The Mars Topside Ionosphere Response to the X8.2 Solar Flare of 10 September 2017. Issue 16 (22nd August 2018) - Thiemann, E. M. B.
Abstract: On 10 September 2017, irradiance from a magnitude X8.2 solar flare impacted Mars while the Mars Atmosphere and Volatile EvolutioN (MAVEN) orbiter was characterizing the Mars upper atmosphere. This solar flare was the largest to occur during the MAVEN mission to date, nearly tripling the ionizing irradiance impacting Mars in tens of minutes, and provides an opportunity to study the planet's response to extreme irradiance changes. This letter reports in situ observations of the Mars topside ionosphere's response to this flare above 155 km made 1.67 hr after the flare soft X‐ray peak. The observed plasma density increase is higher than expected based solely on increased ionization, and the electron temperature decreases below 225 km; both effects can be explained by an expanded neutral atmosphere, which efficiently dissipates any flare‐induced heating of the thermal electrons at altitudes where CO2 is the dominant species. Further, the ion density and composition change significantly at both fixed altitude and pressure level, which can be explained by a change in the O:CO2 density ratio, highlighting the importance this ratio has in determining ionospheric structure. Plain Language Summary: On 10 September 2017, a large solar flare erupted from the Sun sending intense radiation into the upper atmosphere of Mars. This radiation ionized the gases in Mars's upper atmosphere, resulting in significant changes in its structure and composition. Because solar flares are short‐lived events, studying how the Mars atmosphere responds to them can unmask phenomena that may otherwise be hidden when the Sun varies more gradually. This letter reports the first in situ observations of the how the ions and electrons in the Mars upper atmosphere, above 155 km, change during solar flares. This region of the atmosphere interfaces with the space environment, where it can be stripped away and lost. The rate of loss is believed to be strongly dependent on the same radiation released by flares. Therefore, understanding how Mars responds to flares can provide insight into how its atmosphere evolved early in its history, when the Sun is believed to have produced larger flares more frequently, and Mars is believed to have had an atmosphere capable of supporting large amounts of liquid water. In addition to Mars researchers, these results will be of particular interest to those studying space weather, planetary atmospheres, and the habitability of exoplanets. Key Points: Ionizing EUV flux increased by 170% at the flare peak, causing changes in the observed (>150 km) plasma density, temperature, and composition Ionospheric changes are a result of an expanded neutral atmosphere, and the increased relative abundance of O at fixed pressure level Photochemical escape of O increased moderately for observations made 80 min after the flare peak