Modeling the Nightside Martian Ionosphere: Structure, Composition, and Energetics

Abstract The Mars Atmosphere and Volatile Evolution (MAVEN) mission has provided multi-instrument measurements of the Martian nightside ionosphere, yet a self-consistent interpretation of its structure, composition, and energetics remains lacking. Here we extend our previous dayside Martian ionosphere model to the deep-nightside conditions during the Deep Dip 6 (DD6) campaign, by self-consistently solving the continuity, momentum, and energy equations with the background atmospheric structure and suprathermal electron precipitation constrained by MAVEN observations. The model reproduces the representative DD6 electron and ion density distributions, supporting suprathermal electron precipitation as the primary source sustaining the deep-nightside ionosphere. A notable exception is NO + below 170 km, whose long chemical lifetime allows day-to-night plasma transport, neglected in the present one-dimensional model, to become important. Unlike the dayside, where HCO + behaves as a terminal ion, the nightside calculations reveal a distinct chemical regime in which reduced electron densities suppress dissociative recombination, allowing proton transfer to H 2 O to become the dominant HCO + loss pathway. The local heating processes are insufficient to reproduce the MAVEN-derived electron and ion temperatures. Additional nonlocal energy inputs are required and represented by downward conductive heat fluxes imposed at the upper boundary, reflecting thermal coupling of the nightside ionosphere to the hotter overlying plasma. These results demonstrate that while suprathermal electron precipitation largely controls the density and chemistry of the deep-nightside ionosphere, its detailed composition and thermal structure retain important signatures of nonlocal plasma and energy transport beyond the direct effects of electron precipitation.