Day-side ionospheric photoelectrons observed by MAVEN-SWEA at low altitudes and high solar zenith angles on the Martian night-side

Abstract:

Crustal magnetic fields in the northern hemisphere of Mars are generally much weaker than those in the south (Connerney et al. 2005). Over two large regions, Utopia Planitia and the Tharsis rise, the observed magnetic field at 400 km altitude is thought to be dominated by fields induced by the solar wind interaction, although the draping pattern is asymmetric and may be influenced by the presence of crustal sources far from the spacecraft (Brain et al., 2006). The MAVEN mission (Jakosky et al. 2015) provides a comprehensive set of plasma and magnetic field observations to altitudes as low as ~150 km (~120 km during "deep dips") and over a wide range of local times and solar zenith angles. From 12/1/2014 to 2/15/2015, when periapsis was at high northern latitudes, the Solar Wind Electron Analyzer (SWEA) observed ionospheric photoelectrons with energies from 3 to 500 eV at low altitudes (140-200 km) and high solar zenith angles (120-145 deg) on ~35% of the orbits. Since this electron population is unambiguously produced in the dayside ionosphere, these observations demonstrate that the deep Martian nightside is at times magnetically connected to the sunlit hemisphere. We investigated the occurrence rate of ionospheric photoelectrons as a function of altitude, solar zenith angle, and magnetic field orientation, and found that photoelectrons are more likely to be observed at low altitudes and high solar zenith angles when the local field is more vertically oriented. This implies that the magnetic field extends to high altitudes between the night hemisphere, where photoelectrons are observed, and the source region in the dayside ionosphere, thus avoiding significant attenuation in transit. The BATSRUS Mars multi-fluid MHD model (Dong et al., 2014) suggests the presence of closed crustal magnetic field lines over the northern hemisphere that straddle the terminator and extend to high SZA. Simulations with the SuperThermal Electron Transport (STET) model (Xu and Liemohn, 2015) show that photoelectron transport along such field lines can take place without significant attenuation. Precipitation of photoelectrons onto the nightside atmosphere should cause ionization and possibly auroral emissions in localized regions. On one orbit, the O2+ energy flux measured by STATIC correlates well with precipitating photoelectron fluxes.