Numerical simulations of the ionosphere of Mars during a solar flare

Abstract

Electron densities in planetary ionospheres increase substantially during solar flares in response to the increased solar irradiance at soft X‐ray and extreme ultraviolet wavelengths. Here we modify an existing model of the ionosphere of Mars to incorporate time‐dependent solar irradiances and use it to simulate ionospheric conditions during the X14.4 and M7.8 solar flares of 15 and 26 April 2001, respectively. Simulations were validated by comparison to Mars Global Surveyor radio occultation measurements of vertical profiles of ionospheric electron density. Adjustments to the model's representation of the neutral atmosphere were required to adequately reproduce the observations before and during these solar flares. An accurate representation of electron‐impact ionization, an important process in the lower ionosphere of Mars, is required in order to adequately simulate the doubling of electron densities that can occur in the lower ionosphere of Mars during a solar flare. We used the W‐value representation of electron‐impact ionization, in which the number of ion‐electron pairs created per photon absorbed equals the ratio of the difference between photon energy and the ionization potential of carbon dioxide to the W‐value. A range of possible W‐values for Mars have been suggested in the literature, and a value of 28 eV led to the best reproduction of flare‐affected observations. Simulated enhancements in the electron density are largest and persist the longest in the M1 region. We predict that the peak electron density in the M1 region can exceed that of the M2 region for short periods during intense solar flares.