MESSENGER observations of Mercury's planetary ion escape rates and their dependence on true anomaly angle

TL;DR

This paper analyzes Mercury's sodium-group ion escape rates using MESSENGER data, revealing their dependence on Mercury's orbital phase and identifying the magnetosheath as the main escape region, with rates comparable to other inner planets.

Contribution

It provides the first detailed analysis of Na+-group ion escape rates from Mercury and their variation with true anomaly angle based on MESSENGER observations.

Findings

Escape rates range from 0.2 to 1 x 10^{25} atoms/s.

Escape rates peak near Mercury's perihelion.

Magnetosheath is the primary escape region.

Abstract

This study investigates the escape of Mercury's sodium-group ions (Na+-group, including ions with m/q from 21 to 30 amu/e) and their dependence on true anomaly angle (TAA), i.e., Mercury's orbital phase around the Sun, using measurements from MESSENGER. The measurements are categorized into solar wind, magnetosheath, and magnetosphere, and further divided into four TAA intervals. Na+-group ions form escape plumes in the solar wind and magnetosheath, with higher fluxes along the solar wind's motional electric field. The total escape rates vary from 0.2 to 1 times 10^{25} atoms/s with the magnetosheath being the main escaping region. These rates exhibit a TAA dependence, peaking near the perihelion and similar during Mercury's remaining orbit. Despite Mercury's tenuous exosphere, Na+-group ions escape rate is comparable to other inner planets. This can be attributed to several processes, including that Na+-group ions may include several ion species, efficient photoionization frequency for elements within Na+-group, etc.