Abstract
AbstractNonhorizontal localized electron density structures associated with regions of near‐radial crustal magnetic fields are routinely detected via radar oblique echoes on the dayside of Mars with the ionospheric sounding mode of the Mars Advanced Radar for Subsurface and Ionospheric Sounding (MARSIS) radar onboard Mars Express. Previous studies mostly investigated these structures at a fixed plasma frequency and assumed that the larger apparent altitude of the structures compared to the normal surrounding ionosphere implied that they are bulges. However, the signal is subjected to dispersion when it propagates through the plasma, so interpretations based on the apparent altitude should be treated with caution. We go further by investigating the frequency dependence (i.e., the altitude dependence) of the shape of 48 density structure events, using time series of MARSIS electron density profiles corrected for signal dispersion. Four possible simplest shapes are detected in these time series, which can give oblique echoes: bulges, dips, downhill slopes, and uphill slopes. The altitude differences between the density structures and their edges are, in absolute value, larger at low frequency (high altitude) than at high frequency (low altitude), going from a few tens of kilometers to a few kilometers as frequency increases. Bulges dominate in numbers in most of the frequency range. Finally, the geographical extension of the density structures covers a wide range of crustal magnetic fields orientations, with near‐vertical fields toward their center and near‐horizontal fields toward their edges, as expected. Transport processes are suggested to be a key driver for these density structures.
Highlights
The Martian upper atmosphere is not currently protected by a global magnetic dipole, and is exposed to erosion by the incoming supersonic solar wind
Martian dynamo, are able to stand off the solar wind and locally control the magnetic topology at low altitudes in the forms of magnetic arcades with both footpoints anchored in the planetary crust, while in other areas the interplanetary magnetic field (IMF) is free to reach lower altitudes, imposing a mostly horizontal induced magnetic field on the dayside (e.g. Brain et al, 2003)
At low altitude near the ionospheric peak the plasma is in photochemical equilibrium; for ions the transport time is much longer than the chemical loss time
Summary
The Martian upper atmosphere is not currently protected by a global magnetic dipole, and is exposed to erosion by the incoming supersonic solar wind. An induced magnetosphere arises from the interaction between the conductive ionospheric obstacle and the solar wind plasma with its embedded interplanetary magnetic field (IMF) (e.g. Nagy et al., 2004). Martian dynamo, are able to stand off the solar wind and locally control the magnetic topology at low altitudes in the forms of magnetic arcades with both footpoints anchored in the planetary crust (closed field lines), while in other areas the IMF is free to reach lower altitudes, imposing a mostly horizontal induced magnetic field on the dayside (e.g. Brain et al, 2003). Experiment (Picardi et al, 2004) is part of the payload onboard the Mars Express (MEX) orbiter. MEX orbit has period 7.5 h and inclination ~86° around Mars, with periapsis at ~300 km and apoapsis at ~10000 km. MARSIS is able to run in either a Sub-Surface mode or an Active
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