OASIS

Presentation Abstract

Title	Comparisons And Simulations Of Same-day Observations Of The Ionosphere Of Mars By Radio Occultation Experiments On Mars Global Surveyor And Mars Express
Author Block	Paul Withers¹, A. Lollo¹, M. Mendillo¹, M. Paetzold², S. Tellmann² ¹Boston University, ²University of Cologne, Germany.
Abstract	Radio occultation experiments on the Mars Global Surveyor (MGS) and Mars Express (MEX) spacecraft made same-day measurements of vertical electron density profiles in the martian ionosphere between 8 December 2004 and 4 January 2005. The MGS measurements occurred at 66-74N, 79-75 degrees solar zenith angle (SZA), and 4.7 to 5.7 hours local solar time (LST). The MEX measurements occurred at 22-60S, 77-90 degrees SZA, and 7.5 to 8.7 hours LST. These datasets enable controlled investigations of how ionospheric properties, such as the altitudes, shapes and peak electron densities of the main and lower ionospheric layers (M2 and M1, respectively), depend on atmospheric properties. Solar irradiance will not differ between the northern and southern hemispheres, but the chemical composition and thermal structure of the neutral atmosphere will. Spatial variations in the M1 layer are of particular interest. Direct photoionization by solar photons, here soft X-rays whose flux varies on short timescales, accounts for only part of plasma production in the M1 layer; electron impact ionization due to energetic photoelectrons produces a significant proportion of the plasma. The M2 layer is maintained by photoionization by less variable solar extreme-ultraviolet photons. Consequently, the M1 layer is significantly less well-understood than the M2 layer and published simulations of the M1 layer have not yet reproduced its behaviour accurately. After discussing the results of our analyses of these datasets, we shall present numerical simulations of the ionosphere. The purpose of these simulations is to investigate what variations in assumed atmospheric state and assumed parameterization of secondary ionization are required to reproduce the observed hemispheric differences in the ionosphere.