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, sev-eral profiles were measured at a constant solar zenith angle (ϰ) but at different longitudes. However, ϰ varied considerably during each data set and in this paper about 2000 profiles have been analysed to study the height (hm) and density (Nm) of the primary peak as a function of solar zenith angle. A significant decrease of Nm is seen with increasing ϰ, but hm starts increas-ing at ϰ 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。 in the equation Nm = N。(cos ϰ)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。 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.