Negative Hydrogen Ions in the Martian Atmosphere: MAVEN Observations

A portion of Mars' neutral hydrogen exosphere lies outside of the bow shock, exposed to the solar wind. A fraction of the incoming solar wind protons undergoes charge exchange with the neutral hydrogen, resulting in energetic neutral atoms (ENAs) moving with the original solar wind velocity. The ENAs pass through the bow shock undeterred into the Martian atmosphere, where a portion of them are reconverted to a charged state by collisions with atmospheric particles. The result is positive and negative hydrogen ions deep within the atmosphere.

Data obtained from the Solar Wind Electron Analyzer (SWEA) aboard the Mars Atmosphere and Volatile EvolutioN (MAVEN) mission reveals a population of negatively charged ions in the Martian atmosphere. Our analysis indicates that these ions are moving exclusively in the solar wind flow direction. During orbits with a significant flux of negative ions, we find a strong correlation between the upstream solar wind energy and the negative ion energy. We find the flux of negative ions to be correlated with the flux, density, and velocity of positively charged precipitating hydrogen ions. There is an increased signal of negative ions during orbits near perihelion, which we attribute to the extension of the exosphere at these times. Based on this, we propose that the observed negative hydrogen ions are derived from the interaction of the solar wind and the Martian hydrogen exosphere.