UPSTREAM WAVES AT MERCURY, VENUS AND EARTH - COMPARISON OF THE PROPERTIES OF ONE HERTZ WAVES

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-handed 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.