Detection of interplanetary magnetic field fluctuations stimulated by the lunar wake

Abstract

The analysis of detailed measurements at 5.11-second intervals by the NASA-GSFC magnetic field experiment on lunar Explorer 35 in the vicinity of the moon has revealed the presence of rapid fluctuations up to the instrument bandpass of 5 Hz with amplitudes of several gammas. These disturbances are transmitted both up- and downstream from the penumbra into regions of space directly connected to the penumbra by the magnetic field. Similar fluctuations are not observed in the center of the lunar wake, the solar wind plasma umbra, where no plasma is present. Power spectral analyses made in a field-aligned coordinate system reveal that these fluctuations have approximately the same amplitude both parallel and transverse to the average field direction. Statistical studies are presented of the frequency of occurrence of the disturbances as a function of the distance to the point of observation from the lunar wake and of the fluctuation amplitude, and correlation with field magnitude. These observations suggest that the fluctuations have their source where the solar wind proton and electron distribution functions are disturbed by the removal of particles by absorption by the moon. Instabilities induced by this disturbance of the electron distribution function are postulated as the source of the fluctuations, and their propagation with the electron velocity, which is greater than the bulk speed of the solar wind, allows the upstream solar wind to be affected by the wake of the moon.