PROBA2 LYRA Occultations: Thermospheric Temperature and Composition, Sensitivity to EUV Forcing, and Comparisons With Mars

Abstract

The Earth's thermosphere, the upper region of its neutral atmosphere, extends from the temperature minimum at the mesopause (85-100 km;Xu et al., 2007) to geospace. Spanning the regions of both the ionosphere and Low Earth Orbit, the thermosphere and its state have important implications for space weather (Schunk & Sojka, 1996). Photochemical and dynamical processes in the thermosphere change its temperature, density, and composition, directly influencing satellite drag and trans-ionospheric electromagnetic propagation. Geomagnetic storms provide perhaps the most dramatic example of the interplay between the magnetosphere, thermosphere and ionosphere, and the resulting impact on space weather (e.g., Fuller- Rowell et al., 1994;Mayr & Volland, 1973). Currents induced in the magnetosphere cause intense heating in the lower thermosphere, driving an upwelling of N 2 rich air. Satellite drag increases significantly over the poles, resulting in the temporary or even permanent loss of satellite trajectory knowledge. In the ionosphere, the

Solar Rotation Effects in Earth's and Mars' Thermospheric Densities as Revealed by Concurrent MAVEN, Swarm‐C, and GOES Observations

“…The noted m and r values of ∼2.63 ± 0.24 (∼1.70 ± 0.20) and ∼0.67 (∼0.56) at Earth (Mars), respectively, indicate that the terrestrial response to the solar rotation in flux is significantly stronger than that at Mars for a similar density level (Table 1 and Figure 4 contain more quantitative analyses). This result confirms previous findings (e.g., Bougher et al., 1999, 2015, 2017; Forbes et al., 2006, 2007; Keating & Bougher, 1987, 1992; Thiemann & Dominique, 2021), reporting reduced sensitivities to solar irradiance for Mars' thermosphere generally explained by the increased importance of CO 2 cooling in damping solar EUV energy at Mars. The lower correlation for Mars may be due to short‐term density variability not associated with solar effects.…”

“…Consistent with our understanding of the increased importance of CO 2 cooling in damping solar EUV energy at Mars, Earth's daytime thermospheric density sensitivity to flux is found to be ∼10%–50% larger than that of Mars for a similar density level. This result is in excellent agreement with earlier studies (e.g., Forbes et al., 2006) and the recent work by Thiemann and Dominique (2021) that showed Mars' exospheric EUV temperature sensitivities to be about 0.47 as sensitive to EUV variability as those at Earth. Similar to previous Martian (e.g., Hughes et al., 2022) and terrestrial (e.g., Thiemann, Dominique, et al., 2017) studies, Mars' thermospheric density response to EUV forcing is shown to increase with altitude up to ∼240 km (with m values up to ≃2.8%/%).…”

“…This different response is discussed to be mainly associated with a larger damping of solar EUV energy at Mars due to increased CO 2 cooling. These results were more recently confirmed by Thiemann and Dominique (2021), who showed Mars' exospheric EUV temperature sensitivities to be about 0.47 times as sensitive to EUV variability as those at Earth.…”

“…Forbes et al. (2006) and related studies (e.g., Bougher et al., 1999; Forbes et al., 2007; Keating & Bougher, 1987, 1992; Thiemann & Dominique, 2021) established that exospheric temperatures at Mars are about 30%–50% as responsive as Earth to the ∼27‐day solar rotation variation in EUV. This different response is discussed to be mainly associated with a larger damping of solar EUV energy at Mars due to increased CO 2 cooling.…”

“…Forbes et al (2006) employed simultaneous density observations from the Challenging Minisatellite Payload and Mars Global Surveyor satellites to investigate Earth's and Mars' thermospheric responses to the solar rotation variation in flux using the F10.7 solar radio flux as a proxy for EUV and revealed a response that is twice as large for Earth than for Mars. Forbes et al (2006) and related studies (e.g., Bougher et al, 1999;Forbes et al, 2007;Keating & Bougher, 1987, 1992Thiemann & Dominique, 2021) established that exospheric temperatures at Mars are about 30%-50% as responsive as Earth to the ∼27-day solar rotation variation in EUV. This different response is discussed to be mainly associated with a larger damping of solar EUV energy at Mars due to increased CO 2 cooling.…”