MAVEN observations of solar wind hydrogen deposition in the atmosphere of Mars

Abstract

Mars Atmosphere and Volatile EvolutioN mission (MAVEN) observes a tenuous but ubiquitous flux of protons with the same energy as the solar wind in the Martian atmosphere. During high flux intervals, we observe a corresponding negative hydrogen population. The correlation between penetrating and solar wind fluxes, the constant energy, and the lack of a corresponding charged population at intermediate altitudes implicate products of hydrogen energetic neutral atoms from charge exchange between the upstream solar wind and the exosphere. These atoms, previously observed in neutral form, penetrate the magnetosphere unaffected by electromagnetic fields (retaining the solar wind velocity), and some fraction reconvert to charged form through collisions with the atmosphere. MAVEN characterizes the energy and angular distributions of both penetrating and backscattered particles, potentially providing information about the solar wind, the hydrogen corona, and collisional interactions in the atmosphere. The accretion of solar wind hydrogen may provide an important source term to the Martian atmosphere over the planet's history.

The Modulation of Solar Wind Hydrogen Deposition in the Martian Atmosphere by Foreshock Phenomena

“…An energy-dependent charge changing cross section with CO 2 was derived, based on the results of Halekas,Lillis, et al (2015), to determine what fraction of these precipitating ENAs would convert back to penetrating protons. An energy-dependent charge changing cross section with CO 2 was derived, based on the results of Halekas,Lillis, et al (2015), to determine what fraction of these precipitating ENAs would convert back to penetrating protons.…”

MAVEN observations on a hemispheric asymmetry of precipitating ions toward the Martian upper atmosphere according to the upstream solar wind electric field

“…Interestingly, the precipitating ion fluxes in the + E hemisphere are slightly stronger than those in the − E hemisphere around the nightside polar region for the latitude higher than ∼50° with the local time of 21–3 h. While the strong precipitating ion fluxes are generally observed in the dayside subsolar region ( 40°) in both the ± E hemispheres, the precipitating ion fluxes are typically stronger in the + E hemisphere rather than in the − E hemisphere (blue bins at low‐latitude dayside in Figure c). These precipitating ion populations might be partially responsible for the penetrating double‐charge exchange protons seen in the Martian upper atmosphere [ Halekas et al , , ], rather than heavy ions; however, it has never been reported that these penetrating protons display an asymmetric distribution between the ± E hemispheres. A discussion of what ion species may be responsible for the hemispheric asymmetry of the ion precipitation in the MSE coordinates is left in section 4, where we investigate different precipitation maps for protons and heavy ions.…”

MAVEN observations of a giant ionospheric flux rope near Mars resulting from interaction between the crustal and interplanetary draped magnetic fields

“…A portion of the solar wind protons charge exchange with the Martian exospheric constituents, resulting in energetic neutral atoms (ENAs) and allowing them to penetrate to low altitudes in the atmosphere, because they do not feel the ambient electromagnetic fields. Some of these ENAs can be converted back to charged particles that can be measured by SWIA around the MAVEN closest approach [ Halekas et al , , ]. Since these ENAs can penetrate to low altitudes, maintaining the same velocity as that of solar wind, the penetrating protons velocity (Figure b) is the most reliable proxy to determine the upstream solar wind velocity.…”