Magnetic topology dependence of ionizing electrons on the Martian nightside ionosphere

Abstract

The interaction between the solar wind and Martian crustal magnetic fields plays a significant role in shaping electron precipitation in the ionosphere, particularly on the nightside. Using five years of data from the Mars Atmosphere and Volatile EvolutioN (MAVEN) spacecraft, this study explores how magnetic topology influences the distribution of ionizing electrons, quantified through the electron impact ionization frequency (EIIF). Data from MAVEN's Solar Wind Electron Analyzer (SWEA) and magnetometer (MAG) were analyzed, with strong and weak crustal field regions defined at observation altitudes using a crustal field model. As expected, the results demonstrate that magnetic topology critically influences the altitude distribution of EIIF on the nightside. However, variations in EIIF across the terminator reveal dynamic and novel patterns. In open and draped topologies, EIIF decreases gradually from the dayside, across the terminator, and onto the nightside. Conversely, closed topologies exhibit a sharper decrease in EIIF across the terminator, characterized by strong altitude dependence. This is attributed to day-to-night transport along cross-terminator closed field lines. Below 600 km on the nightside, EIIF in closed topologies is up to two orders of magnitude lower than in open and draped topologies due to the shielding effect of crustal fields, with stronger shielding observed at lower altitudes. Above 240 km, EIIF in open and draped topologies shows no dependence on crustal field strength and weak dependence on solar wind dynamic pressure (P_sw). In contrast, the EIIF in closed topologies shows a strong dependence on magnetic field strength and a weak dependence on P_sw. These findings provide critical insights into the role of magnetic topology in electron precipitation, aiding the parameterization of electron dynamics in global circulation models of the Martian ionosphere.