Photoionization Loss of Mercury’s Sodium Exosphere: Seasonal Observations by MESSENGER and the THEMIS Telescope
Jasinski, Jamie M.; Cassidy, Timothy A.; Raines, Jim M.; Milillo, Anna; Regoli, Leonardo H.; Dewey, Ryan; Slavin, James A.; Mangano, Valeria; Murphy, Neil
2021-04-28
Citation
Jasinski, Jamie M.; Cassidy, Timothy A.; Raines, Jim M.; Milillo, Anna; Regoli, Leonardo H.; Dewey, Ryan; Slavin, James A.; Mangano, Valeria; Murphy, Neil (2021). "Photoionization Loss of Mercury’s Sodium Exosphere: Seasonal Observations by MESSENGER and the THEMIS Telescope." Geophysical Research Letters 48(8): n/a-n/a.
Abstract
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.Plain Language SummaryMercury 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 ∼1024 atoms/s of sodium are lost due to photoionization, and that it is a significant loss process in comparison to acceleration by radiation.Key PointsPhotoionization 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 legPublisher
Cambridge University Press Wiley Periodicals, Inc.
ISSN
0094-8276 1944-8007
Other DOIs
Types
Article
Metadata
Show full item recordCollections
Remediation of Harmful Language
The University of Michigan Library aims to describe library materials in a way that respects the people and communities who create, use, and are represented in our collections. Report harmful or offensive language in catalog records, finding aids, or elsewhere in our collections anonymously through our metadata feedback form. More information at Remediation of Harmful Language.
Accessibility
If you are unable to use this file in its current format, please select the Contact Us link and we can modify it to make it more accessible to you.