The Origins of Long‐Term Variability in Martian Upper Atmospheric Densities. Issue 3 (22nd March 2022)

• Record Type: Journal Article
• Authors: Fang, Xiaohua; Forbes, Jeffrey M.; Benna, Mehdi; Montabone, Luca; Curry, Shannon; Jakosky, Bruce
• Is Part Of: Journal of geophysical research. Volume 127:Issue 3(2022)
• Page Start: n/a
• Page End: n/a
• Publication Date: 2022-03-22
• Subjects: Mars -- upper atmosphere -- long‐term variability -- orbital effect -- solar EUV effect; Magnetospheric physics -- Periodicals; Space environment -- Periodicals; Cosmic physics -- Periodicals; Planets -- Atmospheres -- Periodicals; Heliosphere (Astrophysics) -- Periodicals; Geophysics -- Periodicals; 523.01
• DOI: 10.1029/2021JA030145
• Languages: English
• ISSNs: 2169-9380

Abstract

Abstract: We quantify and interpret the long‐term variability of dayside Martian upper thermosphere and lower exosphere densities within 180–275 km altitudes. Atmospheric CO2, N2, O, and Ar densities are from NASA Mars Atmosphere and Volatile EvolutioN (MAVEN) observations during the time period of 2015–2020 near solar minimum. These neutral measurements, together with contemporaneous solar irradiance measurements at Mars, enable disentanglement of the orbital effect (due to the annual Sun‐Mars distance change with solar longitude) and the solar extreme ultraviolet (EUV) effect in atmospheric density variations. The relative importance of these two effects, which is obtained using a statistical method of Dominance Analysis, reveals the competition between the indirect effect of solar infrared (via the upward coupling from the middle atmosphere) and the direct effect of solar EUV (due to local heating). Our results show that, unlike the orbital effect which is relatively constant at low altitudes and then decreases with increasing altitude, the solar EUV effect nearly monotonically increases. These two effects are comparable at high altitudes (about 240/270/205 km for CO2 /N2 /O). This analysis is extended to include long‐term exospheric mass density estimates near 400 km from Mars Global Surveyor and Mars Odyssey data, with a focus on representative solar cycle phases of solar minimum and maximum. It is found that near 400 km, the orbital effect is always a key driver regardless of the solar cycle phase, while the solar EUV effect plays a minor role during solar minimum and is greatly enhanced and slightly exceeds the orbital effect during solar maximum. Plain Language Summary: Observations of the Martian upper atmosphere, which is above ∼100 km altitude and is an important intermediate region between the lower and middle atmospheres below and the space environment above, have been especially scarce. Analyses of neutral density distributions in this region are critical for understanding the long‐term leakage of Mars' atmosphere into space. This paper is the first to analyze long‐term variations of specific atmospheric chemicals (CO2, N2, O, and Ar) within 180–275 km altitudes in Mars' atmosphere using measurements by the NASA Mars Atmosphere and Volatile EvolutioN mission. It is found that most of the long‐term variability in upper atmosphere densities are accounted for by heating due to local absorption of solar extreme ultraviolet (EUV) radiation, and by absorption of solar infrared (IR) radiation in the middle atmosphere which then exerts its influence on the overlying upper atmosphere. Long‐term variability in IR radiation and its associated effects are driven by Mars' eccentric (non‐circular) orbit around the Sun, whereas variability in EUV radiation reaching the planet originates in part at the Sun and is also subject to the orbital effect. The relative importance of these two effects is further partitioned, clearly showing their altitude and solar cycle dependences. Key Points: Orbital and solar extreme ultraviolet (EUV) effects on CO2, N2, Ar, and O densities within 180–275 km are quantified for the first time in Mars Atmosphere and Volatile EvolutioN measurements Orbital (solar EUV) effect decreases (increases) with altitude, reflecting indirect (direct) control of the upper atmosphere by solar infrared (EUV) Near 400 km the orbital effect is a key driver, and from solar min to solar max the solar EUV effect changes from secondary to primary