Total electron content in the Martian atmosphere: A critical assessment of the Mars Express MARSIS data sets

Abstract

The total electron content (TEC) is one of the most useful parameters to evaluate the behavior of the Martian ionosphere because it contains information on the total amount of free electrons, the main component of the Martian ionospheric plasma. The Mars Express Mars Advanced Radar for Subsurface and Ionosphere Sounding (MARSIS) radar is able to derive TEC from both of its operation modes: (1) the active ionospheric sounding (AIS) mode and (2) the subsurface mode. TEC estimates from the subsurface sounding mode can be computed from the same raw data independently using different algorithms, which should yield similar results. Significant differences on the dayside, however, have been found from two of the algorithms. Moreover, both algorithms seem also to disagree with the TEC results from the AIS mode. This paper gives a critical, quantitative, and independent assessment of these discrepancies and indicates the possible uncertainty of these databases. In addition, a comparison between the results given by the empirical model of the Martian ionosphere developed by Sánchez-Cano et al. (2013) and the different data sets has been performed. The main result is that for solar zenith angles higher than 75°, where the maximum plasma frequency is typically small compared with the radar frequencies, the two subsurface algorithms can be confidently used. For solar zenith angles less than 75°, where the maximum plasma frequency is very close to the radar frequencies, both algorithms suffer limitations. Nevertheless, despite the solar zenith angle restrictions, the dayside TEC of one of the two algorithms is consistent with the modeled TEC.

Annual variations in the Martian bow shock location as observed by the Mars Express mission

“…Both modes, although through different means, can characterize the ionosphere (the effective obstacle to the solar wind flow) through its total electron content (TEC). The TEC is a measurement of the number of free electrons in an atmospheric column with vertical extent set by two altitudes and a cross section of 1 m 2 [e.g., Sánchez‐Cano et al , ]. The topside ionospheric TEC (via MARSIS‐AIS) is obtained through numerical integration of the vertical electron density profiles with altitude [e.g., Sánchez‐Cano et al , , ].…”

“…The topside ionospheric TEC (via MARSIS‐AIS) is obtained through numerical integration of the vertical electron density profiles with altitude [e.g., Sánchez‐Cano et al , , ]. The TEC of the entire atmosphere is retrieved as a by‐product of MARSIS‐SS mode radar signal distortions caused by the ionosphere [ Safaeinili et al , ; Mouginot et al , ; Cartacci et al , ; Sánchez‐Cano et al , , ]. Our study uses daily averaged TEC measurements of the entire atmosphere from this latter method at a solar zenith angle (SZA) of 85° .…”

Solar cycle variations in the ionosphere of Mars as seen by multiple Mars Express data sets

“…Note that the steps between periods in the modeled TEC are an artifact of dividing the interval into four subintervals. Taking into account that the uncertainty of the plasma frequency and altitude of the AIS data set is typically 3% or less and 6.9 km, respectively, and that the sensitivity of the derived TEC from subsurface mode is about 3.8 · 10 14 m −2 in the dayside [ Sánchez‐Cano et al , , and references there], the agreement between them is good. This comparison demonstrates that the empirically modeled scale height of each period from the AIS data (section 3.1) is consistent with independent empirical observations, i.e., subsurface‐derived TEC.…”

Variaciones de la ionosfera de Marte debidas al ciclo solar

“…Subsurface TEC data have, however, daylight limitations as they are less trustable in the full dayside when the MARSIS carrier frequencies are close to the maximum plasma frequency of the ionosphere. In these cases, a signal degradation is manifest (Sánchez-Cano et al, 2015a). In this study, subsurface TEC is analysed through the Cartacci et al (2013) algorithm, only with data that fit the Signal-toNoise Ratio (SNR) condition of SNR>25 dB.…”

“…This secondary layer is typically located at around 110-115 km altitude (e.g. Schunk and Nagy, 2009;Sánchez-Cano, 2014), and is considerably weaker than the main peak but it is not negligible since it contributes to about 10% of the Total Electron Content (Sánchez-Cano et al, 2015a). Photochemical processes control the behaviour of the two main ionospheric layers at Mars up to 170-200 km altitude (e.g.…”