Solar Wind Absorption by Venus

The fraction of solar wind which interacts with the dayside ionosphere and atmosphere of Venus was calculated assuming the presence ofmagnetic field fluctuations in the, • ionosheath region and taking into consideration the interaction with the upper neutral atmosphere extending above the ionopause. The Monte Carlo calculations indicated that fluctuations with Brms/(B) of 0.3, a not unreasonable value according to recent observations, implies that about 0.3% of solar wind may be absorbed on the average. Interactions with the upper neutral atmosphere due to charge xchange can decrease the shocked solar wind flux by 1-7%, depending on the ionopause altitude. Most of the fast H atoms produced by charge exchange interactions with the atmosphere escape, and only a negligible fraction can enter the atmosphere. The precipitation f solar wind ions into the atmosphere b cause of magnetic field fluctuations is not uniform over the daysid. e atmosphere but was calculated to be greatest near the terminator. The effect that these penetrating solar wind particles have on the structure of the ionosphere was calculated. It was found that while some of the energy deposition processes l ad to unique and observable signatures (e.g., a narrow Lyman a emission region), these penetrating solar wind particles do not appear to play a significant role in controlling the physical nd/or chemical structure of the daytime Venus ionosphere.