Solar wind energy deposition related to hydrogen energetic neutral atoms in the Martian upper atmosphere observed by MAVEN

¹ Key Laboratory of Planetary Science and Frontier Technology, Institute of Geology and Geophysics, Chinese Academy of Sciences, Beijing 100029, PR China
² College of Earth and Planetary Sciences, University of Chinese Academy of Sciences, Beijing 100049, PR China
³ Beijing National Observatory of Space Environment, Institute of Geology and Geophysics, Chinese Academy of Sciences, Beijing 100029, PR China
⁴ Heilongjiang Mohe Observatory of Geophysics, Institute of Geology and Geophysics, Chinese Academy of Sciences, Beijing 100029, PR China

^★ Corresponding author: chenyd@mail.iggcas.ac.cn

Received: 19 April 2025
Accepted: 1 September 2025

Abstract

The coupling between the solar wind and the Martian upper atmosphere is an important topic for the Martian space environment. Due to the induced magnetosphere of Mars, solar wind particles are largely prevented from entering the Martian ionosphere. Instead, studies suggest that solar wind protons undergo charge exchange with atmospheric hydrogen atoms to produce hydrogen energetic neutral atoms (H-ENAs) that penetrate the upper atmosphere and deposit energy. The H-ENAs at ionospheric altitudes of Mars are difficult to detect directly. Proton aurorae are thought to be a result of H-ENA deposition. Laboratory experiments have demonstrated that collisions between H-ENAs and Martian atmospheric CO₂ produce both H⁺ and H⁻. In this study, we present a particular event in which collision-induced H⁺ and H⁻, along with the Martian proton aurora, were simultaneously recorded by the Mars Atmosphere and Volatile EvolutioN (MAVEN) spacecraft. H⁺ and H⁻ propagate in the anti-sunward direction, and their energies gradually dissipate with decreasing altitudes, accompanied by the auroral emission intensity reaching its maximum. Furthermore, the observed H⁺∕H⁻flux ratio closely matches laboratory data. The results strongly support H-ENA deposition into the Martian upper atmosphere and the correlation between Martian proton aurorae and ionospheric energetic H⁺ and H⁻. This study enhances our understanding of the solar wind interactions with the Martian space environment.