MESSENGER Observations of Planetary Ion Enhancements at Mercury’s Northern Magnetospheric Cusp During Flux Transfer Event Showers

Abstract

At Mercury, several processes can release ions and neutrals out of the planet’s surface. Here we present enhancements of planetary ions (Na+-group ions) in Mercury’s northern magnetospheric cusp during flux transfer event (FTE) “showers.” FTE showers are intervals of intense dayside magnetopause reconnection, during which FTEs are observed in quick succession, that is, only separated by a few seconds. This study identifies 1953 FTE shower intervals and 1795 Non-FTE shower intervals. During the shower intervals, this study shows that the FTEs form a solar wind entry layer equatorward of the northern magnetospheric cusp. In this entry layer, solar wind ions are accelerated and move downward (i.e., planetward) toward the cusp, which sputter upward-moving planetary ions with a particle flux of 1 × 1011 m−2 s−1 within 1 min. The precipitation rate is estimated to increase by an order of magnitude during FTE showers, to 2 × 1025 s−1, and the neutral density of the exosphere could vary by >10% in response to this FTE-driven sputtering. Such rapid large-scale variations driven by dayside reconnection may explain the minute-to-minute changes in Mercury’s exosphere, especially on the high latitudes, observed by ground-based telescopes on Earth. Our MESSENGER in situ observation of enhanced planetary ions in the entry layer likely corresponds to an escape channel for Mercury’s planetary ions. Comprehensive, future multipoint measurements made by BepiColombo will greatly enhance our understanding of the processes contributing to Mercury’s dynamic exosphere and magnetosphere.Plain Language SummaryFor the airless objects in the solar system, energetic ion sputtering is an important process that can release particles out of the planet’s surface. In Mercury’s magnetosphere, the solar wind energetic ions have been suggested to be able to release neutrals out of the surface in a short period by models and simulations. This paper has examined the 4 years of magnetic field and plasma measurements collected by NASA’s MESSENGER mission orbiting Mercury and has led to the discovery of how reconnection between the interplanetary and planetary magnetic fields leads to the formation of solar wind entry layers around Mercury’s magnetospheric cusps. These entry layers act as magnetic channels that cause the solar wind energetic ions to move downward toward the planet and precipitate onto the surface beneath Mercury’s northern and southern magnetospheric cusps where they sputter neutral sodium and sodium ions. This paper concludes that the solar wind sputtering could account for 10% or greater changes in the density of neutral sodium in Mercury’s exosphere, and the sputtered sodium ions likely form an escape channel of Mercury’s planetary species. The ion escape depended on the solar wind driving magnetopause reconnection at Mercury is different from the ion escape at Venus and Mars, where escape ions are ionized by the solar ultraviolet (UV).Key PointsSolar wind entry layer is observed to form equatorward of cusp during flux transfer event showers which enhances precipitation rate by an order of magnitudeSolar wind sputtering can release planetary ions and neutrals efficiently within 1 min of the onset of magnetopause reconnectionA new escape channel for planetary ions is formed by solar wind sputtering with an escape rate of ∼1024 s−1 for Na+-group ions