O⁺ ion beams reflected below the Martian bow shock: MAVEN observations

Abstract

We investigate a generation mechanism of O+ ion beams observed above the Martian bow shock by analyzing ion velocity distribution functions (VDFs) measured by the Suprathermal and Thermal Ion Composition instrument on the Mars Atmosphere and Volatile Evolution (MAVEN) spacecraft. In the solar wind near Mars, MAVEN often observes energetic O+ ion beams (~10 keV or higher). Accompanied with the O+ ion beam events, we sometimes observe characteristic ion VDFs in the magnetosheath: a partial ring distribution. The partial ring distribution corresponds to pickup ions with a finite initial velocity (i.e., not newborn pickup ions), and its phase space density is much smaller than that of local pickup O+ ions of the magnetosheath. Thus, the partial ring distribution is most likely produced by the reflection of pickup O+ ions precipitating from the upstream solar wind below the bow shock. After being injected into the magnetosheath from the solar wind, the precipitating O+ ions are subject to the significantly enhanced magnetic field in this region and start to gyrate around the guiding center of the plasma frame in the magnetosheath. Consequently, a part of precipitating O+ ions are reflected back to the solar wind, generating O+ beams in the solar wind. The beams direct quasi‐sunward near the subsolar region but have large angle with respect to the sunward direction at high solar zenith angles (>50°). The reflected O+ beams are accelerated by the convection electric field of the solar wind and may escape Mars.

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

“…Jarvinen et al [, ] pointed out that the magnetic barrier is stronger on the + E hemisphere than on the − E hemisphere on the dayside, indicating that it can prevent pickup ions from more effectively precipitating in the + E hemisphere rather than in the − E hemisphere by deflecting them. Indeed, Masunaga et al [] reported that some pickup ions precipitating toward the Martian upper atmosphere are reflected back to the solar wind based on MAVEN observations.…”

Statistical analysis of the reflection of incident O+ pickup ions at Mars: MAVEN observations

“…It was recently found that some of incident O + pickup ions are reflected in the Martian magnetosheath and return to the solar wind as a beam with energy of ~10 keV or higher [ Masunaga et al ., ]. In this paper, to evaluate the importance of the reflection of incident O + pickup ions, we statistically estimated a reflection ratio by calculating average inward and outward O + ion fluxes (>10 keV) above the Martian bow shock.…”

“…Since Masunaga et al . [] examined O + ion reflection features only in the magnetosheath, it is important to identify the reflection features in MPR and compare their occurrence frequency between the magnetosheath and MPR.…”

“…It is possible that the incident O + ions are reflected by the crustal magnetic field with the same analogy to the reflection occurred in the magnetosheath shown in Masunaga et al . []. Because the reflection ratio is larger under strong IMF and magnetic fields below the bow shock shown in Figure and the magnitude of the crustal magnetic field typically reaches several hundred nanoteslas, it is possible that O + ion reflections locally occur more frequently than those seen in the magnetosheath.…”

“…The second characteristic distribution is O + ion beams with energy of ~10 keV flowing nearly toward the Sun. These beams are produced as incident O + pickup ions are reflected in the magnetosheath and return to the solar wind [ Masunaga et al ., ].…”