E. W. Greenstadt

Initial ISEE magnetometer results: Shock observation

ISEE-1 and -2 magnetic field profiles across 6 terrestrial bow shocks and one interplanetary shock are examined. The interplanetary shock illustrates the behavior of a low Mach number shock. It had an upstream whistler wave precursor with an apparent wavelength of 180 km. The shock thickness was about 90 km for the thickness of the final field jump or 270 km for the exponential growth of the precursor wave packet. The ion inertial length was 50 km, upstream of the shock.

Spatial distribution of electron plasma oscillations in the Earth`s foreshock at ISEE 3

Electric field oscillations recorded by the 10–56 kHz channels of TRWs plasma wave detector during parts of two of the ISEE 3 circumterrestrial orbits in 1983 have been used to make the first mapping of Earths electron plasma wave foreshock. By combining data from the two trajectory segments, each of which provided relatively meager spatial sampling outside the bow shock, but high variation of interplanetary magnetic field (IMF) direction, a first-order pattern of occurrence of electron plasma waves, hence also backstreaming electrons, has been determined. We depict the pattern with an adaptation of the mapping program previously used for the Venus electron foreshock. As at Venus, plasma wave activity was concentrated most densely along the IMF line tangent to the bow shock where energized electrons stream against the solar wind from the quasi-perpendicular part of the shock. The size of the Earths electron plasma wave foreshock, however, is vastly greater than that of Venus, implying that a foreshocks dimension scales with the size of its diamagnetic obstacle and associated bow shock. Our mappings with three additional ISEE 3 channels surrounding the local electron plasma frequency indicate a richer distribution of waves in the foreshock than the single electron frequency channel of Pioneer Venus Orbiter could detect around Venus.

Weak, quasiparallel profiles of earth's bow shock - A comparison between numerical simulations and ISEE 3 observations on the far flank

Over 200 crossings of the distant downwind flanks of earths magnetosonic bow shock by ISEE 3 included many cases of weak, or low Mach number, quasi-parallel shocks. A consistent feature of the magnetic field profiles was the presence of large amplitude, near periodic to irregular transverse oscillations downstream from even the weakest Q-parallel shocks. Large downstream perturbations with whistler-like features similar to those of the observations appear in 1D simulations when the Alfven Mach number M(A) is greater than 2.5 but not when M(A) = 2.1. The observed cases with downstream waves also occurred when M(A) is greater than about 2.5, suggesting the importance of the Alfven as opposed to magnetosonic Mach number in determining the signature of weak, Q-parallel shocks.

Magnetic and electric field waves in slow shocks of the distant geomagnetic tail: ISEE 3 observations

During the first pass through the distant geomagnetic tail by ISEE 3 the slow shocks encountered on February 2 and 11, 1983, provide particularly clear examples of the magnetic field and plasma wave properties of the shock transition. The magnetic ramp contains transverse polarized magnetic field oscillations with frequencies just below the ion cyclotron frequency and amplitudes of 2-4 nT. These waves are plausibly generated by the electromagnetic ion/ion cyclotron instability predicted by Winske and Omidi (1990). The electric field plasma waves within the shock ramp exhibit two spectral peaks. A midfrequency emission occurs near the ion plasma frequency and electron cyclotron frequency but well below the maximum Doppler shift frequency for electrostatic waves. The midfrequency waves extend into the upstream region where the spectral peak occurs at a slightly higher frequency. A new high-frequency emission with frequencies between the maximum Doppler shift frequency and the electron plasma frequency occurs throughout the downstream region. This emission disappears at the start of the magnetic ramp and is replaced upstream by electron plasma oscillations. The high-frequency emissions are clearly polarized parallel to the magnetic field. The polarization of the midfrequency waves is less certain; both parallel and a fairly broad angular distribution about the parallel electric fields are consistent with the measurements.

The quasiperpendicular environment of large magnetic pulses in Earth's quasiparallel foreshock: ISEE 1&2 observations

ULF waves in Earth;aposs foreshock cause the instantaneous angle ϑBn between the upstream magnetic field and the shock normal to deviate from its average value. Close to the quasiparallel (Q∥) shock the transverse components of the waves become so large that the orientation of the field to the normal becomes quasiperpendicular (Q⟂) during applicable phases of each wave cycle. Large upstream pulses of B were observed completely enclosed in excursions of ϑBn into the Q⟂ range. A recent numerical simulation included ϑBn among the parameters examined in Q∥ runs, and described a similar coincidence as intrinsic to a stage in development of the reformation process of such shocks. Thus, the natural environment of the Q∥ section of Earths bow shock seems to include an identifiable class of enlarged magnetic pulses for which local Q⟂ geometry is a necessary association.

