Mars Express/ASPERA-3/NPI and IMAGE/LENA observations of energetic neutral atoms in Earth and Mars orbit

TL;DR

This paper analyzes neutral atom observations from Mars Express and IMAGE spacecraft, revealing unexpected streams and populations that challenge existing models of heliospheric physics.

Contribution

It identifies and compares neutral atom streams observed by different instruments, suggesting potential new sources and origins not explained by current understanding.

Findings

Detection of unexplained neutral beams unrelated to Mars

Identification of streams with different properties than known populations

Potential new sources of neutral atoms in interplanetary space

Abstract

The low energy neutral atom imagers on Mars Express and IMAGE have revealed that the neutral atom populations in interplanetary space come from a variety of sources and challenge our current understanding of heliospheric physics. For example, both in cruise phase and at Mars, the neutral particle instrument NPD on Mars Express observed "unexplained neutral beams" unrelated to Mars which appear to be either of heliospheric or solar wind origin. Likewise, the NPI instrument on Mars Express has revealed streams of neutral atoms with different properties than those observed by NPD. Independently, IMAGE/LENA has reported neutral atom observations that may be interpreted as a "secondary stream" having different characteristics and flowing from a higher ecliptic longitude than the nominal upstream direction. Both sets of observations do not appear to fit in easily with the neutral atom environment from 1.0-1.57 AU as it is currently understood. In this paper we examine some highly suggestive similarities in the IMAGE/LENA and Mars Express/ASPERA-3/NPI data to try to determine potential origins for the observed signal.