We explore the factors that determine the bow shock standoff distance. These factors include the parameters of the solar wind, such as the magnetosonic Mach number, plasma beta, and magnetic field orientation, as well as the size and shape of the obstacle. In this report we develop a semiempirical Mach number relation for the bow shock standoff distance in order to take into account the shock's behavior at low Mach numbers. This is done by determining which properties of the shock are most important in controlling the standoff distance and using this knowledge to modify the current Mach number relation. While the present relation has proven useful at higher Mach numbers, it has lacked effectiveness at the low Mach number limit. We also analyze the bow shock dependence upon the size and shape of the obstacle, noting that it is most appropriate to compare the standoff distance of the bow shock to the radius of curvature of the obstacle, as opposed to the distance from the focus of the object to the nose. Last, we focus our attention on the use of bow shock models in determining the standoff distance. We note that the physical behavior of the shock must correctly be taken into account, specifically the behavior as a function of solar wind dynamic pressure; otherwise, erroneous results can be obtained for the bow shock standoff distance.

A statistical analysis of 351 independent bow shock crossings and 233 independent magnetopause crossings by the ISEE‐I spacecraft from 1977 to 1980 was performed to determine the average positions and shapes of the bow shock and magnetopause. The standoff distance between the magnetopause and the bow shock depends on the compressibility of the plasma which in the ‘polytropic’ approximation is related to the ratio of specific heats, γ. Standoff distances for the bow shock and magnetopause were found to be 13.7 RE (±0.2 RE) and 10.3 RE (±0.3 RE) respectively. These distances are smaller than those observed during earlier epochs. The observed thickness of the magnetosheath is that expected for the compression of a gas whose polytropic index, γ, is 1.76±0.15. This value, representative of the entire magnetosheath, is consistent with the value of 1.67 deduced from the behavior of the plasma across individual shock transitions. A value of 1.67 is expected for an adiabatic process in a collisional, monatomic gas with three degrees of freedom with lower values for non‐adiabatic processes and higher values for anisotropic heating at the shock. The observed value of 1.76 indicates that heat flux does not much affect the position of the shock while the downstream anisotropy may have a small effect.

ISEE 1 and 2 magnetic field measurements are used to examine the structure of the low beta, quasi‐perpendicular shock. A shock crossing database consisting of ISEE 1 satellite crossings from the beginning of the mission in 1977 to the end of 1980 is utilized to identify shock crossings for this study. A set of 20 low beta, quasi‐perpendicular shock crossings are drawn from the database for study. Analysis of the shock overshoots indicates that the strength of the overshoot of low beta, quasi‐perpendicular shocks increases as the ratio of the Mach number to the first critical Mach number (or ratio of criticality) increases. There are subcritical crossings which have nonnegligible overshoots and other subcritical crossings which exhibit no overshoot. Wave analysis shows that the power of the downstream waves also increases as a function of this ratio of criticality. Upstream of the shock, large‐amplitude, low‐frequency whistler mode and higher‐frequency (f ∼ 1 Hz) whistler waves are evident for subcritical and marginally critical shocks. The lower‐frequency whistlers are right‐hand elliptically polarized and phase stand upstream of the shock, propagating along the shock normal direction. The thickness of the shock is found to be within a factor of 1 and 2 times greater than the wavelength of this precursor wave. This result is inconsistent with the conjecture that the shock is merely the last amplified cycle of the precursor wave, for if this were true, the thickness of the shock from minimum to maximum would be one half of the precursor wavelength. The 1‐Hz waves are right‐hand elliptically polarized and propagate upstream obliquely to the magnetic field direction. Downstream of the marginally critical and supercritical shock, left‐hand elliptically polarized waves are found to propagate along the magnetic field direction and have frequencies of about 0.2‐0.8 fci. These ion cyclotron waves appear to result from the excitation of the Alfvén ion cyclotron (AIC) instability. The AIC instability is driven by the Ti⊥>Ti∥ temperature anisotropy created in front of the shock by the reflection of solar wind ions. Ion cyclotron waves act to pitch angle scatter the ions downstream of the shock and remove the temperature anisotropy. A transitional behavior in the noncoplanar component of the magnetic field occurs at or about the first critical Mach number. Below the critical Mach number, the noncoplanar component is associated with the upstream whistler train. When the ratio of criticality is approximately unity, the noncoplanar component is isolated from any upstream or downstream wave activity. In the supercritical regime, this component of the field is associated with the downstream ion cyclotron wave train. For all ranges of criticality, the noncoplanar component is seen to lie within the shock ramp, and the transitional behavior of this component of the field indicates that it is an inherent part of the shock.

The formation of a hot flow anomaly (HFA) observed near the Earth's bow shock appears to be due to the interaction between the bow shock and an impinging discontinuity in the upstream plasma. Recent single‐particle and two‐dimensional hybrid numerical studies have suggested that such an interaction will produce an HFA only if the motional electric field in the ambient plasma points toward the discontinuity, thereby focusing shock‐reflected ions into it. We perform a test of this electric field orientation for a set of nine HFA events observed by the ISEE spacecraft and described previously in the literature. The principal difficulty with the test is the determination of the normals to the discontinuities. Application of a minimum variance analysis produces discontinuity normals which suggest that the majority of the discontinuities were probably tangential, even though it is doubtful that the conditions for validity are very well satisfied for these events. Under the assumption that the discontinuities were tangential, the predicted electric field orientation is found on at least one side of all nine observed HFAs (on the trailing edge of seven and the leading edge of five, and on both sides of three events). Further, there is evidence that asymmetries in the observed magnetic field signatures are related to the orientation of the motional electric field: The events in which the electric field points toward the discontinuity on both sides tend to be those with fairly symmetric flanking magnetic field enhancements. Two‐dimensional hybrid simulations appear to confirm the association between the magnetic field signature and the electric field orientations, and they show further that rotational discontinuities appear to be less effective at producing distinct HFA‐like signatures than tangential discontinuities.