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

Abstract

Citation:Fallows, K., P. Withers, and M. Matta 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• , 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 2 = 5.2 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 70• to 90• , 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.

Recovery and validation of Mars ionospheric electron density profiles from Mariner 9

“…Other workers using electron density profiles from Mars Global Surveyor have found similar exponents (Fallows et al 2015;Fox and Yeager 2006;Liao et al 2006). The fitted subsolar M1 density is also comparable to previous results (Fallows et al 2015). The result reported here confirms those earlier findings using an independent dataset and also demonstrates that the Squares highlight data points excluded from fits.…”

“…For instance, a fit of peak density, N m , to Ch −k , where Ch is a dimensionless function that reduces to sec SZA for small SZAs and plane-parallel geometry (Chapman 1931a, b;Smith and Smith 1972;Withers 2009b), finds a best-fit subsolar peak density N 0 of (2.07 ± 0.03) × 10 5 cm −3 and exponent k of 0.56 ± 0.01. The fitted subsolar peak density is also comparable to previous results (Fallows et al 2015).…”

“…The M1 peak density is proportional to Ch −k where k = 0.50 ± 0.03. Other workers using electron density profiles from Mars Global Surveyor have found similar exponents (Fallows et al 2015;Fox and Yeager 2006;Liao et al 2006). The fitted subsolar M1 density is also comparable to previous results (Fallows et al 2015).…”

“…Our analysis focused on topics that had not previously been addressed using these profiles, namely the effects of a dust storm on the topside ionosphere, the morphology of the topside ionosphere, the behavior of the M1 layer, and possible meteoric layers. The M1 layer is visible in electron density profiles as a shoulder approximately 25-30 km below the main M2 layer (Fallows et al 2015). As these topics have recently been addressed using other datasets, the analysis reported here helps to place the recovered Mariner 9 electron density profiles in the context of current ionospheric studies.…”

“…The dependence of the M1 peak electron density on solar zenith angle has been studied recently by several workers (Fallows et al 2015;Fox and Yeager 2006;Liao et al 2006). However, all used the same dataset from Mars Global Surveyor.…”