Enhanced energization of plume ions around Mars from interplanetary shocks

Abstract

Heavy ions escaping Mars along the solar wind electric field direction are often referred to as an “ion plume”. This phenomenon represents one of the major ion escape channels on Mars. Spacecraft observations have indicated that the global average of escaping ion fluxes, derived with the aid of models, can be increased by an order of magnitude or more in response to strong solar events. In particular, it has been reported that interplanetary (IP) shocks produce high-energy escaping ion plumes. However, the ion acceleration mechanisms associated with the shock arrival have not yet been fully elucidated. During the passage of an IP shock on Mars on March 3, 2015, the plume O+ ions continuously entered the narrow field of view (FoV) of STATIC on board the MAVEN spacecraft, thanks to favorable FoV configurations. This event provides a unique opportunity to identify plume ion energization processes associated with the shock arrival. Our analysis suggests that the enhanced energization of the plume O+ ions is mainly due to the enhanced convection electric field caused by the IP shock compression. This finding provides a crucial clue towards the understanding of how IP shocks facilitate ion escape through the plume.