The Mars Topside Ionosphere Response to the X8.2 Solar Flare of 10 September 2017

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.

Significant Space Weather Impact on the Escape of Hydrogen From Mars

“…The solar Lyman‐α flux during the solar events was enhanced by ~6% (Thiemann et al, ). The brightness of Martian Lyman‐α observed in both inbound and outbound segments of orbit 5722 was enhanced above the modeled trend.…”

September 2017 Solar Flare Event: Rapid Heating of the Martian Neutral Upper Atmosphere From the X‐Class Flare as Observed by MAVEN

“…Solar events including flares can have significant impacts on the Martian thermosphere; this has significant implications for understanding how space weather events drive atmospheric escape (Brain et al, ; Curry et al, ; Jakosky et al, ; Jain et al, ; Thiemann et al, , ). These planetary ions can also precipitate back into the upper atmosphere and collide with neutrals, giving them enough energy to escape Mars' gravitational well—a process called sputtering (Johnson & Liu, ; Leblanc & Johnson, ).…”

“…O and CO densities saw a significant increase in both associated temperature and scale height (see Table ) and larger than those of Ar. This is likely due to photochemical changes (Thiemann et al, ), making it difficult to compute a temperature for O and CO at these altitudes. In these ranges the following heating and cooling for each species occurred: above ~180 km, collisions become increasingly rare, and the various species are not in thermal equilibrium.…”

“…The 10 September 2017 event was unique in that it is the strongest X‐class flare, at X8.1, observed since MAVEN arrived at Mars in October 2014 and the second largest in the current 11‐year cycle (Thiemann et al, ). Fortunately, neutral NGIMS observations and EUV observations were well timed to coincide with nominal periapsis science.…”

Effects of a Solar Flare on the Martian Hot O Corona and Photochemical Escape

“…FISM‐M is tailored based on the observations by MAVEN Extreme Ultraviolet Monitor (EUVM; Eparvier et al, ). This study uses flare irradiance estimates from Thiemann et al (), which improved the FISM‐M irradiance estimates using additional algorithms and Earth‐based measurements. For each case considered, the thermosphere and ionosphere simulations were conducted on a 5° × 5° latitude and longitude horizontal grid with a constant altitude resolution of 2.5 km from ~100 to ~300 km, which is approximately equivalent to 0.25 scale height spacing, to provide neutral plus ion densities, temperatures (neutral, ion, and electron), and neutral winds.…”