Precipitating Solar Wind Hydrogen as Observed by the MAVEN Spacecraft: Distribution as a Function of Column Density, Altitude, and Solar Zenith Angle

Abstract

Because it lacks a global magnetic field, Mars's atmosphere is subject to direct interaction with the solar wind, which can provide energy for atmospheric constituents to escape the planet's gravitational pull through a variety of processes, including photochemical escape, sputtering, and pick up ion escape Lillis et al., 2015). Evidence suggests that this solar wind interaction has played a major role in changing Mars from a warm, water rich environment to the cold, dry planet we observe today (Jakosky et al., 2018). Mars's atmosphere is comprised primarily of CO 2 but also exhibits a hydrogen corona that extends past the planet's bow shock (Anderson, 1974;Anderson & Hord, 1971). This hydrogen corona is subject to direct interaction with the solar wind by means of charge exchange, providing us with a unique laboratory to observe and characterize various phenomena within the upper Martian atmosphere.As solar wind protons propagate toward Mars, they can charge exchange with neutral hydrogen atoms in the planet's corona, resulting in energetic neutral atoms (ENAs) with the solar wind speed (Kallio et al., 1997). These ENAs can then travel toward the planet without being affected by electromagnetic forces and thus

Influence of Magnetic Fields on Precipitating Solar Wind Hydrogen at Mars

“…To better understand how neutrals within the Martian atmosphere affect precipitating solar wind hydrogen atoms, we examine how their flux changes as a function of column density. We integrate the CO 2 number density measured by NGIMS along a line of sight between each proton observation point and the Sun (Henderson et al., 2021 ; Mahaffy et al., 2014 ). This traces the path of the precipitating solar wind hydrogen from the Sun to the point at which it is observed by SWIA, allowing us to quantify the column density through which the particle has passed.…”

“…The specific details of our calculations are described in Henderson et al. ( 2021 ). Since CO 2 comprises ∼95% of the Martian atmosphere within the altitude range we are examining, we determine that using column density values for this species suffices for a first‐order approximation.…”

“…Previous studies have demonstrated seasonal variability of precipitating solar wind hydrogen flux, in addition to changes in flux with respect to solar wind conditions (Brecht, 1997;Diéval et al, 2012;Halekas, 2017;Halekas et al, 2015;Henderson et al, 2021;Wang et al, 2013Wang et al, , 2018. To mitigate these temporal effects, we normalize the penetrating proton fluxes to gain unbiased measurements of these particles within the Martian ionosphere.…”

“…This traces the path of the precipitating solar wind hydrogen from the Sun to the point at which it is observed by SWIA, allowing us to quantify the column density through which the particle has passed. The specific details of our calculations are described in Henderson et al (2021). Since CO 2 comprises ∼95% of the Martian atmosphere within the altitude range we are examining, we determine that using column density values for this species suffices for a first-order approximation.…”

“…Hydrogen ENAs and their charged byproducts have been observed and characterized by Mars Express (MEX) and the Mars Atmosphere and Volatile EvolutioN (MAVEN) missions. Previous studies have examined the behavior of precipitating hydrogen as a function of various temporal and global parameters (Brinkfeldt et al., 2006 ; Futaana et al., 2006 ; Girazian & Halekas, 2021 ; Gunell et al., 2006 ; Halekas, 2017 ; Halekas et al., 2015 ; Henderson et al., 2021 ). It has not been determined through observations; however, how the local magnetic fields in the Martian environment affect the behavior of these particles.…”