ALBERT

Description: The M1 layer of the Mars ionosphere is one of its most significant features, second only to the M2 layer. Observations have shown how the physical properties of this layer depend on solar zenith angle (SZA) and solar irradiance, but these trends have not yet been explored in detail by numerical simulations. Hence the full implications of the observational findings for the M1 layer's behavior have not been established. Here we use the Boston University Mars Ionosphere Model to simulate the M1 layer over a period of six months. In order to adequately reproduce the SZA-dependence of the observed M2 peak density, an ad hoc isothermal electron temperature profile was required. This representation was motivated by detailed energy balance calculations thatpredict relatively small variations in electron temperature at the M2 peak. We find several model results consistent with observations: the simulated M1 peak density is effectively proportional to Ch(SZA) –0.5 , where Ch is the Chapman function; the ratio of M1 to M2 peak electron densities is independent of SZA; the simulated M1 peak altitude decreases with increasing solar irradiance; the simulated difference in altitude between the M1 and M2 layers increases with SZA at the observed rate. Due to limitations in the assumed neutral atmosphere, the simulated increase in M1 peak altitude with increasing solar zenith angle is significantly greater than observed. In both simulations and observations, limitations in representing the width of the M1 layer prevent meaningful comparisons and connections to the neutral scale height.

Description: The variations in peak properties of the M1 layer (the lower photochemical plasma layer) on 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 70º 〈 SZA 〈 90º, in contrast with previous numerical simulations. The M1 altitude increases with solar zenith angle with a lengthscale, L 1 = 2.5 km, which is about half that of the M2 layer, L 1 = 2.5 km, indicating that the two layers become increasingly separated at high solar zenith angles. The vertical width of the M1 layer, H 1 , decreases from 7 km to 5 km as solar zenith angle increases from 70º to 90º, whereas the vertical width of the M2 layer, H 2 , increases from 10 km to 14 km. The prediction of ideal Chapman theory that both the widths H i and the lengthscales L i 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.