Martian electron density profiles retrieved from Mars Express dual-frequency radio occultation measurements

The S- and X-band dual-frequency Doppler radio occultation observations obtained by the Mars Express Radio Science (MaRS) experiments are reduced in this study. A total of 414 Martian electron density profiles are retrieved covering the period from DOY 93 2004 to DOY 304 2012. These observations are well distributed over both longitude and latitude, with Sun-Mars distance varying from 1.38 AU to 1.67 AU, the solar zenith angle (SZA) ranging from 52 degrees to 122 degrees. Due to the improved vertical resolution for the MaRS experiments, the vertical structures of the retrieved profiles appear to be more complicated than those revealed by early radio occultation experiments. Dayside electron density profiles have primary peaks (M2) typically around 130 km and secondary peaks (M1) around 110 km. Nightside electron density profiles are highly variable, many of which do not have double layer structures. Both the dayside and nightside electron density profiles reveal some atypical features such as topside layering above M2 and bottom-side layering below M1. The former likely represent the plasma fluctuations in response to the solar wind (SW) interactions with the Martian ionosphere, whereas the latter is thought to be induced by the meteoric influx. We fit the peak electron density of profiles up to terminator with a simple power relation (N-m = N-0/ch(k)(x)) with the best-fit subsolar peak electron density being N-0 = (1.499 +/- 0.002) x 10(5) cm(-3), and the best-fit power index being k = 0.513 +/- 0.001. The measured total electron content (TEC) is obtained by integrating the observed electron density profile vertically from 50 km to 400 km, which is then compared with the ideal TEC computed from the one-layer Chapman model. We find that the one-layer Chapman model can generally underestimate the measured TEC up to similar to 0.1 TECU (1 TECU = 1.0 x 10(16) m(-2)) for 55 degrees < SZA < 90 degrees and up to similar to 0.05 TECU for 90 degrees < SZA < 120 degrees. The dayside TEC (SZA <= 75 degrees) varies from 0.1 to 0.6 TECU, whereas the nightside TEC (SZA >= 105 degrees) is usually below 0.2 TECU. Several large TEC values can be found in the terminator region. The corresponding slab thickness varies from 40 km-60 km for the dayside and 40 km-140 km for the nightside, with an average value of 61 km. (C) 2015 COSPAR. Published by Elsevier Ltd. All rights reserved.