Enhancement of planetary ions at Mercurys cusp during flux transfer event showers: MESSENGER observations

Since the discovery of Mercurys neutral exosphere, thermal-stimulated desorption, photon-stimulated desorption, solar wind sputtering (including both ions and electrons), and micrometeoroid impact vaporization have been proposed to release neutrals and ions from the planets surface. Mercurys exosphere, and the planetary ions in the magnetosphere, exhibit complex annual variations that appear driven by changes in the source and loss processes. However, these processes cannot be observed directly and their relative importance is poorly understood. Here we present MESSENGER observations of enhanced Na+-group ions near Mercurys northern magnetospheric cusp during flux transfer event (FTE) "shower" intervals. FTE showers are intervals of frequent flux ropes formation at Mercurys magnetopause. These FTEs accelerate and magnetically channel solar wind protons planetward toward the magnetospheric cusps. A solar wind entry layer in the region about the northern cusp is observed to form in response to these FTE showers. The proton precipitation rate beneath the cusps is observed by MESSENGER to increase by an order of magnitude to ~ 1025 ions/s. Further, anti-planetward-moving Na+-group ions are detected within ~ 1 min of the onset of the FTE shower. This response time is much faster than the minimum response time, ~ 10 min, to support the fastest temporal variations observed in the exosphere remotely from Earth. Our results offer direct evidence that FTE showers strongly enhance solar wind impact on the surface and sputter neutrals and ions into Mercurys surface-bounded exosphere and high-latitude magnetosphere. Comprehensive, multi-point measurements to be returned by ESA and JAXAs BepiColombo dual-orbiter mission will greatly enhance our understanding of all of the processes contributing neutrals and ions to Mercurys dynamic exosphere and magnetosphere after making orbit in late 2025.