On the role of the quasi‐parallel bow shock in ion pickup: A lesson from Venus?

Previous observations at Venus show convincing evidence of planetary (O+) ion pickup by the large‐scale motional (−V × B) electric field in the magnetosheath when the interplanetary magnetic field is perpendicular to the solar wind flow. However, the presence of magnetic field fluctuations in the magnetosheath downstream from the quasi‐parallel bow shock should allow pickup to occur even when the upstream magnetic field B and plasma velocity V are practically coaligned. Single‐particle calculations are used to demonstrate that convecting magnetic field fluctuations similar to those observed in the Venus magnetosheath when the subsolar bow shock is quasi‐parallel can efficiently accelerate cold planetary ions by means of the electric field associated with their transverse components. This ion pickup process, which is characterized by a spatial dependence determined by the bow shock shape and the orientation of the upstream magnetic field, is likely also to occur at Mars and may be effective at comets.