Influence of Magnetic Fields on Precipitating Solar Wind Hydrogen at Mars

Abstract

Solar wind protons can interact directly with the hydrogen corona of Mars through charge exchange, resulting in energetic neutral atoms (ENAs) able to penetrate deep into the upper atmosphere of Mars. ENAs can undergo multiple charge changing interactions, leading to an observable beam of penetrating protons in the upper atmosphere. We seek to characterize the behavior of these protons in the presence of magnetic fields using data collected by the Mars Atmosphere and Volatile EvolutioN spacecraft. We find that backscattered penetrating proton flux is enhanced in regions where the magnetic field strength is greater than 200 nT. We also find a strong correlation at CO₂ column densities less than 5.5 × 10¹⁴ cm^-2 between magnetic field strength and the observed backscattered and downward flux. We do not see significant changes in penetrating proton flux with magnetic field strengths on the order of 10 nT.

Keywords: MAVEN; penetrating protons; solar wind hydrogen.

Figure 1

(a) Example differential energy flux Solar Wind Ion Analyzer (SWIA) spectrum collected at periapsis on 6 November 2016. Characteristic penetrating proton signature can be seen between 10:01 and 10:17 a.m. (b) Example of angular‐averaged profile of downward‐propagating protons. Pink points represent background‐corrected, angle‐averaged flux. Gray points correspond to points that fall between 0.25 and 1.75 E _peak. Blue points are a result of quadratic spline interpolation. Gaussian fit overplotted in black, which is used to find characteristic heights and full widths at half maximum (FWHMs) for each profile (c) Corresponding backscattered proton profile for timestamp in (b) (note different scale).

Figure 2

(a) Median, normalized downward penetrating proton flux as a function of magnetic field strength and elevation angle. (b) Median, normalized backscattered proton flux. (c) Ratio of median normalized backscattered and downward proton flux. (d) Total number of data points per bin.

Figure 3

Behavior of downward and backscattered penetrating proton flux as a function of magnetic field strength for different column density ranges. The column density (altitude) increases (decreases) from Panels a to i. Slopes (m) and their standard errors (SEs) are displayed to the right of Panels c, f, and i. (a) Median normalized downward and backscattered penetrating proton flux with Q1 and Q3 displayed as lower and upper error bars, respectively, in 10 nT bins. These statistical quantities were chosen due to a nonnormal distribution of penetrating proton fluxes. Data were collected at column densities 10¹³–5.5 × 10¹³ cm⁻². Panels d and g follow this format and are collected at column densities indicated in titles. (b) Ratio of median backscattered and downward fluxes for data collected at column densities 10¹³–5.5 × 10¹³ cm⁻². Error bars displayed are Q1 and Q3 computed over 10 nT bins. Panels e and h follow this format and are collected at column densities indicated in titles. (c) Distribution of backscattered and downward data at column densities 5.5 × 10¹³–10¹⁴ cm⁻². Panels f and i follow this format and are collected at column densities indicated in titles.