The Bow Shock

Galileo approached Venus from high latitude on the dawn flank, making its closest approach to the planet at 05:59:03 UT on February 10, 1990 [ Johnson et al., 1991]. Although the trajectory did not pass through the planetary ionosphere or the magnetotail, it did fortuitously skim the planetary bow shock at antisolar distances from 4.2 to 9.0 Venus radii. At Venus, the obstacle to the solar wind flow is its ionosphere whose characteristic dimension is 1 R 1 R (R radius of Venus; R radius of Earth), whereas at Earth the obstacle is the dayside magnetosphere whose characteristic dimension is 10 R. Thus these shock encounters are in some sense equivalent to encounters with Earth's bow shock at antisolar distances between 40 and 90 R where relatively few observations have been made.

Shock motion produced repeated crossings into the magnetosheath. These crossings gave the fields and particle investigators a unique opportunity to learn about the geometry and structure of the distant bow shock. As Galileo moved towards the subsolar region, accelerated ions were encountered, giving new information on the role of the shock ion acceleration.

The first hint of bow shock encounters came from the magnetometer [ Kivelson et al., 1991, 1992 (instrument description)]. Sharp transitions (separated by minutes to tens of minutes) between two different field magnitudes occurred repeatedly. The two regimes corresponded to the solar wind and the magnetosheath in the weak shock regime of M2 where M is the Mach number based on the normal component of the solar wind flow. Small M is common on the distant flanks of the bow shock where the normal component of the solar wind flow is small relative to its value at the subsolar point. The magnetic and plasma [ Frank et al., 1991, 1992 (instrument description)] signatures both showed evidence that the shocks became stronger with approach to the day side. The plasma detector reported heating and compression across the shock, with electron temperature increasing by 20% and density increasing by factors of 2 to 3 at the crossings nearest to the day side and by smaller factors further downstream from the planet [ Frank et al., 1991]. Plasma waves were measured during only a portion of the flyby interval which fortunately included the first two entries into the magnetosheath. The shock crossings were confirmed by plasma wave spectra similar to those found in the terrestrial magnetosheath [ Gurnett et al., 1991, 1992 (instrument description)].

Thousands of crossings of the Venusian bow shock under a range of solar wind conditions have been studied in the Pioneer Venus Orbiter (PVO) data. The general shape [ Tatrallyay et al., 1983, 1984] and the cross section in the terminator plane [ Zhang et al. 1991] have been established. The new feature of the Galileo data is that multiple crossings occurred at different distances down the tail in what appeared to be rather steady solar wind conditions. However, the direction of the solar wind magnetic field changed just before each crossing. This observation led Kivelson et al. [1991] and Khurana and Kivelson [1994] to suggest that the multiple shock crossings could be understood by quantifying an asymptotic shock model with a non-circular cross section. The shape and orientation of this cross section in the direction transverse to the (aberrated) solar wind flow direction is controlled by the direction of the interplanetary magnetic field (IMF). The model is consistent with the PVO observations of the response of the shock cross section in the terminator plane to the IMF orientation [ Zhang et al., 1991]. The quasi-elliptical cross section can be understood by recognizing that the speed of fast magnetohydrodynamic waves varies with the angle between the propagation vector and the magnetic field [see, for example, Spreiter and Stahara, 1985]. Propagation is fastest at 90 to the magnetic field and correspondingly the bow shock stands farthest from the center of the magnetotail in directions perpendicular to the solar wind magnetic field. For steady plasma conditions, as the IMF rotates, the shock shape changes as if it were rotating about its axis. Along the Galileo trajectory, this rotation swept the shock back and forth across the spacecraft. The variable cross section model of Khurana and Kivelson [1994] uses three parameters to express the instantaneous shock position, and consequently the distance D between the shock and Galileo, as a function of the IMF angles. The twelve Galileo shock crossings occur within minutes of the times when D=0 is predicted, consistent with rapid reconfiguration in response to IMF rotations. Below I discuss the relation between these observations and Galileo's observations of the Earth's distant bow shock.

During intervals in the solar wind upstream of the terminator, the Galileo Energetic Particle Detector [ Williams et al., 1992], the first to measure ions in the 20-280 keV energy range in the Venus environment, observed intermittent streams of ions with energies extending into the top range (120 keV to 280 keV) of the detector. The particles were streaming along the magnetic field direction away from the direction of Venus, presumably accelerated at the bow shock [ Williams et al., 1991]. Ions of lower energy, measured by the plasma instrument were also detected streaming away from the bow shock [ Frank et al., 1991]. Similar ion beams have been observed upstream of Earth's bow shock where it has been suggested that they may be outstreaming "trapped ions". As Venus is not expected to contain a trapped energetic particle population, a magnetospheric source can be ruled out, requiring that a mechanism be identified to explain the significant acceleration of a solar wind ion population. The location of the observations relative to the magnetic field direction suggests that the intercept of field line through the spacecraft was downstream of the subsolar region. Williams et al. ruled out the possibility that the observed fluxes were pickup ions and concluded that the most likely mechanism for acceleration of the ions was shock-drift acceleration. In this mechanism, ions undergo magnetically-driven drift along the shock. Magnetic drift occurs when the motion of an ion (charge q, mass m, and velocity, v perpendicular to the magnetic field) is modified by magnetic field gradients. In a uniform magnetic field (field magnitude B), a charged particle moves along the field on a helical orbit of radius mv/qB (referred to as a gyroradius). Non-uniformity of the field leads to drifts perpendicular to the field, but the process requires that changes of the field be small over distances of order the particle gyroradius. The gyroradii of the observed ions, whose energies exceeded 100 keV, were 1/4 to 1 R if they were protons and larger if they were heavier ions. The dilemma in this interpretation is to understand how ions can remain within a shock front with a subsolar radius of order of the ion gyroradius for long enough to be significantly accelerated.