Photoionization Loss Of Mercury'S Sodium Exosphere: Seasonal Observations By Messenger And The Themis Telescope

We present the first investigation and quantification of the photoionization loss process to Mercury's sodium exosphere from spacecraft and ground-based observations. We analyze plasma and neutral sodium measurements from NASA's MESSENGER spacecraft and the THEMIS telescope. We find that the sodium ion (Na+) content and therefore the significance of photoionization varies with Mercury's orbit around the Sun (i.e., true anomaly angle: TAA). Na+ production is affected by the neutral sodium solar-radiation acceleration loss process. More Na+ was measured on the inbound leg of Mercury's orbit at 180 degrees-360 degrees TAA because less neutral sodium is lost downtail from radiation acceleration. Calculations using results from observations show that the photoionization loss process removes similar to 10(24) atoms/s from the sodium exosphere (maxima of 4 x 10(24) atoms/s), showing that modeling efforts underestimate this loss process. This is an important result as it shows that photoionization is a significant loss process and larger than loss from radiation acceleration.Plain Language Summary Mercury has a thin sodium collision-less atmosphere (i.e., an exosphere). A variety of processes add or subtract sodium particles to and from the exosphere. Photoionization is a loss process, and we investigate it in this paper by analyzing data from NASA's MESSENGER spacecraft and ground-based observations made by the THEMIS telescope. Mercury has an eccentric (noncircular) orbit, which means the planet's distance from the Sun changes throughout its orbit. This, first of all, affects how much sodium is lost due to acceleration of neutral sodium by radiation (i.e., how much sodium is accelerated away from Mercury by radiation from the Sun). This subsequently affects how much sodium is left to be photoionized. Therefore, the amount of sodium lost due to photoionization varies throughout a Mercury-year. We calculate that similar to 10(24) atoms/s of sodium are lost due to photoionization, and that it is a significant loss process in comparison to acceleration by radiation.