Very-low-frequency electric fields in the interplanetary medium: Pioneer 8

Abstract

We present Pioneer 8 observations of magnetosheath and interplanetary VLF electric fields, consisting of hourly ranges of the potential amplitude in a. broadband channel (0.1 to 100 kHz) and in a 15% bandpass channel centered on 400 Hz. Significant signals are correlated with position with respect to earth, and with solar wind plasma parameters obtained from the lunar orbiting Explorer 35 spacecraft. We detect two principal features: noise bursts or spikes with duration less than approximately 10 sec, and persistent signals with durations typically 1 day or more. The noise bursts coincide with plasma and magnetic field discontinuities where these data are available for comparison. The persistent signals correlate loosely with solar wind density, and this correlation holds whether the density increases are due to interplanetary shocks, the 'snow plow' effect, or other processes. Although the experiment is too limited to provide unambiguous determination of the wave modes, at present it appears most likely that ion acoustic waves have been detected.Pioneer 8, launched into solar orbit on December 13, 1967, provided the first opportunity for long-term monitoring of the interplanetary ¾LF electric field. The Pioneer 8 electric field experiment was the forerunner of a series of increasingly sophisticated instruments designed to investigate electric field phenomena in space. Its purpose was to provide the initial survey of characteristics of solar wind VLF electric fields, and thus it was designed mainly to detect signals and to obtain crude frequency and amplitude information. This report presents the results of that survey.Electric field measurements offer new possibilities for exploration of basic problems in the physics of the solar wind and of collisionless plasmas. Problems that can be studied relate to the microscopic processes that cause the interplanetary plasma to behave collectively and the processes that govern the collisionless dissipation and transport properties. Knowledge of the electric field behavior is useful in the study of such solar wind problems as field line merging at discontinuities, the dissipation of hydromagnetic waves and of suprathermal particles, and the control of the pressure anisotropy. The measurements, of course, bear on other fundamental problems in the physics of magnetized plasmas, such as the description of the collisionless magnetohydrodynamic shock. This paper presents the electric field data from the first 10 months of the flight and emphasizes the general characteristics of the interplanetary electric fields over time scales greater than I hour. However, special events are examined at higher resolution. The early flight data are compared with nearly simultaneous solar wind plasma data obtained with the lunar orbiter, Explorer 35. For the time scales of interest here, a comparison of data between the two spacecraft should be meaningful at least during the early part of the flight. The data and correlations are then used to discuss possible electric field generation mechanisms...

Waves and instabilities in the solar wind

“…High-frequency electric field fluctuations in the solar wind have been observed by Pioneer 8 and 9 in both narrow band (370 < f< 430 Hz) and broad band (100 < f< 105 Hz) channels Siscoe et al, 1971 ]. The experimenters present arguments that the observed fluctuations are ion acoustic waves, but they recognize that precise identification of the wave mode is difficult from a single spacecraft, since Doppler frequency shifts can confuse the mode identification.…”

Interplanetary shock waves generated by solar flares

“…Additional, but still limited, evidence of piston-driven sh~cks has come from the VLF electric field observations of Pioneers 8 and 9 (Scarfet al, 1970;Siscoe et al, 1971). These investigators have suggested that significant waveparticle interactions occur on a time scale of hours following passage of interplanetary shock waves.…”

Analyses of techniques for measuring DC and AC electric fields in the magnetosphere

“…Measurements at higher frequencies require higher time resolution in the experimental data which, among the techniques described earlier, has been achieved only for double probe potential difference measurements on rockets and satellites. Such experiments have been performed on more than 15 rockets [121,122,123,124,125,126,127,128,129] and 15 satellites in the ionosphere, magnetosphere, and interplanetary medium [130,131,132,133,134,135,136,137,138,139,140,141,142,143,144,145,146,147,148] over a frequency range that has generally been ~ 10 to ~ 105 Hz. Indirect measurements of the electric fields in waves can be made over a similar frequency range by other techniques.…”