MESSENGER observations of planetary ion enhancements at Mercury's northern magnetospheric cusp during Flux Transfer Event Showers

TL;DR

This study reports on Mercury's magnetospheric cusp experiencing ion enhancements during flux transfer events, revealing how magnetopause reconnection influences planetary ion escape and exosphere variability.

Contribution

It provides the first in situ observations linking flux transfer events to planetary ion enhancements and exosphere changes at Mercury.

Findings

Enhanced planetary ions during FTE showers

Exosphere neutral density varies >10% during events

Magnetopause reconnection drives ion escape channels

Abstract

At Mercury, several processes can release ions and neutrals out of the planet's surface. Here we present enhancements of dayside planetary ions in the solar wind entry layer during flux transfer event (FTE) "showers" near Mercury's northern magnetospheric cusp. The FTE showers correspond to the intervals of intense magnetopause reconnection of Mercury's magnetosphere, which form a solar wind entry layer equatorward of the magnetospheric cusps. In this entry layer, solar wind ions are accelerated and move downward (i.e. planetward) toward the cusps, which sputter upward-moving planetary ions within 1 minute. The precipitation rate is enhanced by an order of magnitude during FTE showers and the neutral density of the exosphere can vary by >10% due to this FTE-driven sputtering. These in situ observations of enhanced planetary ions in the entry layer likely correspond to an escape channel of Mercury's planetary ions, and the large-scale variations of the exosphere observed on minute-timescales by Earth observatories. Comprehensive, future multi-point measurements made by BepiColombo will greatly enhance our understanding of the processes contributing to Mercury's dynamic exosphere and magnetosphere.