The interaction and evolution of interplanetary shocks
from 1 to beyond 60 AU
Chi Wang
1
2
and John D. Richardson
1 Center for Space Science and Applied Research, Chinese Academy of Sciences, Beijing, China
2 Center for Space Research, Massachusetts Institute of Technology, Cambridge, MA 02139, USA
During the current solar maximum (Cycle 23), several major CMEs associated with solar
ares produced large transient ows and shocks which were observed by widely-separated spacecraft such
as Wind at Earth and Voyager 2 beyond 60 AU. Using data from these spacecraft and numerical models,
we study shock propagation and interaction in the outer heliosphere. We demonstrate that a strong shock
in the distant heliosphere could be an outer heliospheric remnant of a strong shock in the inner heliosphere
(\one to one" relationship), or it could be an outcome of the successive interaction and merging of a series
of interplanetary shocks (\one to many" relationship).
Abstract.
INTRODUCTION
With the accumulation of Voyager observations
in the outer heliosphere, we now have the opportunity to study the evolution of a transient ow sysWhile Voyagers continue to make solar wind meatem (2) and its shocks from the inner to distant hesurements in the outer heliosphere (Voyager 2 is loliosphere where pickup ions play an important role
cated at 67 AU as of June, 2002), several Earthin the ow dynamics and shocks (3-5). Especially
orbited spacecraft such as ACE, Wind are monitorduring the current solar maximum (Cycle 23), Voying solar wind conditions at 1 AU. With the developager 2 observed serval relatively strong shocks bement of both observational and theoretical studies,
yond 60 AU, including the well-known Bastille Day
our insight into the dynamical processes in the solar
2000 CME-driven shock and the October 2001 shock,
wind throughout the heliosphere has been much imthe strongest one have been recorded in the outer
proved. Shocks are an important component of the
heliosphere since 1991. In order to nd their inner
solar wind structures. Generally speaking, there are
heliospheric origins, we take advantage of the Wind
two classes of shocks observed in the solar wind: coobservations at 1 AU and use numerical models to
rotating shocks and transient shocks. Co-rotating
propagate the solar wind structures from 1 AU to the
shocks result from the interaction of the fast and
location of Voyager 2. We follow the evolution and
slow solar wind streams as a consequence of the sointeraction of shocks until they pass Voyager 2, and
lar rotation and the tilt of the solar dipole. They are
compare the model predictions with Voyager 2 obsermore likely to occur near solar minimum when the
vations. The solar wind in the outer heliosphere is
solar wind has a relatively simple conguration with
fundamentally dierent from that in the inner heliohigh-speed streams at high latitudes and slow-speed
sphere, with the inuences from the local interstellar
streams at low latitudes. On the other hand, tranmedium becoming profound. In this study, we emsient shocks are in general produced near the Sun
ploy an one-dimensional multi-uid MHD model(6),
by fast ejecta from violent events on the Sun. They
which assumes a spherically symmetry of the helioare most frequent near solar maximum when the Sun
sphere and takes into account the interaction of the
is most active. The observations made by a eet of
solar wind protons with the interstellar neutral hyspacecraft during past three decades have inspired
drogen. We follow the approach of Isenberg (7) to
enormous interests in studying the evolution and inassume the solar wind consists of there co-moving
teraction of shocks (see the review by Whang(1)).
there particle populations: protons, pickup ions and
These studies, however, are limited to the distance
electrons. The solar wind protons are coupled with
within 10 AU or do not appreciate the importance of
the neutral hydrogen via charge exchange. We also
pickup ions in the distant heliosphere (beyond 30
International
Solar Wind
Conference,
CP679, Solar Wind Ten: Proceedings of the Tenthallow
the energy
transfer
between the solar wind proAU).
edited by M. Velli, R. Bruno, and F. Malara
© 2003 American Institute of Physics 0-7354-0148-9/03/$20.00
725
on July 15 with a speed jump from 600 to over
1050 km s 1 and a few small streams. The propagation and evolution of the Bastille Day CMEdriven shock in the outer heliosphere and their interaction with the heliospheric boundaries have been
studied by serval authors with dierent approaches
(8-11). Figure 1 shows the speed proles observed
by Wind at 1 AU (top panel) and predicted by
the model with the Wind data as input at dierent distances from 10 AU to Voyager 2. The in-
V (km/s)
1000
WIND
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600
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650
V (km/s)
600
10 AU
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600
V (km/s)
550
20 AU
500
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600
V (km/s)
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30 AU
500
450
4
400
350
600
40 AU
500
Compression Ratio
V (km/s)
550
450
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600
V (km/s)
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50 AU
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3
2
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V (km/s)
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Voyager 2
. . . . Observation
500
1
700
450
400
350
250
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DOY (Day 1 = January 1 of 2000)
350
400
1. Speed proles observed by Wind (top
panel) and predicted by the model with the Wind data
as input at various radial distance. The comparison of
the model result with Voyager 2 ( 63 AU) is shown in the
bottom panel. Note that the strong shock observed by
Voyager 2 is the outer heliospheric remanet of the strong
shock in the inner heliosphere.
