A Case Study of the Density Structure over a Vertical Magnetic Field Region in the Martian Ionosphere

Abstract:

One of the discoveries made by Mars Advanced Radar for Subsurface and Ionospheric Sounding (MARSIS) on the Mars Express spacecraft is the existence of magnetically controlled structures in the ionosphere of Mars, which result in irregularities in the ionospheric electron density contours. These irregularities lead in turn to oblique echoes, which show up as hyperbola-shaped features on the plots of echo intensity measured by MARSIS as a function of altitude and universal time. The study of a hyperbola-shaped feature observed in a pass over an isolated region of strong crustal magnetic field shows that this kind of feature can be associated with a plasma cavity in the upper ionosphere and a corresponding density enhancement in the lower levels of the ionosphere. At the location where the hyperbola-shaped echo is observed, the electron and ion fluxes measured by ASPERA-3 at the location of the spacescraft are depleted and the local electron density from MARSIS shows a small decrease, as well. However, the peak ionospheric density obtained by MARSIS remote sounding shows a clear increase as Mars Express passes over the same region. We conclude that through the open magnetic field lines, the electrons are accelerated downward and ions are accelerated upward in a manner similar to the field-line driven auroral acceleration at Earth. This heating due to precipitating electrons causes a bulge at the altitude of the main ionosphere, which in turn leads to a hyperbola shaped echo, and loss of ionospheric plasma at high altitudes.