PROBLEMS OF GEOCOSMOS

Proceedings of the 7th International Conference "Problems of Geocosmos" (St. Petersburg, Russia, 26-30 May 2008)
INTERACTION OF OBLIQUE INTERPLANETARY SHOCKS WITH THE
BOW SHOCK
A.A. Samsonov 1 , Z. Němeček 2 , J. ˇSafránková 2 , L. Pˇrech 2
1 Physical Faculty, St. Petersburg State University, 198504, Russia, e-mail:
samsonov@geo.phys.spbu.ru; 2 Charles University, Prague, Czech Republic
Abstract. This is first work where propagation of oblique interplanetary shocks from the solar wind
through the magnetosheath is investigated by a three-dimensional MHD simulation. The oblique
shocks are characterized by a normal diverged from the Sun-Earth line; they are usually driven by
corotating interaction regions. We simulate variations in the dusk (quasi-perpendicular) and dawn
(quasi-parallel) magnetosheath after propagation of an artificial shock and show a real event with
an oblique shock observed by Themis and Cluster spacecraft. We find that MHD variations in the
magnetosheath for the oblique interaction are generally similar to those for the direct interaction (i.e.
when the shock normal is along the Sun-Earth line) that has been studied previously. However values
of plasma and magnetic field parameters are different on the dawn and dusk sides, and consequently
the pressure pulse magnitudes affecting the opposite magnetopause flanks are different too.
1 Introduction
Interplanetary shock (IS) observed as a simultaneous steep increase of the solar wind dynamic pressure and
magnetic field magnitude compresses the Earth’s magnetosphere and results in sudden impulses in magnetic
field observations. Most strong ISs connected with coronal mass ejection (CME) occur during solar maximum.
Another type of ISs dominated near solar minimum is often observed as recurrent events with periodicity about
27 days. These shocks are explained by interaction of the fast stream emanating from a coronal hole with the
low-speed stream associated with the heliosperic current sheet. The fast stream compresses the slow solar wind,
creating a corotating interaction region (CIR) [see review Pizzo, 1985].
Using spacecraft observations of ISs near 1 AU, it was found that the shock normals of CME-generated
shocks have a scattered distribution (in a few tens of degrees) with a maximum at the Sun-Earth line [e.g. Chao
and Lepping, 1974]. The normals of CIR-generated shocks near 1 AU are assumed to lay in the heliospheric
equatorial plane, and a mean angle between the normals and the Sun-Earth line is about 45 degrees [ref. Pizzo,
1991]. Such oblique shocks strike the Earth’s magnetopause first on the dusk (more often) or dawn flank,
rather then near the subsolar point. In this work, we present results of a new MHD simulation to understand
difference of the magnetospheric impact of the oblique shocks from the impact of the direct shocks studied in
several previous works [e.g. Samsonov et al., 2006; Samsonov et al., 2007].
Interaction of a forward fast IS with the bow shock was theoretically studied by Grib and Pushkar (2006).
They obtained solutions of the Rankine-Hugoniot (R-H) relations on the both flanks and showed a spatial
density distribution in the magnetosheath after the interaction. Assuming that the IS normal has a duskward
component (the angle to the solar wind velocity is 40 degrees), the ratio of the mean densities from the dawn to
dusk flanks was found to be equal to 1.16. The authors assumed that this difference may explain a statistically
observed dawn-dusk assymmetry in the magnetosheath density profiles [Paularena et al., 2001; Němeček et al.,
2002], however the last is a controversial point because the ISs occur too rarely to make a real input in the
statistical profiles.
Pˇrech et al. (2008) studied an event when the oblique shock (with the angle between the shock normal and
the Sun-Earth line in the equatorial plane equal to about 41 o ) was observed by Themis in the magnetosheath and
by several spacecraft upstream of the bow shock. They found a deceleration of the shock in the magnetosheath
and generation of a new discontinuity characterized by an increase of the magnetic field and plasma density
and by a drop of temperature. These results agree with those obtained previously from the MHD simulations
in the case of an IS strictly aligned with the Sun-Earth line [Samsonov et al., 2006]. In particular, Samsonov
et al. (2006) predicted a discontinuity at the Sun-Earth line consisted from the combination of a forward slow
expansion wave, a contact discontinuity, and a reversed slow shock. The three discontinuities propagate from
the bow shock to the magnetopause with velocities nearly equal to the bulk flow speed, therefore they can not
be separated both in the 3-D simulations and in observations. The above-mentioned variations of the MHD
parameters observed by Pˇrech et al. (2008) are obtained as a combination of variations through the three
discontinuities. Similar discontinuities in the magnetosheath measurements from Interball and Geotail were
found by ˇSafránková et al. (2007) after the interaction between non-oblique ISs and the bow shock.
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