PROBA2 LYRA Occultations: Thermospheric Temperature and Composition, Sensitivity to EUV Forcing, and Comparisons With Mars. Issue 7 (20th July 2021)

• Record Type: Journal Article
• Authors: Thiemann, Edward M. B.; Dominique, Marie
• Is Part Of: Journal of geophysical research. Volume 126:Issue 7(2021)
• Page Start: n/a
• Page End: n/a
• Publication Date: 2021-07-20
• Subjects: thermosphere -- occultations -- EUV -- space weather -- comparative planetology; Magnetospheric physics -- Periodicals; Space environment -- Periodicals; Cosmic physics -- Periodicals; Planets -- Atmospheres -- Periodicals; Heliosphere (Astrophysics) -- Periodicals; Geophysics -- Periodicals; 523.01
• DOI: 10.1029/2021JA029262
• Languages: English
• ISSNs: 2169-9380

Abstract

Abstract: A method for retrieving temperature and composition from 150 to 350 km in Earth's thermosphere using total number density measurements made via extreme ultraviolet (EUV) solar occultations by the Project for OnBoard Autonomy 2/Large Yield Radiometer (PROBA2/LYRA) instrument is presented. Systematic and random uncertainties are calculated and found to be less than 5% for the temperature measurements and 5%–20% for the composition measurements. Regression coefficients relating both temperature and the [O]/[N2 ] abundance ratio with EUV irradiance at 150, 275, and 350 km are reported. Additionally, it is shown that the altitude where [O] equals [N2 ] decreases with increasing solar EUV irradiance, an effect attributed to thermal expansion. Temperatures from 2010 to 2017 are compared with estimates from the MSIS empirical model and show good agreement at the dawn terminator but LYRA is markedly cooler at the dusk terminator, with the MSIS‐LYRA temperature difference increasing with solar activity. Anthropogenic cooling can explain this discrepancy at periods of lower solar activity, but the divergence of temperature with increasing solar activity remains unexplained. LYRA measurements of the exospheric temperature sensitivity to EUV irradiance are compared with contemporaneous measurements made at Mars, showing that the exospheric temperature at Mars is approximately half as sensitive to EUV variability as that of Earth. Plain Language Summary: The Large Yield Radiometer (LYRA) instrument onboard the Project for OnBoard Autonomy 2 (PROBA2) satellite measures the structure of the upper atmosphere by tracking the absorption of sunlight as it passes through the atmosphere as the Sun sets or rises as viewed from the satellite. This paper presents a method for determining atmospheric temperature to a high degree of accuracy from 150 to 350 km from the PROBA2/LYRA measurements. Additionally, a method for determining the relative abundance of atomic Oxygen and molecular Nitrogen is presented. Both temperature and relative abundance are known to change with changing extreme ultraviolet (EUV) radiation from the Sun. The dependence of these values on solar EUV radiation is reported, and is shown to be different from predictions of the MSIS model, frequently used to estimate atmospheric temperature and composition. Additionally, these results for Earth are compared with similar measurements made at Mars and show that the Earth's atmosphere heats twice as efficiently from solar EUV radiation as the Mars atmosphere. Key Points: Temperature and composition from 150 to 350 km from 2010 to 2017 are presented Dusk temperatures are markedly cooler than MSIS predictions Mars exospheric temperature is half as sensitive to extreme ultraviolet variability as that of Earth