Venus ion outflow estimates at solar minimum: Influence of reference frames and disturbed solar wind conditions

Abstract

Recent estimates of ion escape rates from Venus, based on ASPERA‐4 data, differ by more than a factor of 4. Whereas the ASPERA‐4 instrument provides state‐of‐the art observations, the limited field of view of the instrument and the strongly limited geographical coverage of the spacecraft orbit means that significant assumptions must be used in the interpretation of the data. We complement previous studies by using a method of average distribution functions to obtain as good statistics as possible while taking the limited field of view into account. We use more than 3 years of data, more than any of the previous studies, and investigate how the choice of a geographical reference frame or a solar wind electric field oriented reference frame affects the results. We find that the choice of reference frame cannot explain the difference between the previously published reports. Our results, based on a larger data set, fall in between the previous studies. Our conclusion is that the difference between previous studies is caused by the large variability of ion outflow at Venus. It matters significantly for the end result which data are selected and which time period is used. The average escape rates were found to be 5.2±1.0×1024 s−1 for heavy ions (m/q ≥16) and 14±2.6×1024 s−1 for protons. We also discuss the spatial distribution of the planetary ion outflow in the solar wind electric field reference frame.