The Transition of Interplanetary Shocks through the
Magnetosheath
A. Szabo∗ , C. W. Smith† and R. M. Skoug∗∗
∗
Laboratory for Extraterrestrial Physics, NASA Goddard Space Flight Center, Greenbelt, MD 20771
†
Bartol Research Institute, University of Delaware, Newark, DE 19716
∗∗
Los Alamos National Laboratory, Los Alamos, NM 87545
Abstract. WIND, ACE, IMP 8 and Geotail data shows that the magnetosheath signature of IP shocks is
primarily a fast-mode shock or pressure pulse with a wide ramp. No model predicted secondary discontinuities
could be identified above the magnetosheath background fluctuation level. Even though the interplanetary
surface geometry of IP shocks could be significantly corrugated, this study suggests that there is no significant
deceleration of the pressure front in the magnetosheath.
INTRODUCTION
of the magnetic field on the interaction of the IP
shock and bow shock by using various MHD and
hybrid formulations. The one-dimensional model of
Whang [10] was very successful at describing outer
heliospheric observations of IP shocks. It allowed
the merger of two IP shocks if they are both forward or reverse, and predicted the transmission of
the two interacting shocks if one is forward and
the other reverse with a tangential discontinuity
(TD) forming between them. This model, however,
is limited to the treatment of perpendicular shocks.
Cargill [11] relaxed this requirement with the use
of a one-dimensional hybrid code. He showed that
while the collision between two perpendicular collisionless shocks gives rise to a TD located between the two transmitted shocks, as predicted by
one-dimensional MHD theory, the collision between
two oblique shocks produces a much more extensive and turbulent region between the two transmitted shocks, possibly a contact discontinuity (CD).
The CD, like the TD, shows jumps in the plasma
and magnetic field components and therefore should
be identifiable in observational data. Moreover the
transmitted shock is deflected from its original orientation that should also be observable. Aside for
Zhuang et al. [12] reporting a case of ISEE 1 and
3 measurements where such a sequence of disturbances was possibly observed in the Earth’s magnetosheath, observational evidence in the literature
remains rather limited and is one of the topics of
this paper.
Next the data sets and analytical techniques used
in this study is discussed followed by the results
A strong correlation of interplanetary (IP) shocks
impinging on the magnetosphere and geomagnetic
disturbances have been reported by many observers
[1, 2, 3]. IP shocks, as all solar wind pressure events,
tend to disrupt the magnetopause surface leading
to magnetopause transient events [4] that, in turn,
can initiate magnetic reconnection resulting in substorm onset [5]. IP shocks have also been connected
to sudden commencements and auroral brightening
[6]. However, before IP shocks reach the magnetopause, they have to cross the Earth’s bow shock
and traverse the magnetosheath introducing both
geometrical and physical modifications. Following IP
shocks through the magnetosheath with in-situ observations is the topic of this paper.
The interaction of IP shocks with the bow shock
and its transmission through the magnetosheath
to the magnetopause, has been studied mainly by
gas dynamic modeling [7, 8, 9]. These models, by
construction, allow the generation and propagation
of only one type of wave in the magnetosheath.
Therefore, it is not surprising that they find only
a single, fast mode pressure pulse (or fast shock
in the supersonic flanks) propagating through the
magnetosheath. Interestingly the predicted disturbance geometry or shape in the magnetosheath remains nearly planar (the shape of the undisturbed
IP shock) propagating with the same speed as the
undisturbed IP shock [9]. This prediction is investigated in detail in this paper.
Attempts have been made to include the effect
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
782
of the interplanetary and magnetosheath shape and
signatures of shocks. Finally, a short summary will
be presented.
DATA AND ANALYSIS
TECHNIQUES
During 1998–1999, twelve IP shocks have been identified that were observed by at least 2 solar wind
monitors and by another spacecraft in the magnetosheath. This interval was selected because IP
shocks were not very frequent and clearly observable
(unlike the later solar maximum years), and there
was nearly continuous coverage by WIND, ACE,
IMP 8 and Geotail. Moreover, the relatively large
separation between consequent IP shocks assured
that there was only minimal possibility of interaction between them before reaching 1 AU keeping
their surface geometry the simplest possible. In order
to establish the undisturbed surface geometry of the
incoming IP shocks the observed solar wind plasma
and magnetic field data before and after the shocks
were fitted by the non-linear least squares ”RankineHugoniot” technique originally developed by Vinas
and Scudder [13] and further enhanced by Szabo [14].
This fitting technique provides the best possible local shock normal direction and speed determination
by a single spacecraft with the associated uncertainties. Data from the WIND Magnetic Field Investigation (MFI) [15], and Solar Wind Experiment (SWE)
[16], IMP 8 magnetic field and plasma experiments
[17], and ACE magnetic field experiment (MAG) [18]
and solar wind plasma instrument (SWEPAM) [19]
were used for the analysis. Preliminary use was also
made of the Geotail magnetic field (MGF) [20] and
solar wind (CPI) [21] data.
