An observational study of the influence of solar zenith angle on properties of the M1 layer of the Mars ionosphere

Abstract

AbstractThe variations in peak properties of the M1 layer (the lower photochemical plasma layer) with solar zenith angle (SZA) are important relationships for understanding the physical processes which control this region of the Mars ionosphere. The behavior of the M1 layer has been poorly characterized to date. Here we introduce an automated and repeatable method for determining properties of the M1 and M2 layers simultaneously in 5600 Mars Global Surveyor radio occultation profiles of dayside electron density. The results support previous findings for M1 and M2 subsolar peak densities and the dependence of peak densities on solar zenith angle. The ratio of M1 peak density to M2 peak density remains constant at 0.4 for 70° <SZA< 90°, in contrast with previous numerical simulations. The M1 altitude increases with solar zenith angle with a lengthscale, L1 = 2.5 km, which is about half that of the M2 layer, L2 = 5.2 km, indicating that the two layers become increasingly separated at high solar zenith angles. The vertical width of the M1 layer, H1, decreases from 7 km to 5 km as solar zenith angle increases from 70° to 90°, whereas the vertical width of the M2 layer, H2, increases from 10 km to 14 km. The prediction of ideal Chapman theory that both the widths Hi and the lengthscales Li equal the neutral scale height is not supported by observations. These findings provide meaningful observational constraints for numerical models, which are known to have trouble reproducing observations and observed trends associated with the M1 layer.