Local plasma processes and enhanced electron densities in the lower ionosphere in magnetic cusp regions on Mars

Abstract

Both the MARSIS ionospheric sounder and the charged particle instrument package ASPERA-3 are experiments on board the Mars Express spacecraft. Joint observations have shown that events of intense ionospheric electron density enhancements occur in the lower ionosphere of magnetic cusp regions, and that these enhancements are not associated with precipitation of charged particles above a few hundred electron volts (<300 eV). To account for the enhancement by particle precipitation, electron fluxes are required with mean energy between 1 and 10 keV. No ionizing radiation, neither energetic particles nor X-rays, could be identified, which could produce the observed density enhancement only in the spatially limited cusp regions. Actually, no increase in ionizing radiation, localized or not, was observed during these events. It is argued that the process causing the increase in density is controlled mainly by convection of ionosphere plasma driven by the interaction between the solar wind and crustal magnetic field lines leading to excitation of two-stream plasma waves in the cusp ionosphere. The result is to heat the plasma, reduce the electron–ion recombination coefficient and thereby increase the equilibrium electron density.

Introduction

The topside radio wave sounder MARSIS (Mars Advanced Radar for Subsurface and Ionosphere Sounding) (Picardi et al., 2004) on board the Mars Express spacecraft (Chicarro et al., 2004) measures reflections from electron densities in the Martian ionosphere as a function of frequency and radio wave propagation time (Gurnett et al., 2005). Several different kinds of targets giving rise to reflections have been identified, and a variety of them are poorly understood. Single and double peaks in the vertical profile of electron density have been observed, oblique echoes from upward bulging ionosphere in cusp regions (see Duru et al. (2006); Nielsen et al. (2007)), diffuse echoes from off-nadir directions, and possible holes in the ionosphere are indicated. The most common observation is vertical reflections from nadir. Electron densities in the Martian ionosphere are generally horizontally stratified and the dominating observation by the sounder is vertical reflections from these layers. Techniques have been developed whereby the observations are inverted to the vertical profile of electron densities (Nielsen et al., 2006). The primary density peak in the profile conforms to the predictions of photochemical equilibrium between ionization and recombination in a plasma. For example, the peak density conforms to a predicted spatial variation controlled by the solar zenith angle (Chapman, 1931; Gurnett et al., 2005; Nielsen et al., 2006). However, at times sudden short lasting strong increases are observed in the maximum electron density in the primary density peak. This separates the events from the overall behavior of the background ionosphere observed along the spacecraft trajectory. Such intense events have been mentioned before (Gurnett et al., 2007) and are analyzed in this work invoking charged particle measurements by the particle detector ASPERA-3 (Analyzer of Space Plasmas and Energetic Atoms-3) on board Mars Express (Barabash et al., 2006; Lundin et al., 2004). An absence of extra ionizing radiation during these events points to local processes in the ionosphere as the cause for the increase in electron density.