Precipitating Solar Wind Hydrogen as Observed by the MAVEN Spacecraft: Distribution as a Function of Column Density, Altitude, and Solar Zenith Angle. Issue 7 (30th June 2021)

• Record Type: Journal Article
• Authors: Henderson, Sarah; Halekas, Jasper; Lillis, Robert; Elrod, Meredith
• Is Part Of: Journal of geophysical research. Volume 126:Issue 7(2021)
• Page Start: n/a
• Page End: n/a
• Publication Date: 2021-06-30
• Subjects: MAVEN -- SWIA -- penetrating protons -- ENAs; Planets -- Periodicals; Geophysics -- Periodicals; 559.9
• DOI: 10.1029/2020JE006725
• Languages: English
• ISSNs: 2169-9097

Abstract

Abstract: The Mars Atmosphere and Volatile EvolutioN (MAVEN) mission observes precipitating solar wind hydrogen ENAs in the upper Martian ionosphere with various instruments onboard. These precipitating solar wind hydrogen atoms result from the direct interaction between the solar wind and Mars's extensive hydrogen corona, creating a beam of particles traveling at solar wind speed that can penetrate deep into the ionosphere of Mars. Using particle data collected by both the Solar Wind Ion Analyzer (SWIA) and Neutral Gas and Ion Mass Spectrometer (NGIMS) onboard MAVEN, we seek to characterize the global behavior of these particles. By using a combination of energy spectra and neutral gas density data, we are able to determine the behavior of this particle population as a function of altitude, column density, and solar zenith angle. Through our analysis, we see that these precipitating solar wind hydrogen ENAs undergo multiple charge exchange interactions and ultimately scatter significantly at an altitude of 160 km or column density of 5 × 10 16 cm −2 . This observed column density at which equilibrium is reached is at a value ∼17 times greater than the theoretical value. We also demonstrate that this particle population's energy flux follows a cosine dependence of solar zenith angle, as is predicted by models. Plain Language Summary: Positively charged particles in the solar wind can interact with neutral hydrogen atoms in the Martian atmosphere. Through a process of charge exchange, these solar wind ions can become neutral and are able to navigate their way into the atmosphere of Mars, forgoing various boundaries around the planet. These new neutral particles retain the energy they had as ions, which allows them to penetrate far into the atmosphere. As they travel deeper into the atmosphere, these energetic neutrals begin to interact with other particles, like CO2, through a process of charge‐changing reactions, where they can again become positively charged. This beam of positively charged particles can deposit energy in the atmosphere, resulting in auroral emission. Using particle data from two instruments onboard the Mars Atmosphere and Volatile Evolution (MAVEN) spacecraft, we seek to characterize how these particles behave as a function of column density, altitude, and solar zenith angle. We find that these penetrating protons interact with the atmosphere and ultimately scatter significantly at an altitude of 160 km. We also find that the observed energy flux of these particles as a function of solar zenith angle follows the same trend that has been predicted by models. Key Points: Precipitating hydrogen ENAs experience energy loss and spatial scattering below altitudes of 160 km and at column densities above 5 × 10 16 cm −2 Column density at which precipitating hydrogen ENAs reach an equilibrium charge fraction is ∼17 times greater than the theoretical value Precipitating solar wind hydrogen flux follows cosine of the solar zenith angle, as predicted by models