Ulf waves upstream of the Venus bow shock: Properties of one‐hertz waves

Abstract

Pioneer Venus orbiter data are used to examine the properties of a class of ULF upstream waves with relatively high observed frequencies (1.1–1.3 Hz). In the spacecraft frame these waves are most often left‐hand elliptically polarized. They have amplitudes up to 3 nT (near the bow shock) and propagate obliquely to the magnetic field at angles from 10° to 50°. These waves show significant similarity in their properties to “one‐Hertz” waves identified at the Earth in the ISEE 1 and 2 observations and the whistler waves identified earlier with IMP 6 observations. The waves appear almost immediately after the spacecraft crosses the magnetic field tangent line to the bow shock surface into the region of connected field lines. The amplitude of these waves decreases with distance from the shock measured along the magnetic field line. We have used the cold plasma dispersion relation in order to study the propagation of these waves and to explain their observed polarization. Calculated group velocities indicate that those waves have sufficient upstream velocities to propagate from the shock into the solar wind. The totality of observations, including the observed wave damping and the observation of right‐handed waves, seems best explained by a source of right‐handed whistler mode waves at the bow shock.