Space Weather Influence on High Energy Pick-up Ions at Venus

Understanding the effect of extreme solar wind conditions on atmospheric escape at Venus is crucial to understanding the evolution of the planet, and other unmagnetized bodies in the solar system such as Mars. Because Venus lacks a global dynamo magnetic field, the solar wind interacts directly with its ionosphere, leading to the loss of atmospheric constituents (particularly oxygen) by processes such as ion pick-up. This study uses data from the Venus Express (VEX) ASPERA-4 Ion Mass Analyzer (IMA)instrument for measurement of escaping oxygen and the VEX magnetometer to give the solar wind context. Previous analyses of VEX IMA data have shown populations of planetary ions at low energies (~10’s of eV) and distinctly beam shaped classical pick-up ion features (“beams”) at higher energies (>1 keV). This study focuses on the high energy pick-up ion features. A survey of 389 VEX orbits during 2006 and 2007 reveals 265 samples of these features in 197 separate orbits. Corresponding magnetometer data during the same period shows that 17 ICMEs (Interplanetary Coronal Mass Ejections) impacted Venus, engulfing it in the characteristic strong, rotating interplanetary magnetic fields and enhanced solar wind of these disturbances. The high energy pick-up ions organize according to the solar wind conditions in that they accelerate to higher energies at low altitudes during ICMEs relative to periods of quiet solar wind. There is not a clear dependence of the escaping oxygen flux on ICMEs. This may be because these events occurred during solar minimum when ICMEs typically do not have high velocity.