FIGURE
Shock Speed (km/s)
200
600
500
400
10
20
30
40
Distance (AU)
50
60
(a)The shock strength (represented by the
density compression ratio) and (b) the shock propagation
speed decay with distance of the leading forward shock.
FIGURE 2.
tons and pickup ions. The energy partition ratio (6)
is taken be 0.05, which means about 5% of the total energy from the pickup process goes to the solar
wind wind protons, in order to reproduce the temperature prole observed by Voyager 2 in the outer heliosphere. Furthermore we use the hydrodynamical
approach to calculate the distribution of the neutral
hydrogen in an self-consistent manner. The interstellar neutral hydrogen density is chosen as 0.09 cm 3
at the termination shock to match the slowdown of
the solar wind.
ONE TO ONE RELATIONSHIP
A large solar event took place on the Sun on July
14 (Bastille Day), 2000. Many aspects of this storm
event can be found in the December 2001 topical
issue of Solar Physics. The passage of the ejecta
at Earth produced a very large high-speed stream
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teraction and evolution of the dominant large highspeed stream and serval small streams observed by
Wind at Earth has evolved into a well-dened strong
shock in the front by 10 AU, followed by a complicated solar wind structure. As they continue to
propagate into the heliosphere, the speed prole becomes a relatively simple \jump-ramp" structure.
The leading forward shock (we called it the \Bastille
Day shock") decays signicantly with distance, while
other shocks/discontinuties almost disappear at Voyager 2. Figure 2 plots the shock strength (indicated
by the density compression ratio) and propagation
speed as functions of distance.
The compression ratio decreases steady from 4 in
the inner heliosphere to 1.8 at Voyager 2, and the
shock propagation speed decreases from above 650
to 460 km s 1 at Voyager 2 (63 AU). The model
predicted the Bastille Day shock would arrive at Voy-
800
V (km/s)
700
WIND
600
500
400
V (km/s)
300
700
10 AU
600
500
400
V (km/s)
600
20 AU
550
500
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V (km/s)
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30 AU
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V (km/s)
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40 AU
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V (km/s)
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V (km/s)
ager 2 on January 9, 2001 with a speed jump of 70 km s 1 . Within a few days of the predicted date,
Voyager 2 saw a relatively strong shock on January
12, 2001 with a speed jump of 65 km s 1 . The
thermal pressure increases across the shock by a factor of 2.5.
The bottom panel in gure 1 shows the comparison of the model results (dotted line) with the Voyager 2 observations. The timing and speed prole are
in reasonably good agreement with the observations.
Therefore, we conclude the strong shock observed by
Voyager 2 at 63 AU on January 13, 2001 is naturally the outer heliospheric remanet of the strong
Bastille Day shock in the inner heliosphere. Not surprisingly, there exists an one to one relationship between an strong shock in the outer heliosphere and
a strong shock in the inner heliosphere driven by a
big solar event. However, not all strong shocks in
the outer heliosphere can nd their counterparts in
the inner heliosphere. There exists another type of
relationship which we will discuss in the following
section.
450
Voyager 2
. . . . Observation
400
350
300
100
ONE TO MANY RELATIONSHIP
150
200
250
DOY (Day 1 = January 1 of 2001)
300
350
3. Speed proles observed by Wind (top
panel) and predicted by the model with the Wind data
as input at various radial distance. The comparison of
the model result with Voyager 2 ( 65 AU) is shown in the
bottom panel. Note that the strong shock observed by
Voyager 2 is the outcome of the interaction and merging
of a series of interplanetary shocks.
FIGURE
On October 16, 2001, Voyager 2 recorded a strong
shock at 65 AU with a speed jump of 105 km s 1
across the shock, and the thermal pressure increases
by a factor of 5.9. In contrast to the Bastille Day
shock at Voyager 2 (with a speed jump of 65 km s 1
and a thermal pressure increases by a factor of 2.5),
it is a much stronger shock. As a matter of fact, it
is the strongest shock has been observed by Voyager
2 in the outer heliosphere since 1991. In an attempt
to identify its inner heliospheric source, initially we
search the Wind data for a big event similar to the
Bastille Day 2000 event, but failed to pinpoint a single solar source which could have been responsible.
