ISEE 1, 2, 3, IMP 8, and Prognoz 7 observations of interplanetary shocks in 1978 and 1979 provide five instances where a single shock is observed by four spacecraft. These observations are used to determine best‐fit normals for these five shocks. In addition to providing well‐documented shocks for future investigations these data allow us to evaluate the accuracy of several shock normal determination techniques. When the angle between upstream and downstream magnetic field is greater than 20°, magnetic coplanarity can be an accurate single spacecraft method. However, no technique based solely on the magnetic measurements at one or multiple sites was universally accurate. Thus, we recommend using overdetermined shock normal solutions whenever possible, utilizing plasma measurements, separation vectors, and time delays together with magnetic constraints.

We have studied the structural elements, including shock ramps and precursor wave trains, of a series of oblique 1ow-Mach number terrestrial bow shocks. We used magnetic field data from the dual ISEE 1 and 2 spacecraft to determine the scale lengths of various elements of shock structure as well as wavelengths and wave polarizations. Bow shock structure under these conditions is essentially that of a large-amplitude damped whistler mode wave which extends upstream in the form of a precursor wave train. Shock thicknesses, which are determined by the dispersive properties of the ambient plasma, are too broad to support current-driven electrostatic waves, ruling out such turbulence as the source of dissipation in these shocks. Dissipative processes are reflected in the damping of the precursors, and dissipative scale lengths are ~200-800 km (several times greater than shock thicknesses). Precursor damping is not related to shock normal angle or Mach number, but is correlated with Te/T •. The source of the dissipation in the shocks does not appear to be wave-wave decay of the whistlers, for which no evidence is found. We cannot rule out the possibility of contributions to the dissipation from ion acoustic and/or lower-hybrid mode turbulence, but interaction of the whistler itself with upstream electrons offers a simpler and more self-consistent explanation for the observed wave train damping. (inward) motion. :[The sign of the wave velocity indicates the polarization of the waves as observed in the spacecraft frame: +(-) corresponding to right-handed (left-handed) waves.õSpacecraft separation too small to allow accurate measurements.

The evolution of the ion and electron distribution functions across a set of 10 low‐Mach number, nominally subcritical, quasi‐perpendicular shocks is examined in high time resolution (full distribution every 3 s) using data from the ISEE 1 and 2 spacecraft. Both ions and electrons sometimes show slight preheating upstream of the shock, but otherwise the ion and electron temperatures rise together in the magnetic ramp and show no further increase downstream. Contrary to the usual assumption based on early laboratory and theoretical work that at subcritical shocks the bulk of the energy dissipation occurs as resistive heating of the electrons, it is found that the ion temperature increase exceeds that of the electrons. This difference is attributed to the distinction between dispersive shocks, such as those studied here, and resistive shocks, such as those observed in most laboratory studies. The increase in ion temperature is predominantly in the perpendicular direction and is due to heating of the entire distribution rather than to the formation of a high‐energy tail. The perpendicular temperature increase is typically a factor of 10–20, much greater than the usual assumption of adiabatic heating. The downstream to upstream ratio of perpendicular electron temperature is equal to the magnetic field ratio (∼2–2.5). The electrons also show significant heating in the parallel direction, with the downstream T∥/T⊥ ∼1–1.2. The downstream electron distribution exhibits the characteristic flattop seen downstream of supercritical shocks, and there is evidence for the field‐aligned electron beam identified previously within those shocks. As previously reported, the downstream ion and electron total temperatures are nearly equal. These observations are interpreted as evidence for the simultaneous operation of several plasma instabilities, including the modified two‐stream instability, driven by the cross‐field current within the shock, and the ion acoustic instability, driven by the field‐aligned electron beam.

The electron heating at collisionless shocks in near-Earth space is normally found to be relatively small. We report here on two sets of bow shock crossings observed by the ISEE 1 and ISEE 2 spacecraft in which very large electron temperature increases were found. The two sets of shocks are part of a larger set of 52 bow shock crossings compiled from the ISEE data set. When the shocks with the large electron heating are compared to the rest of the shocks in the compiled set, it is found that the only upstream parameter which is out of the ordinary is the upstream solar wind flow speed V u. Indeed, for the entire shock set the only upstream parameters which correlate well with the electron heating are the solar wind speed and quantities derived from it. The best correlation among those tested is between the temperature increase (Tea-Teu ) and the difference in the square of the bulk flow speed (V• 2 -Va 2) across the shock, which is proportional to the total amount of bulk flow energy dissipated by the shock. A subsequent search for bow shock crossings which occur during intervals of high solar wind speed confirms that the electron heating is very large under high-speed conditions. The first-order dependence of the electron temperature increase on the available bulk flow energy is consistent with a heating process which is dominated by the macroscopic cross-shock electrostatic potential jump. This study suggests that the temperature difference is the appropriate measure of electron heating at shocks (rather than the ratio T•a/T•,,) and that the first-order dependence on (V•2 _ Va2) should be normalized out in future studies of electron heating at space shocks.

We investigated the role played by low‐frequency turbulence in the determination of magnetic field overshoots in collisionless shock waves. The data set used in this study included magnetometer and solar wind data from the ISEE 1 and 2 spacecraft for ∼65 quasi‐perpendicular bow shocks. Overshoots were calculated from both high‐resolution data and from data averaged to eliminate the effects of turbulence with frequencies greater than the ion cyclotron frequency. Overshoots determined by the two methods exhibited generally similar behavior, although those calculated from the high‐resolution data were generally larger by a factor of ∼2. Overshoot size correlated well with shock Mach number and electron beta in both cases. The size of overshoots calculated from the high‐resolution data increased strongly with Mach number and beta, while those calculated from the averaged data showed less dramatic increases. The behavior of overshoots calculated from the average data was generally consistent with hybrid simulation results. The difference between overshoots measured using averaged and unaveraged data was generally consistent with the presence of a component of overshoot magnitude due to low‐frequency turbulence. Measurable overshoots were observed for all shocks in the data set, although those associated with the weakest shocks were small. Neither set of overshoots showed any particular change in behavior at the first critical Mach number.