Previous studies have shown that the Venus foreshock region contains low‐frequency upstream waves similar to those in the terrestrial foreshock, but perhaps with different amplitudes than at Earth. Herein, we compare the properties of a second class of upstream waves, analogous to the so‐called 1 Hz waves at Earth. The waves observed at Mercury, Venus, and Earth have very similar properties, i.e., propagation angles less than 55 degrees to the magnetic field and less than 35 degrees to the solar wind flow direction. The waves occur exclusively on the field lines connected to the bow shock. They are most commonly left‐hand elliptically polarized with similar fractional amplitudes, approximately 0.1 of the background field strength. Their amplitudes decrease with increasing distance from the shock. The observed frequencies are similar for Mercury, Venus and Earth when scaled by the interplanetary magnetic field. If, as generally assumed at Earth, these waves arise in regions of backstreaming electrons, these results imply that similar electron foreshocks occur at Earth, Venus and Mercury despite differences in bow shock size and the nature of the obstacle to the solar wind.
Plasma waves are observed in the solar wind upstream of the Venus bow shock by the Pioneer Venus Orbiter. These wave signatures occur during periods when the interplanetary magnetic field through the spacecraft position intersects the bow shock, thereby placing the spacecraft in the foreshock region. The electron foreshock boundary is clearly evident in the data as a sharp onset in wave activity and a peak in intensity. Wave intensity is seen to drop rapidly with incresing penetration into the foreshock. The peak wave electric field strength at the electron foreshock boundary is found to be similar to terrestrial observations. A normalized wave spectrum was constructed using measurements of the electron plasma frequency and the spectrum was found to be centered about this value. These results, along with polarization studies showing the wave electric field to be field aligned, are consistent with our interpretation of the waves as electron plasma oscillations.
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