Statistical imaging of the Venus foreshock using VLF wave emissions

Strong VLF wave emissions have been shown to be an intrinsic property of the Venus foreshock and foreshocks in general. In this work, we use these measured wave emissions to construct statistical “images” of the Venus foreshock. This analysis method allows us to develop a macroscopic picture of the wave properties, the inferred particle distributions, and their evolution as a function of position within the foreshock as well as compare our observations to Earth. The electron foreshock emissions at Venus are parallel polarized Langmuir mode waves with the same peak amplitude at the foreshock boundary as terrestrial emissions (10 mV/m). However, the wave characteristics differ markedly between the upstream and downstream foreshocks for near Parker spiral interplanetary magnetic field (IMF) orientations (∼35° for Venus). Additionally, there is a dramatic decrease in wave intensity for distances beyond ∼15 Rv from the point of tangency along the foreshock boundary. These characteristics in the wave emissions provide strong observational evidence supporting reflection and energization of solar wind electrons at the shock as the dominant source for providing upstream electrons, not leakage. In the ion foreshock the wave emissions consist of parallel polarized ion acoustic-like waves with similar intensities and spectral characteristics to terrestrial emissions. However, these waves are situated much deeper in the foreshock than expected from terrestrial observations. Surprisingly, no emissions are observed in regions where field aligned ion distributions are expected. Rather, the emissions are confined to a region where diffuse ion distributions are expected.

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