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°–360° TAA because less neutral sodium is lost downtail from radiation acceleration. Calculations using results from observations show that the photoionization loss process removes ∼1024 atoms/s from the sodium exosphere (maxima of 4 × 1024 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.
Key Points Photoionization can be a significant loss process to the sodium exosphere with peak loss estimates of 4 × 1024 atoms/sThe photoionization loss process of Mercury's sodium exosphere varies throughout the planet's orbit around the SunMore sodium is lost due to photoionization on the inbound leg (true anomaly angle of 180°–360°) of Mercury's orbit than the outbound leg