Instead, we notice the active regions on the Sun produced a series of solar ares and CMEs in April, 2001.
The consequences of these solar events at Earth are
the group of high-speed streams observed by Wind
during this time period, each separated by only a
few days and last for almost one solar rotation. By
comparing the solar wind plasma measurements and
other aspects such as the geomagnetic impact and
Forbush decrease at Earth, none of these events by itself is as large as the Bastille Day 2000 event. Therefore we hypothesize that as a group they can coalesce
and evolve into a stronger shock in the outer heliosphere (12). As before, we insert the Wind data in
April into the inner boundary at 1 AU.
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Figure 3 shows the interaction and merging of
shocks in the same format as that in gure 1. By the
distance of 10 AU, the stream structures observed by
Wind has evolved into a big triangular speed structure with a leading forward shock. Most obviously,
the big triangular speed structure is then slitted into
a forward and reverse shock pairs. (In fact, numerous interplanetary shocks have developed from the
stream interactions and we will not follow the detail
here). The forward shock overtakes the leading shock
and forms a strong shock by 30 AU in the outer heliosphere. The reverse shock collides with the trailing
shocks and discontinuities and produces lots of small
scale solar wind structures. From 30 to 60 AU, the
leading forward shock decays slightly but not signicantly because the leading triangle structure continues to interact with the small trailing shocks . The
bottom panel of the gure 3 shows the comparison
of the model predictions with the Voyager 2 observations. Both the timing and overall character of the
propagated Wind speed prole match the Voyager 2
observations quite well. Hence we believe that the
strong shock observed by Voyager 2 in October 2001
is the result of the interaction and merging of a series
of the interplanetary shocks evolved from the highspeed streams observed by Wind at Earth in April
2001. These exists the one to many relationship between a strong shock in the outer heliosphere and a
series of interplanetary shocks as results of multiple
solar events.
DISCUSSION AND SUMMARY
During the current solar maximum (Cycle 23),
several major CMEs associated with solar ares produced large transient shocks which were observed by
widely-separated spacecraft such as Wind at Earth
and Voyager 2 beyond 60 AU. Using data from
these spacecraft and numerical models which include the interaction between solar wind protons,
pickup ions and interstellar neutrals, we study shock
propagation and interaction in the outer heliosphere.
The model we used is a one-dimensional multi-uid
model. Since Wind and Voyager 2 are not generally
radially aligned, the radial projection of the speed
prole at Wind to Voyager 2 is impossible. However,
considering the latitudinal dependence of the solar
wind is small near solar maximum according to the
Ulysses observations (13), to rst approximation, we
only need to worry about the longitudinal separation between the two spacecraft near solar maximum
. For the the Bastille Day event case, Wind was, fortunately, in the similar longitude as Voyager 2. As
for the October shock case, considering the magnitude of the solar activity in April 2001, we believe
that the spatial extent of those events is likely large
enough to make our 1-D assumption feasible. Nevertheless, the model predictions at Voyager 2 are in
good agreements with observations.
A strong shock at Earth undergoes a dramatic
change while propagating outward. For example, the
Bastille Day 2000 CME shock had a speed jump of
over 400 km s 1 at Earth and was detected by Voyager at 63 AU with a speed jump of 65 km s 1 about
6 months later. However, a strong shock at Voyager
2 does not necessarily correspond to a strong shock
at Earth. On October 16, 2001, Voyager 2 at 65 AU
observed a strong shock with a speed jump over 100
km s 1 , the strongest shock recorded since 1991, we
could not nd a single solar event which is directly
linked to this shock. Instead, a series of solar events
in April 2001 is found to be responsible. The model
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results show that successive merging and interaction
of relatively small interplanetary shocks could form a
well-developed strong forward shock beyond 30 AU.
In a world, a strong shock in the distant could be
a outer heliospheric remnant of a strong shock in
the inner heliosphere such as the Bastille Day shock
(\one to one" relationship), or it could be a outcome
of the interaction and evolution of a series of interplanetary shocks such as the October shock(\one to
many" relationship). Our demonstration that large
shocks can and do form from the merging mechanism
may have important consequences for the formation
of merged interaction regions and the triggering of
the heliospheric radio emission.
ACKNOWLEDGMENTS
This work was supported under NASA contract
959203 from JPL to MIT and NASA grant NAG511623. C. Wang is grateful to the one-hundred talent
program of the Chinese Academy of Sciences.
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