Observations and Modeling of the Mars Low‐Altitude Ionospheric Response to the 10 September 2017 X‐Class Solar Flare

• Published in: Geophysical research letters Vol. 45; no. 15; pp. 7382 - 7390
• Main Authors: Xu, Shaosui, Thiemann, Ed, Mitchell, David, Eparvier, Frank, Pawlowski, David, Benna, Mehdi, Andersson, Laila, Liemohn, Michael W., Bougher, Stephen, Mazelle, Christian
• Format: Journal Article
• Language: English
• Published: Washington John Wiley & Sons, Inc 16-08-2018; American Geophysical Union
• Subjects: Accuracy; Altitude; Atmosphere; Atmospheric evolution; Atmospheric models; Augers; carbon Auger electrons; Corona; Electron transport; Energy spectra; Evolution; Fluxes; ion densities; Ion density (concentration); Ionization; Ionosphere; ionospheric response; Irradiance; Kinetic energy; Low altitude; Mars; Mars atmosphere; Modelling; Neutral particles; Photochemicals; Photochemistry; Photoelectrons; Photoionization; Photons; Planetary ionospheres; Sciences of the Universe; Solar corona; Solar energy; solar flare; Solar flares; Solar irradiance; Spectra; Statistical methods; Transport; Ultraviolet radiation; Wavelength; Mars; photoelectrons; solar flare; carbon Auger electrons; ionospheric response; ion densities
• ISSN: 0094-8276; 1944-8007
• DOI: 10.1029/2018GL078524

Solar extreme ultraviolet and X‐ray photons are the main sources of ionization in the Martian ionosphere and can be enhanced significantly during a solar flare. On 10 September 2017, the Mars Atmosphere and Volatile EvolutioN orbiter observed an X8.2 solar flare, the largest it has encountered to date. Here we investigate the ionospheric response before, during, and after this event with the SuperThermal Electron Transport model. We find good agreement between modeled and measured photoelectron spectra. In addition, the high photoelectron fluxes during the flare provide adequate statistics to allow us to clearly and repeatedly identify the carbon Auger peak in the ionospheric photoelectron energy spectra at Mars for the first time. By applying photochemical equilibrium, O
2+ and CO
2+ densities are obtained and compared with Mars Atmosphere and Volatile EvolutioN observations. The variations in ion densities during this event due to the solar irradiance enhancement and the neutral atmosphere expansion are discussed.
Plain Language Summary
Solar extreme ultraviolet and X‐ray photons are the main source of ionization in the Martian ionosphere, photoionizing the neutral particles and producing photoelectrons and ions. These short‐wavelength photon fluxes can be enhanced by a factor of a few to orders of magnitudes during a solar flare (the result of the rapid conversion of magnetic energy to kinetic energy in the solar corona). On 10 September 2017, the Mars Atmosphere and Volatile EvolutioN mission encountered the largest solar flare (X8.2) to date. The comprehensive measurements from Mars Atmosphere and Volatile EvolutioN provide us with an opportunity to evaluate the ionospheric response to this flare event in detail with models. In particular, we investigate the photoelectron flux and ion density response to the flare with an electron transport model. The modeled and measured photoelectron fluxes are in a good agreement. Ion density enhancement at a fixed altitude is from tens of percent to 1500% due to a combination of intensified solar photon fluxes and the heated and then expanded neutral atmosphere during this flare event.
Key Points
The modeled and measured photoelectron spectra are in good agreement during an X8.2 solar flare event
The carbon Auger peak is clearly and repeatedly identified in electron energy spectra of the Martian ionosphere for the first time
The ion density enhancement due to the flare at a fixed altitude is from tens to 1,500%