The effect of surface potential on reflected ions at Mercury observed by BepiColombo SERENA/MIPA

Mercury's lack of atmosphere allows magnetospheric and solar wind plasma direct access to the surface through precipitation, altering the surface and plasma composition. In this way, its space plasma environment interactions resemble that of the Moon. Lunar measurements show that such exposed surfaces are subject to electrostatic charging, where photoelectrons generated on the dayside creating a positive potential of a few V, while electron transport to the nightside induces a negative surface potential up to -100s V. Given Mercury's similar surface properties and long diurnal cycle, we expect a similar pattern in surface potential; indeed, the nightside potential is likely even more negative at Mercury than at the Moon due to high energy electron precipitation.

Lunar Prospector electron reflectrometry data indicates that this surface potential can be estimated remotely from pitch angle distributions of reflected particles, as the potential adds an energy-dependent term to the particle loss cone [1]. For positively charged ions and a negative surface potential, the size of the upward loss cone becomes inversely proportional to ion energy and can include all pitch angles < 90 degrees for sufficiently negative potentials. Thus, this effect has important implications for the ability of Mercury's magnetic field to trap particles on the nightside. We examine nightside pitch angle distributions from the Miniature Ion Precipitation Analyzer (MIPA) in the SERENA package from BepiColombo's second Mercury flyby for evidence of this loss cone energy dependence and find a noticeably visible effect for upward-going ions. Lunar surface potential analysis shows a strong dependence on electron temperature [2], so we use Mercury Electron Analyzer (MEA) electron temperature measurements from MFB2 [3] as a basis for fitting the loss cone and find good agreement with the MIPA data. This case study shows that estimation of surface potential from ion measurements is indeed possible, and promises interesting results to come from the BepiColombo orbital phase.

[1] Halekas, J. S., G. T. Delory, R. P. Lin, T. J. Stubbs, and W. M. Farrell. “Lunar Prospector Observations of the Electrostatic Potential of the Lunar Surface and Its Response to Incident Currents.” JGR: Space Physics 113, no. A9 (2008). https://doi.org/10.1029/2008JA013194.
[2] Stubbs, T. J., W. M. Farrell, J. S. Halekas, et al. “Dependence of Lunar Surface Charging on Solar Wind Plasma Conditions and Solar Irradiation.” P&SS 90 (January 2014): 10–27. https://doi.org/10.1016/j.pss.2013.07.008.
[3] Rojo, M., N. André, S. Aizawa, et al. “Structure and Dynamics of the Hermean Magnetosphere Revealed by Electron Observations from the Mercury Electron Analyzer after the First Three Mercury Flybys of BepiColombo.” A&A 687 (July 2024): A243. https://doi.org/10.1051/0004-6361/202449450.