FIGURE 1. Angular deviation between shock normal
directions for the same event as observed by WIND and
ACE (solid circles) and WIND and IMP 8 (open circles).
Crosses mark those shocks that were clearly driven by
magnetic clouds.
served by at least two solar wind monitors during
the rising phase of the previous solar cycle (1997–
1999)have been fitted and the shock normal directions compared. Figure 1 shows the angular deviation between the corresponding shock normal directions as a function of the inter-spacecraft separation perpendicular to the Sun-Earth line. Solid
circles mark shocks observed by WIND and ACE,
while open circles refer to WIND and IMP 8 observations (note the significantly larger associated uncertainties). Considerable deviation from planarity is
apparent. Also the degree of curvature does not appear to be a simple function of the inter-spacecraft
separation. In addition, those IP shocks that were
clearly driven by magnetic clouds are marked by a
solid cross indicating that some clouds drive shocks
that are very nearly planar, on the other hand, some
cloud driven shocks can be as corrugated as some of
those for which the drivers are uncertain. This result complicates the analysis of the magnetosheath
propagation of IP shocks as clearly planarity cannot
be assumed (unlike for computer simulations) and
some allowance for local curvature will have to be
made.
IP SHOCK GEOMETRY IN
INTERPLANETARY SPACE
Generally it is assumed that incoming IP shocks are
planar on the scale size of the magnetosphere. Indeed, in a recent study Russell et al. [23] analyzed
a single IP shock with four solar wind satellites and
found that three of them were consistent with the
planarity assumption. However, deviation from planarity has been reported before [24, 25]. Specifically,
Szabo et al. [25] have found that IP shocks driven
by small magnetic clouds have a highly corrugated
surface geometry on the scale-size of the magnetosphere. To further illustrate that significant deviations from planarity is possible, 17 IP shocks ob-
IP SHOCKS IN THE
MAGNETOSHEATH
During the time period of 1998–1999 twelve IP
shocks have been identified that were observed by
at least 2 solar wind monitors (WIND and ACE)
to place some limit on the interplanetary curva-
783
FIGURE 3. Difference in the predicted and observed
arrival times in the magnetosheath as a function of the
spacecraft separation. See the text for details.
eral magnetosheath background fluctuations could
easily mask small variations. However, it does point
out that for magnetospheric energy and momentum
input the leading fast-mode pressure jump or shock
is the most significant.
In order to make some assessment of the magnetosheath geometry of the transmitted disturbance,
the IP shock surface normal directions and speeds,
fitted in the solar wind data, were used to estimate
a predicted arrival time at the magnetosheath monitor. This predicted arrival time was compared to the
actually observed time delay. The thus obtained difference is plotted in Figure 3. Positive difference time
refer to the actual magnetosheath observation being
later than predicted based on the upstream shock fit
results. This would be the expected case if the pressure front is decelerated in the sheath as is the case
for TDs. Negative difference times correspond to earlier than expected arrival times. This could be due
to an unlikely acceleration of the front or more likely
to the intrinsic curvature of the IP shock. All time
differences are calculated with respect to the beginning of the sheath pressure pulse. The length of the
pressure ramp is indicated by the gray bars. A time
difference within the gray bars would be consistent
with an unaltered shock disturbance front. The same
procedure is repeated for both solar wind monitors.
The results corresponding to the solar wind monitor closest to the sheath monitor perpendicular to
the Sun-Earth line is plotted as a solid circle (the
other result is plotted as a cross) as a function of
this cross-wind separation. Just as in the case of the
undisturbed solar wind IP shocks (Figure 1), there
is no clear dependence on the spacecraft separation.
The same data is plotted in a similar format in
Figure 4 as a function of the distance of the sheath
monitor behind the bow shock. The location of the
bow shock was estimated using the Peredo model
FIGURE 2. WIND, ACE and IMP 8 magnetic field
magnitude and GSE Cartesian and spherical components
on May 29, 1998. The WIND and ACE data is time
shifted to line up the IP shock marked by the dashed
line.
ture of the incoming shock, while IMP 8 provided
magnetosheath observations. Some tentative Geotail sheath events were also identified. For some IP
shocks a corresponding pressure pulse is clearly identifiable in the sheath observations. Figure 2 shows
90 minutes of observations of the magnetic field and
its components by WIND, ACE and IMP 8 on May
29, 1998. The WIND and ACE data has been time
shifted by 29 and 37 minutes, respectively to line
up the shock observations with the sheath pressure
pulse event (the higher overall field values correspond to the IMP 8 compressed sheath measurements). The sheath pressure pulse is very clear in
both magnetic field and plasma observations. Also
it should be noted that the nearby large field rotation corresponding to a TD has a markedly different advection time delay. This is consistent with the
pressure pulse corresponding to the IP shock that
travels faster than the strictly advecting TD. Such
a clear sheath signature was not always apparent.
