agree
as well with the data as the velocity and sector do
because the temperature and density are highly
variable parameters.
In order to compare the average variations in the
mapped density and temperature with the Ulysses
observations, we calculated radial profiles. In Figure 2
the model results from February 5, 1998 through
December 31, 1999 are binned in radial distance and
averaged over time. The ACE and Ulysses
measurements are analyzed similarly. The radial
variation of density and temperature observed in the
Ulysses data is consistent with the density being
proportional to R-2 and temperature proportional to R-1
as predicted by the Zeus model.
It appears that the series of peaks observed at ACE
either do not merge properly in the model, or parts of a
CME could evolve differently as some CME model
results show [Riley et al,.1997]. The LARP method
works particularly well for the CME at 1998.34. In
total we have examined 15 counterstreaming intervals
from February 1, 1998 to October 31, 1998, and find
that the LARP method improves the sector alignment
for 10 of those intervals. We plan to investigate more
CMEs to determine if the LARP method is generally a
better method to use for comparing the ACE Ulysses
data than the RP method. If a CME stays magnetically
connected to the Sun for a long time then it becomes
aligned along a spiral as depicted by McComas et al.
[1992] such that the westward flank becomes
elongated more than the eastward flank. Also, the
spiral angle of a CME will depend on the speed of the
CME.
CONCLUSIONS
We find that the mapped ACE data agrees well
with average plasma properties at Ulysses when the
latitude and longitude separations of the spacecraft are
small. In addition, we find that adjusting the source
longitude clearly improved the mapping of the largescale structure. Thus, at small latitude separations, the
sector and plasma profiles can be adjusted for source
longitude and mapped from 1 to 5.4 AU such that
good agreement is reached between these mapped
plasma profiles and the Ulysses observations.
We plan to extend our analysis to examine the
latitudinal structure of these solar wind structures by
comparing the mapped ecliptic observations with
Ulysses observations at times when Ulysses is out of
the ecliptic plane. The ability to compare such
observations will elucidate the large-scale solar wind
structures. We will test our ability to extend this work
to larger latitude separations (which occurs during
solar maximum) by comparing ACE and Ulysses
measurements with coronal hole maps [Elliott et al.,
2003]. Such comparisons will help determine when
ACE and Ulysses are observing the same or different
coronal holes. Our techniques can be applied to learn
more about CMEs as well. For example, our two
methods of propagation will allow us to probe the
In the lower panels of Figure 1 we compare
Ulysses density (third panel), temperature (forth
panel), and magnetic polarity (bottom panel)
measurements to the LARP ACE data. The LARP
ACE speeds, average density, and temperature agree
fairly well with the Ulysses data; however, the high
frequency fluctuations in density and temperature do
not. We would not expect high frequency fluctuations
to agree well because while the two spacecraft observe
the same large-scale solar wind structures, they never
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shape of CMEs, and will provide information about
how long a CME stays magnetically connected to the
Sun.
Ulysses magnetic field observations 1990-1994., J.
Geophys. Res., 101, 395, (1996).
Gosling, J. T., et al., A CME-driven solar wind disturbance
observed at both low and high heliographic latitudes
Geophys. Res. Lett., 22, 1753, (1995).
Gosling, J. T., Corotating and transient solar wind flows in
three dimensions, Annu. Rev. Astron. Astrophys, 34, 3573, (1996).
Gosling, J. T., P. Riley, D. J. McComas, and V. J. Pizzo,
Overexpanding coronal mass ejections at high
heliographic latitudes: Observations and simulations, J.
Geophys. Res., 103, 1941, (1998).
McComas, D. J., J. T. Gosling, and J. L. Phillips,
Interplanetary Magnetic Flux: Measurement and Balance
J. Geophys. Res., 97, 171-177, (1992).
McComas, D. J., et al. Solar wind electron proton alpha
monitor (SWEPAM) for the Advanced Composition
Expolorer, Space Sci. Rev., 86, 563, (1998).
Figure 2. Density and temperature data divided into radial
bins and averaged. Simulation results are shown in grey,
ACE and Ulysses data are shown in black.
McComas, D. J., et al., Solar wind observations over
Ulysses' first full polar orbit, J. Geophys. Res., 105,
10419, (2000).
ACKNOWLEDGMENTS
McComas, D. J., et al., Complexity near solar maximum and
the reformation of polar coronal holes, Geophys. Res.
Lett., 29(9), 10.1029/2001GL014164, (2002).
We would like to thank N. Ness, C. Smith, and A.
Balogh for use of the ACE and Ulysses magnetometer
data. Our work is supported by NASA's ACE and
Ulysses programs as a part of the SWEPAM and
SWOOPS data analysis efforts. Pete Riley greatfuly
acknowledges the support of NASA (SEC-GI
program).
Neugebauer, M., and R. Goldstein, Coronal Mass Ejections
Geophysical Monograph 99, edited by N. Crooker, et al.,
Washington D. C., AGU, pp. 245, (1997).
Riley, P., J. T. Gosling, and V. J. Pizzo, A two-dimensional
simulation of the radial and latitudinal evolution of a
solar wind disturbance driven by fast, high-pressure
CME ,J. Geophys. Res., 102, 14,677, (1997).
Riley, P., J. T. Gosling, V. J. Pizzo, Investigation of the
polytropic relationship between density and temperature
within interplanetary coronal mass ejections using
numerical simulations, J. Geophys. Res., 106, 8291,
(2001.)
REFERENCES
Balogh A., et al., The magnetic field investigation on the
Ulysses Mission: Instrumentation and preliminary
scientific results, Astron. and Astrophys., Suppl. Ser., 92,
221, (1992.)
Smith, C. W., et al., The ACE magnetic fields experiment
Space Science Reviews, 86, 613, (1998).
Bame S. J. et al., The Ulysses solar wind plasma experiment,
Astron. and Astrophys., Suppl. Ser., 92, 237, (1992).
De Keyser, J., M. Roth, R. Forsyth, and D. Reisenfeld,
Ulysses observations of sector boundaries at aphelion, J.
Geophys. Res., 105, 15,689, (2000).
Stone, J. M., and M. L. Norman, ZEUS-2-D: A radiation
magnetohydrodyanmics code for astrophysical flows in
two dimensions, I, The hydrodynamic algorithms, and
tests, Astrophys. J., 80, 753. (1992).
Elliott, H. A., D. J. McComas, and P. Riley, Latitudinal
extent of large-scale structures in the solar wind, Ann.
Geophys., “Ulysses and Beyond”, accepted (2003).
Richardson, J. D., and K. I. Paularena, Plasma and magnetic
field correlations in the solar wind, J. Geophys. Res.,
106, 239, (2001).
Forsyth, R. J., A. Balogh, E. J. Smith, G. Erdös, and D. J.
McComas, The underlying Parker spiral structure in the
Crooker et al., Scales of heliospheric current sheet coherence
between 1 and 5 AU, J. Geophys. Res. 106, 15963,
(2001).
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