Energy dependent response of the dayside Martian ionospheric electrons to solar forcing

Abstract

We study the response of Auger electrons, He-II photoelectrons, and thermal electrons in the dayside Martian ionosphere to the solar soft X-rays (SXR; 0.1–7 nm), 30.5 nm, and the extreme ultraviolet (EUV; 20–90 nm) irradiances, respectively. For this purpose, the suprathermal electron fluxes measured by the Solar Wind Electron Analyzer (SWEA) and densities of the thermal electrons measured by Langmuir Probe and Waves (LPW) instruments, both on the Mars Atmosphere and Volatile Evolution (MAVEN) mission, have been used. The data used in the present study span from January 2015 (Martian Year (MY) 32, solar longitude, Ls = 263°) to December 2019 (MY 35, Ls = 110°), which falls in the declining phase of solar cycle 24. The solar irradiances are taken from the extreme ultraviolet monitor instrument on MAVEN and also from the Flare Irradiance Spectral Model for Mars. The results of the present study show that the fluxes of the suprathermal electrons and the densities of the thermal electrons are drastically reduced during solar minimum. The Auger electrons show a significant correlation and have an almost linear relationship with the SXR irradiance; both of which are independent of altitude. The response of the He-II photoelectrons to 30.5 nm solar irradiance deviates slightly from the linear relationship, particularly in the altitude range of 225–350 km and the solar zenith angle (SZA) range of 45°-65°. At these altitude and SZA ranges, the thermal electrons show a power law dependence on the EUV irradiance. The responses of the He-II photoelectrons and thermal electrons to their respective solar irradiances decrease on either side of the altitude and SZA range of maximum response (225–350 km and 45°-65°, respectively). The energy dependent response of the electrons to the solar irradiances and their altitude and SZA variation are explained by considering their dependence on electron temperature, and ionization and the neutral heating efficiencies. The responses of the He-II photoelectrons and thermal electrons to their respective solar irradiances decrease near the terminator.