Electromagnetic instabilities in the collisional lower Martian ionosphere: Theory and observations

Abstract

Understanding electromagnetic instabilities in weakly ionized planetary environments is essential for describing ionospheric structure, energy transport, and plasma-neutral coupling. The lower Martian ionosphere provides a favorable regime for this physics: it is weakly ionized and contains short-scale density gradients, together with a strong magnetization asymmetry between electrons and ions. These conditions allow electromagnetic processes controlled by electron–neutral and ion–neutral collisions to develop and be examined using MAVEN observations. While electrostatic instabilities are well documented at Earth, electromagnetic instabilities in this collisional region at Mars remain largely unexplored.

This study combines theoretical development and MAVEN observations to investigate wave generation in the lower Martian ionosphere. A two–fluid collisional framework is formulated, and the full electromagnetic dispersion relation is derived. The model recovers known limits, including electromagnetic Alfvénic and magnetosonic modes and electrostatic Farley–Buneman and gradient-drift branches. It also extends the theory to a regime where magnetic pressure, density gradients, and collisional coupling jointly drive instability. Analytical limits and numerical solutions show that growth maximizes at intermediate perpendicular scales, controlled by ion–neutral collisionality, electron magnetization, and gradient strength.

Guided by these theoretical predictions, MAVEN magnetic field data are analyzed using time–series and spectral techniques, magnetic field–aligned decomposition, and Minimum Variance Analysis to determine wave polarization and propagation relative to the background field. The observed waves are mainly compressional and propagate nearly perpendicular to B0, occurring within regions of positive modeled growth. A detailed case study shows that enhanced magnetic wave power occurs at the same locations and times as positive modeled growth along the MAVEN trajectory. Additional events show that similar plasma conditions recur during multiple periapsis passes and fall within the predicted instability region.

These results provide theoretical and observational evidence for a collisional electromagnetic instability in the lower Martian ionosphere. MAVEN observations show compressional magnetic fluctuations with near-perpendicular propagation within regions of positive modeled growth, typically at altitudes of ∼140–160 km near the terminator. The results provide a framework for interpreting MAVEN electromagnetic wave observations and support future studies of collisional plasma processes in planetary ionospheres.