MESSENGER X‐Ray Observations of Electron Precipitation on the Dayside of Mercury

Abstract

The first maps of electron‐induced X‐ray emission from the dayside of Mercury’s surface are presented, generated by the development of a solar X‐ray flux filter. This enables the isolation of the X‐ray fluorescence of calcium driven by probable electron precipitation. A catalog of such events has been generated and dayside maps of implied electron precipitation zones have been produced. We find that, similar to electron induced emission events on the nightside, these zones are strongly organized by latitude and magnetic local time. The majority of the dayside events appear in the southern hemisphere and there is a strong enhancement observed centered about local dawn (06:00 LT). There is apparent poleward continuation of emission in the north, but very few events were observed on the duskward hemisphere. These results carry implications for Mercury’s magnetosphere by constraining zones of electron precipitation, for the exosphere as a potential source of exospheric species, and for surface science as an additional source of X‐ray fluorescence.Plain Language SummaryMercury has a magnetic field which is similar to Earth’s, and charged particles (electrons and protons) within it move in similar ways. At Earth, particles which reach the atmosphere generate the aurora borealis and aurora australis (northern and southern lights). At Mercury, there is no atmosphere so these particles reach the surface, where they produce X‐rays. At night, it is relatively simple to detect these X‐rays and map the regions where they are being produced, because the only source of X‐rays is the particles reaching the surface. During the day, however, the signal can easily be overwhelmed because at the same time X‐rays are being produced in response to illumination by the Sun. We have been able to tentatively isolate the X‐ray signal coming from charged particles during the day on Mercury by applying a filter which takes into account the brightness of the Sun in X‐rays at the same time. When the sun is dim in X‐rays but the surface is bright, we can be confident that the surface signal comes from charged particles, and vice versa.Key PointsWe have isolated electron‐induced X‐ray fluorescence on the dayside of Mercury to a confidence level dependent on solar X‐ray fluxThe areas of X‐ray emission show where electrons reach the surface, and clarify the interaction between Mercury’s magnetosphere and surfaceInstead of identifying specific events as solar‐ or electron‐induced, we gradually relax a solar filter to see the changes in precipitation