Effects of a Large ICME on Atmospheric Escape at Venus

Understanding the effects of large solar wind disturbances on the ion escape rate at Venus is critical for bounding the history of water on the planet. Previous studies have suggested that solar wind disturbances can cause an enhancement in the the ion escape rate, but did not look specifically at the effect of the sheath region of Interplanetary Coronal Mass Ejections, when the highest dynamic pressures in the solar wind are encountered. The high dynamic pressure region is of specific interest because it pushes the ionopause to a lower altitude exposing more ions to the magnetic fields in the solar wind. We will present a case study from a large (fast and high magnetic field) ICME that hit Venus on November 5, 2011. This event had the highest piled up magnetic field yet encountered by VEX (>250 nT) and also is the best event for studying the effects of the high dynamic pressure sheath region on escape because VEX was near the planet measuring escaping ions during the time period when the ICME sheath passed Venus. During this time period MESSENGER and STEREO B were aligned with Venus, allowing additional measurements of this event. We will present details of this ICME with data from VEX, MESSENGER and STEREO B. The ion escape was enhanced during this event, which we will show by comparison with undisturbed days with similar interplanetary magnetic field directions and orbit geometry.