The Solar Wind in the Outer Heliosphere at Solar
Maximum
John D. Richardson and Chi Wang
Center for Space Research, Massachusetts Institute of Technology, Cambridge, MA 02139
Abstract. This paper reviews solar wind observations in the outer heliosphere, concentrating on the
recent data near solar maximum. The speed and temperature tend to be lower at solar maximum, due to
the lack of coronal holes. The near-absence of a latitudinal speed gradient at solar maximum allows us to
measure the speed decrease of the solar wind and nd a value for the H density in the local interstellar
medium (LISM) at the termination shock of 0.09 cm;3 . The temperature prole is well-matched by a model
using pickup ion heating and a speed dependence of the temperature. The density prole at solar maximum
is dominated by MIRs we show one case where converging CME ejecta form a MIR.
INTRODUCTION
In June 2002 Voyager 2 was 68 AU from the Sun
and at 24 S heliolatitude. The Voyager 2 trajectory
is shown in Figure 1, which illustrates the radial distance of Voyager 2 and its heliolatitude. The state of
the heliosphere changes dramatically with the solar
cycle. At solar minimum the slow (400 km/s) solar
wind is conned to a thin strip of half-width 10-20
near the heliographic equator, but at solar maximum
the slow wind lls almost the entire heliosphere. In
this paper we describe plasma conditions in the outer
heliosphere in the context of their solar cycle variation, use the unique conditions at solar maximum to
determine the interstellar neutral density from the
slowdown of the solar wind, model the temperature
prole using a combination of pickup ions and a dependence of temperature on speed, and show how the
merged interaction regions (MIRs) that are prevalent
at solar maximum can be formed by transient solar
events.
FIGURE 1. The Voyager 2 trajectory showing distance
from the SUN in AU (top) and heliolatitude (bottom)
lation of speed and temperature in the solar wind.
The density structure at solar maximum is increasingly dominated by MIRs as Voyager 2 moves farther
from the Sun. The solar wind dynamic pressure at
1 AU (IMP and Wind data) and at Voyager 2 are
shown in Figure 3. The dynamic pressure of the solar wind is what balances the pressure of the LISM
and thus determines the locations of the heliopause
and termination shock. In the past two solar cycles,
the pressure has been at a minimum near solar maximum, then risen sharply near the end of the solar
maximum period. This factor of two increase in pressure drives out the termination shock as of mid-2002
the increase in pressure has not been observed at 1
VOYAGER 2 DATA OVERVIEW
Figure 2 shows the solar wind speed, normalized proton density (NR2 ), proton temperature, and
sunspot number. The hashed regions show the three
solar maximums observed by Voyager 2 to date. The
speed during solar maximum tends to be lower than
at other times, due to the presence of only low-speed
solar wind. The temperature is also lower at solar
maximum, probably due to the well-known corre-
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
71
AU, after which it will take roughly a year to reach
the termination shock.
FIGURE 3. The solar wind dynamic pressure observed
at 1 AU by IMP 8 and WIND and from 1-65 AU by
Voyager 2. The bottom trace is the sunspot number
2002.2. The bottom panel shows the heliolatitude
of Earth, Ulysses, and Voyager 2 during this time
period. Earth (and IMP 8) are near the helioequator, Ulysses moves from 20 to 45 S heliolatitude,
and Voyager 2 is always at intermediate latitudes.
The speeds observed by IMP 8 and Ulysses track
each other very well and have nearly the same magnitude. The Voyager 2 speeds, however, are signicantly below those observed by the other spacecraft,
clear evidence of the slowing of the solar wind from
the incorporation of the pickup ions. The density
proles at IMP 8 and Ulysses are also very similar,
more evidence that the same solar wind is present
at low and medium latitudes. The Voyager 2 density
prole is dominated by a large interaction region. To
determine the magnitude of the solar wind speed decrease and the LISM H density at the termination
shock, a 1-D MHD model was used to propagate the
solar wind from IMP 8 to Ulysses and then from
Ulysses to Voyager 2 (2). Table 1 shows the average
observed solar wind speeds and densities at Earth,
Ulysses, and Voyager 2. The density of the termination shock was adjusted to nd the value which best
t the data, and that value was 0.09 cm;3 .
The model values based on propagating IMP 8
data out to Ulysses and Voyager 2 are also shown.
The model and observed speed and density
FIGURE 2. The solar wind speed, density normalized
to 1 AU, temperature and sunspot number. The shaded
regions show solar maxima
SOLAR WIND SLOWDOWN
The solar wind interacts with the neutral material from the LISM as it moves outward. These neutrals, mostly H, are ionized and then accelerated to
the solar wind speed, with the energy for this acceleration coming from a slowdown of the solar wind.
