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

Nemec, Frantisek and Morgan, David and Dieval, Catherine and Gurnett, Donald (2015) Intensity of nightside MARSIS AIS surface reflections and implications for low-altitude ionospheric densities. Journal of Geophysical Research: Space Physics, 120 (4). pp. 3226-3239. ISSN 2169-9402

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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.

Item Type:
Journal Article
Journal or Publication Title:
Journal of Geophysical Research: Space Physics
Additional Information:
© 2015 American Geophysical Union. All rights reserved.
ID Code:
76664
Deposited By:
Deposited On:
13 Nov 2015 14:36
Refereed?:
Yes
Published?:
Published
Last Modified:
27 Oct 2020 05:12