Variability in ionospheric total electron content at Mars

The Mars Express (MEX) mission includes a multi-purpose radio instrument called the Mars Advanced Radar for Subsurface and Ionospheric Sounding (MARSIS). When used in its ionospheric-penetrating subsurface sounder (SS) radar mode, a by-product of the MARSIS observations is the ray-path-integral of electron densities, called the total electron content (TEC). We have used the initial TEC database of approximately 1.2 million TEC values spanning the period June 2005 to September 2007 to study the basic characteristics of TEC morphology and variability. We find quantitative agreement between the TEC values measured and those computed from model simulations of global diurnal behavior. With the basic photo-chemistry of the martian ionosphere a well understood process, it is the departures from average conditions that need specification and modeling. Here we use MARSIS TEC to do this quantitatively. We explore the specification of variability using different ways to define it: standard deviations from sample averages versus departures from control curves. For global studies, we computed the standard deviation (sigma in %) of mean values of TEC (in TECU of 10(15) e(-)/m(2)) sorted by latitude, longitude, solar zenith angle (SZA), local time, season, and locations with/without strong crustal magnetic fields (50 nT at 150 km). For daytime conditions (SZA < 75 degrees), the global average (TEC) is similar to 6 TECU with sigma = similar to 20%, while for nighttime (SZA > 105 degrees) (TEC) is similar to 0.3 TECU with sigma = similar to 75%. Daytime variability is enhanced in the latitude region 0-30 degrees S, a pattern that needs validation by later observations before its source can be identified. Nighttime variability is noticeably larger in regions of strong crustal magnetic fields (B)-an effect noted by previous authors. For regional studies, high resolution latitude patterns of variability in the southern hemisphere - within the longitude sector 150-210 degrees of strong crustal-B values - were computed as percentage changes with respect to zonally-averaged patterns outside the region of interest. We present evidence for the first time of B-fields affecting the variability of the daytime ionosphere by small amounts (similar to +/- 5%). Under nighttime conditions, the B-field associated variability is similar to +/- 20%. The results also reveal an anti-correlation between daytime and nighttime variability ordered by the inclination angle (I) of the B-fields. TEC variability is greater as I approaches vertical at night, but higher during the day (by smaller amounts) where I approaches horizontal patterns. (C) 2013 Elsevier Ltd. All rights reserved.