Interplanetary Hydrogen Properties Observed From Mars

Observations of the Lyman-a emissions from Interplanetary Hydrogen (IPH) atoms are made from Mars' orbit using a high spectral resolution instrument in echelle configuration. The measurements can uniquely be used to resolve IPH from planetary H emissions and to subsequently determine the brightness, velocity, and thermal broadening of the IPH flow along the instrument line of sight. Planned observations conducted during special IPH campaigns as well as serendipitous observations made of the planetary limb and a comet sighting, both upwind and downwind of the bulk IPH flow direction, are analyzed to determine these properties and to examine the variability of IPH brightness with solar activity through the declining phase of Solar Cycle 24. The results show that the IPH brightness trends with solar irradiance, the flow is fainter downwind than upwind, the IPH brightness is variable and non-negligible compared with planetary emissions, and that deriving thermal properties of IPH requires higher spectral resolution than is presently available. A heliospheric interface model was used to simulate and further interpret the derived IPH properties. These results can improve the theoretical understanding of solar system dynamics, between the solar wind and the local interstellar medium, by providing empirical constraints to simulations of the global heliosphere from the inner boundary region near 1.6 AU and can guide the development of future interplanetary missions.