This slowdown depends of the density of the LISM
H at the termination shock. The problem with measuring this slowdown is that the solar wind speed
varies with time, latitude, and longitude. Generally one needs spacecraft at the same latitudes in the
inner and outer heliosphere to determine the speed
decrease, since latitudinal speed gradients are large
and systematic. At solar maximum, however, the
latitudinal speed gradient is small (1). Thus the solar wind speed decrease can be determined using any
two spacecraft at dierent radial distances.
Figure 4 shows 50-day running averages of the solar wind speed and normalized density from 1999 to
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Table 1: Solar Wind Parameters
ACE Uly (5 AU) V2 (60 AU)
data model data model data
V (km/s) 441 432 428 379 378
N (cm ;3 ) 7.0
7.3
7.2
6.1
6.6
are almost identical at Ulysses, giving credence to
our assumption that the solar wind is independent of
latitude. The model speed matches that observed at
Voyager 2, and the densities are also close. The solar
wind speed decrease is 53-62 km/s, the largest yet
observed. The LISM value of 0.09 cm;3 is consistent
with other derivations of this number (3).
TEMPERATURE PROFILE
The temperature of the thermal solar wind protons measured by Voyager 2 is shown in gure 5,
where we have plotted 50-day running averages. The
temperature decreases out to about 30 AU, then increases to 50 AU, then starts to decrease again from
50-65 AU. The decrease in temperature is much less
than an adiabatic model would predict (dashed line).
Smith et al. (4) describe a model which incorporates
stream interactions and pickup ion heating which
gives a reasonable t to the overall shape of the data
The large temperature structures with scales of a
FIGURE 5. The solar wind proton temperature ob-
served by Voyager 2 compared with several model predictions
Richardson (5) investigated whether a combination
of the Smith et al. model and a temperature dependence on speed could match the observations. The
upper dotted prole shows the result of this work.
Much of the ner structure is now matched by the
model curve the correlation coecient between the
model prediction and data is 0.86. In this case the
model curve is produced by adding (V - 440 km/s)*80
to the Smith et al. curve, where V is a 100-day running average of the speed. Although this model generally works very well it does not explain all the features of the temperature prole, such as the large
rise near 10 AU and the period near 50 AU which
corresponds to the 1996 solar minimum.
DEVELOPMENT OF A MIR
MIRs dominate the density structure near solar
maximum in the outer heliosphere. These regions
of enhanced magnetic eld and often plasma density
restrict the inward diusion of energetic particles,
resulting in step decreases in cosmic ray intensities.
Transient events at the Sun which pile up the plasma
and eld ahead of them have been suggested as the
drivers of these MIRs. In general it is dicult to test
this hypothesis since signatures of transient events
such as CMEs dissipate by the time they reach the
FIGURE 4. The solar wind speed, normalized density
observed at IMP 8, Ulysses, and Voyager 2 and the heliolatitudes of these spacecraft
few AU are not reproduced by the model. The speed
and temperature of the solar wind are correlated, and
73
CONCLUSION
outer heliosphere. However, large alpha particle ratios were used to track one CME from Ulysses to
Voyager 2 (6). We found another case where a CME
could be tracked from Earth to Ulysses to Voyager 2
these two cases happened to bracket a MIR observed
at Voyager 2 (7).
The Voyager mission continues to explore the heliosphere at a pace which allows study of solar cycle
changes. The combination of Voyager, Ulysses, and
Earth-orbiting spacecraft allows us to look dierentiate between solar cycle and spatial changes, giving
information on the global solar wind outow and in
this case allowing the best determination yet of the
eect of pickup ions on the solar wind, in terms of
heating and slowing the ow. As we enter the declining phase of the solar cycle we expect to see how the
stream structure observed last solar cycle at 35 AU
has evolved, providing new science for SW11.
ACKNOWLEDGMENTS
This work was supported under NASA grant
NAG5-11623 and NASA contract 959203 from the
Jet Propulsion Laboratory to the Massachusetts Institute of Technology.
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FIGURE 6. From top to bottom, Ulysses density data
(which was used as input to the 1-D MHD model), model
density proles at 10, 20, 30, 40, 50 AU and 58 AU, and
the Voyager 2 data at 58 AU. The model proles are timeshifted to roughly align the density decreases associated
with the CME ejecta. Tick marks are every 10 cm;3 . The
vertical dotted lines show the enhanced alpha abundance
regions at Ulysses and Voyager 2.
Figure 6 shows the evolution of the solar wind
from Ulysses at 5 AU to Voyager 2 at 58 AU. A 1-D
MHD model was used to propagate the Ulysses data
outward. The locations of the CME ejecta are shown
by the two vertical dashed lines. The CMEs were initially about 60 days apart. As they propagate outward they converge, piling up plasma between them,
and are about 30 days apart at Voyager 2. The bottom trace shows the measured Voyager 2 density it
is very similar to the model prediction in the trace
above. The density in the MIR is about a factor of 2
above the surrounding solar wind density, consistent
with the factor of 2 decrease in the spacing of the
CME ejecta.
74