Model calculation of ionization efficiency in the Martian dayside ionosphere using MAVEN observations

Abstract

The ionization efficiency (η), which is defined as the ratio of the electron impact to the photon impact ionization rates, is calculated for the dayside Martian ionosphere using Mars Atmosphere Volatile EvolutioN (MAVEN) observations of neutral density and solar flux as input to the model. To calculate the secondary (electron impact) ionization rates, the photoelectron flux is computed using the Analytical Yield Spectrum (AYS) approach. Model calculations suggest that η should increase consistently as altitude decreases in the lower ionosphere. However, when the secondary ionization rates are calculated using the SWEA/MAVEN measured photoelectron flux, the ionization efficiency remains constant in the region 160–250 km. This behaviour is not in agreement with the theory of the altitude variation of ionization efficiency. The reason for this discrepancy is examined in detail. It is suggested that this disagreement is caused by the altitude-independent nature of MAVEN measured photoelectron flux. In two MAVEN deep dip orbits SWEA measured flux showed an altitudinal variation and the η curve estimated using these observations showed the variation in agreement with the theory. This suggests that the ionization efficiency calculated using SWEA/MAVEN measured photoelectron flux will show an altitude-dependent behaviour only when the instrument can resolve the altitudinal variation in the electron flux. We also calculated the ionization efficiency for two MAVEN dayside deep dip campaigns. The efficiency of CO2 and O showed an increase of 30 per cent and 60 per cent, respectively, at 130 km as compared to its value at 200 km.