Near-terminator Martian ionosphere during sunspot cycle 23 from Mars Global Surveyor radio science measurements

Abstract

Radio Science experiment on the Mars Global Surveyor (MGS) has measured a large number of electron density pro-files in the near-terminator Martian ionosphere, from December 1998 to March 2005, thus covering rising, maximum and declining phase of sunspot cycle 23. More than a dozen data sets, EDS 1 to EDS 13, are now available at the public website and these sets contain profiles numbering from a few tens in some sets to several hundreds in others. On any one day, several profiles were measured at a constant solar zenith angle (x) but at different longitudes. However, x varied considerably during each data set and in this paper about 2000 profiles have been analysed to study the height (h_m) and density (N_m) of the primary peak as a function of solar zenith angle. A significant decrease of N_m is seen with increasing x, but h_m starts increas-ing at x above 80° only. To examine how close the Martian primary peak is to an ideal Chapman layer, values of exponent, k and sub-solar peak density, N_o in the equation N_m = N_o (cos x)^k were obtained by using the largest number of profiles em-ployed ever before. A value of 0.45 for the exponent k deduced is somewhat smaller than the value of 0.5 expected for an ideal Chapman layer. Value of N_o was found to be 1.75 × 10¹¹ m^-3. Anomalous features in the Mars ionosphere seen in the earlier MGS data are found to be present in the latest data also.