Radio Wave Reflections from Magnetized Plasma Bulges in the Martian Ionosphere

In this paper we propose a quantitative explanation of a special type of radio wave reflection phenomena observed by MARSIS (Mars Advanced Radar for Subsurface and Ionosphere Sounding), in light of the cold plasma theory. The phenomena in question appear as a type of traces in the AIS (Active Ionosphere Sounding) ionograms. The traces show the following characteristics: (1) They may appear only when the spacecraft is near to a magnetic cusp region (around 300km altitude) on dayside; (2) They are "C"-shaped curves, with their open ends pointing to the increasing frequency direction. Obviously, these traces represent 'reflection pairs' (two echoes corresponding to one transmission from the antenna). The two echoes of a 'pair' have approximately the same time delay at the lowest propagating frequency, and have increasing time delay separation with increasing wave frequency; (3) Their positions and sizes in ionograms (i.e., their frequency ranges and time delay ranges) change regularly with spacecraft motion; (4) They represent quite rare events, since they are clearly observed only in a few orbit segments among thousands of orbits of Mars Express. In order to investigate the origin of these features, we employ a 2D spatial configuration model of the magnetized plasma bulge to simulate the behavior of the AIS radio waves. In the model the magnetic field is assumed to be a deformed vertical cylinder (corresponding to the patched crustal field of Mars), with its transverse size expanding upward. Magnetic flux density decreases upward and sideward continuously into a low background field value (while the total flux is conserved). Electron density is positively related to the field flux density, meanwhile decreases upward in an exponential manner. Equilibrium between magnetic pressure and plasma pressure is assumed to hold the density bulge. A ray tracing method based on the cold plasma dispersion relation is used to produce artificial ionograms. We find that under some specific conditions (spatial variations of magnetic flux density and electron density, and position of the spacecraft relative to the bulge), the observed curved traces, as well as other coexisting features (such as the main plasma layer echoes in the same observation record), can be reproduced. When the conditions are changed, the curved features disappear, or are replaced by other features that are also practically observed by the radar. The simulation suggests that the two echoes of a 'reflection pair' are those of waves propagating parallel (right polarized) and perpendicular to the field lines, respectively. We conclude that the observed "C"-shaped features and their analysis may uncover some important information about the dayside Martian ionosphere properties, especially those associated with the magnetic field. Key words: Mars; MARSIS; Ionosphere; Cold plasma dispersion relation; Ray tracing