Shapes of Magnetically Controlled Electron Density Structures in the Dayside Martian Ionosphere

Abstract

Nonhorizontal 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.

Isolated High‐Density Structures in Magnetic Anomaly Regions of the Martian Lower Ionosphere

“…In all these cases, the peak frequencies of the oblique echoes are much greater than those of the vertical echoes agreeing with the earlier analysis by Venkateswara Rao et al (2019). An in-depth analysis of these observations indicates that the enhanced densities are in fact the density structures of the type reported previously near the ionospheric main peak (Andrews et al, 2014;Diéval et al, 2018;Duru et al, 2006;Gurnett et al, 2005;Mohanamanasa et al, 2018;Němec et al, 2011;Venkateswara Rao et al, 2017). Most importantly, the density structures in the lower ionosphere are observed in a background ionosphere whose density is much smaller than those of the density structures.…”

Ion‐Neutral Coupling in the Upper Atmosphere of Mars: A Dominant Driver of Topside Ionospheric Structure

“…The distribution of uncoupled profiles indicates more plasma‐specific perturbations are occurring near regions of strong crustal fields than in other regions (e.g., Brain et al, ). This is likely due to field‐aligned dynamics that either enhance or deplete regional plasma (e.g., Diéval et al, , ; Dubinin et al, ; Matta et al, ; Modolo et al, ; Shinagawa & Cravens, ). Longitudinal distributions, shown in Figure C, are generally flat for uncoupled profiles, with a slight enhancement between 80° and 280° east longitude.…”

Enhanced Ionization in Magnetic Anomaly Regions of the Martian Lower Ionosphere Associated With Dust Storms

“…In fact, the echoes appear merged on several consecutive ionograms. The characteristics of these ionization bulges, their occurrence at multiple altitudes and their formation mechanisms are well studied by several authors (Andrews et al, ; Diéval et al, ; Duru et al, ; Gurnett et al, ; Mohanamanasa et al, ; Nielsen et al, ; Venkateswara Rao et al, ; Venkateswara Rao et al, ). One important aspect that can be clearly noted from Figure is that the peak frequencies of the vertical and oblique echoes in an ionogram are nearly the same or differ slightly.…”

“…In addition to layers in its vertical structure, the Martian ionosphere also displays spatial inhomogeneities in its horizontal structure. The Mars Advanced Radar for Subsurface and Ionospheric Sounding (MARSIS) instrument aboard the Mars Express spacecraft regularly detects bulges of ionization in the Martian ionosphere (Andrews et al, ; Diéval et al, ; Duru et al, ; Gurnett et al, ; Mohanamanasa et al, ; Nielsen et al, ; Venkateswara Rao et al, ; Venkateswara Rao et al, ). These bulges are predominantly observed in regions of strong vertical magnetic fields.…”