Weaker and reverse shocks had significantly broader
ramps, some reaching over 10 minutes. On the other
hand, even for the clearest cases no other discontinuity nearby could be identified. This does not
prove that the secondary discontinuities, predicted
by MHD and hybrid codes, do not exist as the gen-
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ACKNOWLEDGMENTS
The authors are grateful to A. J. Lazarus, J. D.
Richardson and the MIT IMP 8 and WIND team
for providing plasma data, to S. Kokubun and L.
A. Frank for the use of the Geotail key parameter
magnetic field and plasma data.
REFERENCES
1.
FIGURE 4. Difference in the predicted and observed
arrival times in the magnetosheath as a function of the
distance of the sheath monitor behind the bow shock.
See the text for details.
2.
3.
[26] and the measured solar wind conditions. Clearly,
better determination could be made of the sheath
position of a spacecraft, but this plot serves to show
that some of the better timing agreements happened
for cases where the sheath monitor was deep inside
the sheath, hence any systematic deceleration of the
pressure front would be most noticeable.
4.
5.
6.
7.
8.
9.
SUMMARY
While the data set presented is very limited and the
question of the transition of IP shocks through the
magnetosheath complicated, at least a few preliminary assessments could be made. A clear fast-mode
shock or pressure pulse with a wider ramp could be
identified for most solar wind observed IP shocks.
However, the model predicted secondary discontinuities were not apparent in the highly fluctuating
sheath background indicating that for energetics the
leading pressure pulse is the most relevant. Even
though the intrinsic curvature or corrugation of IP
shocks complicates the determination of the geometrical effects of the magnetosheath on the transmitted
shocks, the data presented suggests that there is no
systematic deceleration of the pressure front. That
is, the uncertainty of the arrival time of an IP shock
to the magnetopause based on upstream solar wind
observations (a value that could be near 10 minutes)
is not due to the effects of the magnetosheath but
most likely to the unknown interplanetary geometry
of the shock surface fronts.
10.
11.
12.
13.
14.
15.
16.
17.
18.
19.
20.
21.
23.
24.
25.
26.
785
Tsurutani, B.T., et al., J. Geophys. Res. 100,
21,717(1995).
Gonzales, W.D., Tsurutani, B.T., and Gonzales,
C.de, Space Sci. Rev. 88, 529–562 (1999).
Gonzales, W.D., and Tsurutani, B.T., ”The
interplanetary causes of magnetic storms: A
Review,” in Magnetic Storms, AGU Monograph 98,
Washington, D.C., 1998.
Sibeck, D.G., and Newell, P.T., J. Geophys. Res.
100, 21,773 (1995).
Wu, C.C., J. Geophys. Res. 105, 7533 (2000).
Tsurutani, B.T., et al., J. Atmos Sol.-Terr. Phys.
63, 513 (2001).
Shen, W.W., and M. Dryer, J. Geophys. Res. 77,
4627 (1972).
Dryer, M., Radio Sci. 8, 893 (1973).
Spriter, J.R., and Stahara, S.S., ”Computer
modeling of solar wind interaction with Venus and
Mars,” in The Comperative Study of Venus and
Mars: Atmospheres, Ionospheres and Solar Wind
Interactions, AGU Monograph 66, Washington,
D.C., 1992, pp. 345–383.
Whang, Y.C., Space Sci. Rev. 57, 339 (1991).
Cargill, P.J., J. Geophys. Res. 95, 20,731 (1990).
Zhuang, H.C., Russell, C.T., Smith, E.J., and
Gosling, J.T., J. Geophys. Res. 86, 5590 (1981).
Vinas, A.F., and Scudder, J.D., J. Geophys. Res.
91, 39 (1986).
Szabo, A., J. Geophys. Res. 99, 14,737 (1994).
Lepping, R.P., et al., Space Sci. Rev. 71, 207 (1995).
Ogilvie, K.W., et al., Space Sci. Rev. 71, 55 (1995).
Lepping, R.P., et al., NASA/GSFC LEP int. doc.,
(1992).
Smith, C.W., et al., Space Sci. Rev. 86, 613 (1998).
McComas, D.J., et al., Space Sci. Rev. 86, 563
(1998).
Kokubun, S., et al., J. Geomag. Geoelectr. 46, 7
(1994).
Frank, L.A., et al., SES-TD-92-007SY, (1992).
Russell, C.T., et al., J. Geophys. Res. 105, 25,143
(2000).
Russell, C.T., et al., J. Geophys. Res. 88, 9941
(1983).
Szabo, A., et al., ”The evolution of interplanetary
shocks driven by magnetic clouds”, in Proceedings
of ”Solar Encounter: The First Solar Orbiter
Workshop”, ESA SP-493, The Netherlands, 2001,
pp. 383–387.
Peredo, M., Slavin, J.A., Mazur, E., Curtis, S.A.,
J, Geophys. Res. 100, 7907 (1995).