Ionospheric Electron Densities at Mars: Comparison of Mars Express Ionospheric Sounding and MAVEN Local Measurements

Abstract

We present the first direct comparison of Martian ionospheric electron densities measured by the Mars Advanced Radar for Subsurface and Ionospheric Sounding (MARSIS) topside radar sounder on board the Mars Express spacecraft and by the Langmuir Probe and Waves (LPW) instrument on board the Mars Atmosphere and Volatile Evolution Mission (MAVEN) spacecraft. As low electron densities are not measured by MARSIS due to the low power radiated at low sounding frequencies, MARSIS electron density profiles between the local electron density and the first data point from the ionospheric sounding (≈104 cm−3) rely on an empirical electron density profile shape. We use the LPW electron density measurements to improve this empirical description and thereby the MARSIS‐derived electron density profiles. We further analyze four coincident events, where the two instruments were measuring within a 5° solar zenith angle interval within 1 h. The differences between the electron densities measured by the MARSIS and LPW instruments are found to be within a factor of 2 in 90% of measurements. Taking into account the measurement precision and different locations and times of the measurements, these differences are within the estimated uncertainties.

Characterizing Average Electron Densities in the Martian Dayside Upper Ionosphere

“…The inversion of MARSIS ionospheric traces is thus intrinsically ambiguous, and different electron density profiles are obtained depending on the assumed electron density profile shape at low densities undetectable by the radar sounding. There are two basic options which allow us to minimize related errors and obtain reliable electron density profiles: (i) A realistic shape of the electron density profile in the region unaccessible by the ionospheric sounding can be employed (Němec, Morgan, & Gurnett, ; Němec et al, ). (ii) Only electron density profiles obtained at not too high spacecraft altitudes can be used.…”

“…In this regard, one should be, however, aware of the discrete sounding frequency steps of the instrument, which result in the very ends of the ionospheric traces not observed by MARSIS. Although this is partly suppressed by a requirement of the time delay at the maximum frequency of the ionospheric trace to be larger than the time delay at the sounding frequency just below (Němec et al, ), the parts of the electron density profiles in the very vicinity of the ionospheric peak are still missing. Considering the relative sounding frequency steps Δ f / f ≈0.02, along with the frequency bandwidths of the sounding pulse and the reception channel, we can estimate that the peak frequencies are on average underestimated by less than about 1%.…”

“…Having constructed the empirical model of the topside ionospheric densities based on the MARSIS radar sounding data set, it is of interest to compare its results with other available electron density measurements. This is particularly important as the evaluation of electron density profiles from the MARSIS radar sounding has to rely on additional assumptions and empirical profile shapes at higher altitudes (Morgan et al, ; Němec, Morgan, & Gurnett, ; Němec et al, ). A comparison of the model performance with local electron densities evaluated based on electron plasma oscillations excited by MARSIS (Duru et al, ) is shown in Figure .…”

“…A distribution of the ratios between the measured and model electron densities is shown in Figure b. While the MARSIS data obtained primarily during the postnoon local time do not allow for such an analysis, there appears to be a systematic difference between MAVEN electron densities measured in the prenoon and postnoon local time sector (Benna et al, ; Fowler et al, ; Němec et al, ). We have thus plotted the results obtained for the Mars‐centered solar orbital (MSO) local time interval 6–12 hr (“morning”) by the blue line, and the results obtained for the MSO local time interval 12–18 hr (“afternoon”) by the red line.…”

Ion‐Neutral Coupling in the Upper Atmosphere of Mars: A Dominant Driver of Topside Ionospheric Structure

“…At the altitudes above the main ionospheric peak (which can vary between ~135 and 150 km), the timescale for diffusion and other plasma dynamics dominate over photochemical timescales, and the plasma becomes more responsive to the complex electric field and magnetic topological environment unique to Mars (Matta et al, ; Mendillo et al, ). At these upper altitudes, ionospheric plasma is also affected by the solar wind and is most susceptible to escape into outer space (Dubinin et al, ; Ledvina et al, ; Ma et al, ; Nemec et al, ). In addition to techniques that measured the lower ionosphere, the upper atmosphere of Mars has been measured for over a solar cycle using topside ionospheric sounding (Picardi et al, ).…”