Energy Spectral Properties of Hydrogen Energetic Neutral Atoms Emitted from the Dayside Atmosphere of Mars

The energy spectrum of hydrogen energetic neutral atoms (HENA) emitted from the dayside atmosphere of Mars reveals the physical processes of their origin and provides means for remote-sensing on the Mars plasma environment. We first compile all available energy spectra of sun-ward flowing HENA using high energy resolution data from the Neutral Particle Detectors on Mars Express from February to June 2004. The median value of the HENA directional flux is 1.12×10⁵cm⁻²s⁻¹sr⁻¹ from 0.3 to 5 keV, making Mars a bright source of HENAs. The energy spectra of the measured HENAs can be approximated by a two-segment power-law spectra. Then we compared the measured statistical energy spectra with those from a kinetic particle transport code. The comparison indicates that the measured HENA energy spectrum is harder (more high-energy components) than the modeled backscattered HENAs. We propose that a population of sunward-flowing, charge-exchanged HENAs produced in the dayside magnetosheath is the most probable reason for this difference. This HENA population with unexpected energy spectrum highlights the kinetic nature of the dayside magnetosheath of Mars