Intensity of nightside MARSIS AIS surface reflections and implications for low‐altitude ionospheric densities

Abstract

Spacecraft radar sounding signals at frequencies higher than the ionospheric peak plasma frequency are not reflected by the ionosphere. Instead, they make it to the ground where they are reflected by the planetary surface. We analyze the intensity of the surface reflections measured by the Mars Advanced Radar for Subsurface and Ionosphere Sounding (MARSIS) ionospheric radar sounder on board the Mars Express spacecraft. Apart from the surface reflectivity and the spacecraft altitude, the detected intensity of surface reflections is controlled primarily by the signal attenuation during the ionospheric propagation. We focus on the nightside region, where the ionospheric densities in the main layer are too low to cause a significant attenuation and allow sampling of the surface reflections at frequencies down to 3 MHz. The attenuation occurs mainly at altitudes below 100 km, where the electron-neutral collision frequency is a maximum. The intensity of surface reflections can thus serve as a proxy for electron densities at low altitudes not accessible by the direct ionospheric radar sounding. We analyze the intensity of surface reflections as a function of relevant controlling parameters. The intensity of surface reflections is lower at higher solar zenith angles on the nightside and during the periods of larger solar activity. Moreover, it exhibits a seasonal variation that is related to the dust storm occurrence. The intensity of surface reflections is lower in areas of closed magnetic field lines, suggesting that nightside electron densities behave rather differently at low altitudes than at higher altitudes. This is confirmed by comparison with simultaneous observations of the main ionospheric layer.

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

“…In this season, as Mars approaches the Sun, increased solar irradiance and temperatures lead to a warmer and more circulated Martian atmosphere, which then lifts dust off the surface of Mars and causes the so-called dust storms. These dust storms have been previously shown to interact with the upper atmosphere and ionosphere of Mars in various ways, including the possibility of modulating the neutral densities and TEC [e.g., Wang and Nielsen, 2004;Mouginot et al, 2008;Liemohn et al, 2012;Withers, 2009;Withers and Pratt, 2013;Withers et al, 2015;Xu et al, 2014;Haider et al, 2015;Němec et al, 2015]. However, we do not draw any further conclusions on how the dust storms could directly impact the location of the Martian bow shock and leave such an investigation to a future study.…”

Origin of the Extended Mars Radar Blackout of September 2017

“…In the case of space weather events, it is not clear whether the main sources of ionization are solar wind electrons or protons. Most of the previous studies suggested that energetic solar wind protons seem to be responsible (e.g., Morgan et al, ; Němec et al, ; Sheel et al, ). However, Ulusen et al () found that in four of the six events that they studied, an increase in electron density below 100 km occurred caused by electrons of 10–20 keV energy, which is consistent with the event in the present study.…”

Precipitation of Hydrogen Energetic Neutral Atoms at the Upper Atmosphere of Mars

“…Modern UV spectrometers might have sufficient sensitivity to identify such an aurora. The ionization and electron stripping processes caused by the precipitating H‐ENAs might contribute to the increased ionization in the lower ionosphere implied by the ground reflection signals of the topside ionosonde on MEX (Němec et al, ).…”