The bow shock is a shock wave formed at a distance of 3-4 Earth radii or so in front of the nose of the magnetopause by the encounter of the supersonic solar wind with the "obstacle" to its flow presented by the Earth's magnetic field. Passing through the shock, which ranges in thickness from roughly 100 km to 2 Earth radii, the solar wind is slowed, compressed, and heated. The region downstream of the bow shock, between the shock and the magnetopause, that is occupied by the shocked solar wind plasma is known as the magnetosheath.

The bow shock is so named by analogy to the wave formed by the bow of a ship as it passes through water. Unlike the shock wave that precedes an airplane flying at supersonic speeds, the bow shock is a stationary (i.e., non-propagating) shock. Moreover, it differs fundamentally from the familiar shocks in the Earth's atmosphere in that it is a "collisionless" shock. That is, the bow shock occurs in a medium--the solar wind--that is so tenuous that collisions among the charged particles that make up the solar wind plasma are exceedingly rare and have no significant influence on the formation of the shock and the dissipation of the solar wind's kinetic energy that occurs there. In this collisionless regime, wave-particle interactions take over the role played by particle collisions in a collisional shock.

Quasi-perpendicular and quasi-parallel shocks

Shocks are classified as "quasi-perpendicular" and "quasi-parallel" according to whether the angle (theta) between the interplanetary magnetic field (IMF) upstream of the shock and the shock normal is greater or less than 45 degrees. This angle changes with location at the curved shock front, so that the bow shock consists of both a quasi-perpendicular (theta > 45 degrees) and a quasi-parallel region (theta < 45 degrees). Moreover, because the orientation of the IMF constantly changes, the location of the quasi-perpendicular and quasi-parallel components of the shock varies as well.

The two kinds of shock are quite different in their structure and behavior. In the case of quasi-perpendicular shocks, the transition from upstream to downstream is stable and characterized by a steep rise in magnetic field strength known as the "ramp." In contrast, at the quasi-parallel portion of the bow shock, the transition from the upstream state to the downstream state occurs over a broad (1-2 Earth radii thick), turbulent region. Computer simulations of high-Mach-number quasi-parallel shocks suggest that such shocks are unsteady, continuously forming and re-forming in a cyclical manner as sharp transitions alternate with more extended ones.

The turbulent foreshock region

Some of the inflowing solar wind particles are reflected from the shock instead of being transmitted through it. These reflected particles--together with some magnetosheath particles that have "leaked" back across the shock--travel upstream along the interplanetary magnetic field lines and populate a region upstream of the shock and magnetically connected to it. This region is known as the foreshock. There are, in fact, two foreshock regions: one populated preferentially by reflected energetic electrons (the electron foreshock) and one populated preferentially by suprathermal ions (the ion foreshock). (This spatial distribution results from the velocity differences between the faster electrons and the slower ions.) The foreshock is characterized by extensive wave activity, which results from the interaction of the backstreaming particles with the inflowing solar wind. This interaction gives rise to instabilities, which in turn excite ultra-low-frequency (ULF) magnetohydrodynamic waves, ion acoustic waves, and electron plasma oscillations. (In addition to local generation upstream, some waves are also generated at the shock and then propagate upstream.) ULF waves generated upstream of the quasi-parallel shock have been shown, in numerical simulations, to be convected back to the shock front, where they contribute to its destabilization and re-formation. In addition, such waves appear to be an important source of waves and turbulence downstream, in the magnetosheath.