Solar Rotation Effects in Martian Thermospheric Density as Revealed by Five Years of MAVEN Observations. Issue 1 (17th January 2022)

• Record Type: Journal Article
• Authors: Hughes, Joseph; Gasperini, Federico; Forbes, Jeffrey M.
• Is Part Of: Journal of geophysical research. Volume 127:Issue 1(2022)
• Page Start: n/a
• Page End: n/a
• Publication Date: 2022-01-17
• Subjects: MAVEN -- thermosphere -- EUV flux -- density; Planets -- Periodicals; Geophysics -- Periodicals; 559.9
• DOI: 10.1029/2021JE007036
• Languages: English
• ISSNs: 2169-9097

Abstract

Abstract: The quasi‐periodic 27‐day solar rotation variation in extreme ultraviolet radiation represents the most prominent source of short‐term ( < $< $ 40‐day) solar‐driven variability in Mars' thermosphere (ca. 125–250 km). Despite recent advances in our understanding of solar‐driven variations in Mars' thermosphere density and composition associated with solar flares, orbital eccentricity, and solar cycle afforded by new Mars Atmosphere and Volatile EvolutioN (MAVEN) observations, relatively little is known about this response to short‐term periodic solar flux variability. This work presents a detailed investigation of the solar rotation effects on the whole Martian thermosphere by applying correlation analysis techniques to over five years of coincident MAVEN Neutral Gas and Ion Spectrometer and Extreme Ultraviolet Monitor observations. Least squares methods are used to estimate the response of C O 2 ${\mathrm{C}\mathrm{O}}_{2}$, Ar, and N 2 ${\mathrm{N}}_{2}$ densities to the quasi‐27‐day solar rotation variability over the 0–7 nm, 17–22 nm, 0–45 nm, and 117–125 nm spectral bands. Results clearly reveal the presence of prominent solar rotation effects in the Martian thermosphere density for all species, irradiance bands, and altitudes. These effects are strongest at higher altitudes (200–250 km), while the dependence of the slope on solar zenith angle depends on the species, with C O 2 ${\mathrm{C}\mathrm{O}}_{2}$ having a higher slope on the nightside, Ar having a higher slope on the dayside, and N 2 ${\mathrm{N}}_{2}$ having a different behavior depending on altitude. Plain Language Summary: Mars' thermosphere (125–250 km) connects the atmosphere with the near‐space environment and its primary energy input is solar Extreme Ultra Violet (EUV) radiation. Since the sun rotates with a period of 27 days and occasionally has active regions which emit more EUV radiation, the EUV received at Mars can also have a 27 day period. The Mars Atmosphere and Volatile EvolutioN orbiter has a EUV Monitor and a Neutral Gas and Ion Mass Spectrometer instrument measuring the abundance of neutrals and ions. This study uses both datasets and least squares methods to investigate the solar rotation effects on the entire Martian thermosphere over different conditions. Results reveal the presence of strong solar rotation effects with density increasing as EUV flux increases. These effects are strongest at higher altitudes (200–250 km) for all species. The relative strength of this effect on the dayside and nightside varies between species. These results provide valuable insights into density variability in Mars' aerobraking region. Better understanding these sources of variability will ultimately improve our ability to predict aerobraking performance that will aid mission planners in orbit determination and entry into the atmosphere by landers, both manned and unmanned. Key Points: Large (∼50%) quasi‐27‐day solar rotation effects are revealed in Mars' thermosphere density (150–250 km) Solar rotation effects are strongest at higher altitudes (200–250 km) CO2, the most abundant species, has a higher slope on the nightside than the dayside