PLASMA INSTABILITY MODES RELATED TO THE EARTH'S BOW SHOCK

The present paper is an attempt to give a very brief outline of the status of physical interpretations of some of the microscopic plasma physical phenomena occurring in bow shock structures. We shall not give any extensive discussion of collisionless shock models and theories in the short space available here, since there exist excellent detailed sources for such information. For general collisionless shock theories, we refer the reader to the recent monograph by Tidman and Krall (1971), who in addition to their own contributions, give bibliographies containing all of the important source material up to perhaps 1970. The main advances in collisionless shock theory since that time have come primarily from computer simulation work.

Plasma waves downstream of weak collisionless shocks

In September 1983 the ISEE 3/ICE spacecraft made a long traversal of the distant dawnside flank region of the Earths magnetosphere and had many encounters with the low Mach number bow shock. These weak shocks excite plasma wave electric field turbulence with amplitudes comparable to those detected in the much stronger bow shock near the nose region. Downstream of quasi-perpendicular (quasi-parallel) shocks, the E field spectra exhibit a strong peak (plateau) at midfrequencies (1-3 kHz); the plateau shape is produced by a low-frequency (100-300 Hz) emission which is more intense behind quasi-parallel shocks. Polarization measurements made in the very steady magnetic field conditions downstream of two quasi-perpendicular shocks show that the low frequency signals are polarized parallel to the magnetic field, whereas the midfrequency emissions are unpolarized or only weakly polarized. A new high frequency (10-30 kHz) emission which is above the maximum Doppler shift frequency is clearly identified as a separate wave component. High time resolution spectra often exhibit a distinct peak at high frequencies; this peak is often blurred by the large amplitude fluctuations of the midfrequency waves. The high-frequency component is strongly polarized along the magnetic field and varies independently of the lower-frequency waves.

Plasma wave profiles of Earth's bow shock at low Mach numbers: ISEE 3 observations on the far flankTtethered Satellite System

The Earths bow shock is weak along its distant flanks where the projected component of solar wind velocity normal to the hyperboloidal surface is only a fraction of the total free stream velocity, severely reducing the local Mach number. We present a survey of selected crossings far downstream from the subsolar shock, delineating the overall plasma wave (pw) behavior of a selected set of nearly perpendicular crossings and another set of limited Mach number but broad geometry; we include their immediate upstream regions. The result is a generalizable pw signature, or signatures, of low Mach number shocks and some likely implications of those signatures for the weak shocks plasma physical processes on the flank. We find the data consistent with the presence of ion beam interactions producing noise ahead of the shock in the ion acoustic frequency range. One subcritical case was found whose pw noise was presumably related to a reflected ion population just as in stronger events. The presence or absence, and the amplitudes, of pw activity are explainable by the presence or absence of a population of upstream ions controlled by the component of interplanetary magnetic field normal to the solar wind flow.

On the absence of plasma wave emissions and the magnetic field orientation in the distant magnetosheath

In early September, 1983 ISEE-3 made a long traversal of the distant dawnside magnetosheath starting near x = −150 RE downstream. The distant magnetosheath often contains moderately intense plasma wave emissions at frequencies from several hundred Hz to 5 kHz. However, over time scales of many days, a clear correlation exists between the occurrence of the plasma waves and the cone angle (θxB) between the magnetic field and the plasma flow velocity (x-direction). For θxB large (small), the plasma wave amplitudes are near background (high). Sudden ( < 1 minute) changes in the local magnetic field orientation produce correspondingly sudden changes in the wave amplitudes. Statistically, the wave amplitudes decrease continuously with increasing θxB.

Observations of plasma waves in the solar wind interaction region of comet Giacobini‐Zinner at high time resolution

High-time-resolution spectra of plasma wave emissions detected in the interaction region of comet Giacobini-Zinner with the solar wind reveal a wave phenomenology much more complicated than first reported. Spectra often exhibit three or more independent peaks, which become more prominent the deeper into the interaction region the spacecraft traversed. The main peaks correspond to whistler emissions below the electron cyclotron frequency, a midfrequency peak near the maximum Doppler shift frequency for waves with kλD= 1, a high-frequency peak above the Doppler shift maximum frequency, and electron plasma oscillations at the plasma frequency. Similar multipeaked spectra are also observed downstream from weak shocks at Earth, which suggests that the plasma wave generation mechanisms responsible need not require particle populations created by photoionization.