Evidence for Crustal Magnetic Field Control of Ions Precipitating Into the Upper Atmosphere of Mars

Abstract

We present the effects of the local magnetic field configurations on ions precipitating into the upper atmosphere of Mars using Mars Atmosphere and Volatile EvolutioN (MAVEN) observations. Precipitating pickup planetary heavy ions (O + , O + 2 , and CO + 2 ) are of particular interest in the Martian plasma environment because they potentially enhance the sputtering loss of ambient neutral particles. In addition, solar wind protons (and H + pickup ions) penetrate into the dayside atmosphere due to the direct interaction with the Martian obstacle. We present a statistical study showing that precipitating ion fluxes are typically enhanced by a factor of 2-3 under radial field configurations. We also show that the crustal fields have a shielding effect; the precipitating fluxes are significantly reduced by ∼50% under the strong crustal fields (≳ 100 nT), where the local magnetic field is oriented with a more horizontal component to the surface. These trends are seen consistently regardless of ion species, as well as the observed locations including dayside/nightside, subsolar longitudes, and ±E hemispheres in the Mars-centered solar electric (MSE) coordinates. In particular, the local magnetic field configurations control precipitating ions with energies lower than a few keV, while precipitating high-energy ion fluxes are likely independent of the local magnetic field configurations. Precipitating ion fluxes are known to vary by at least an order of magnitude depending on the upstream solar wind. Therefore, the local magnetic field configurations turn out to be the secondary factor in modulating precipitating ion fluxes at Mars.

On Mars's Atmospheric Sputtering After MAVEN's First Martian Year of Measurements

“…As an example, Hara et al (2018) and Hara et al (2017) have shown that large variation of the precipitating flux can be expected in correlation with the solar wind pressure, the orientation of the electric field of convection, and with the presence of crustal magnetic fields. As an example, Hara et al (2018) and Hara et al (2017) have shown that large variation of the precipitating flux can be expected in correlation with the solar wind pressure, the orientation of the electric field of convection, and with the presence of crustal magnetic fields.…”

“…The first evidence of heavy ion precipitation for nominal solar wind conditions has been reported by Leblanc et al (2015), highlighting that sputtering should clearly occur at Mars. Hara et al (2018) also highlighted the role of the crustal magnetic field in the local increase of this precipitating flux. Hara et al (2018) also highlighted the role of the crustal magnetic field in the local increase of this precipitating flux.…”

Variability of Precipitating Ion Fluxes During the September 2017 Event at Mars

“…Moreover, the dynamic pressure compresses Mars' magnetosphere and favors the acceleration and precipitation of the planetary picked up ions. Also, the presence of crustal magnetic fields (e.g., Acuña et al, ) can also influence locally the precipitating flux as shown by Hara, Luhmann, Leblanc, Curry, Halekas, et al ().…”

Influence of Extreme Ultraviolet Irradiance Variations on the Precipitating Ion Flux From MAVEN Observations

“…We also restrict our sample of precipitation measurements to those obtained in a solar zenith angle interval between 70° and 130°. Furthermore, in order to avoid the potential influence of the crustal fields (Hara, Luhmann, Leblanc, Curry, Halekas, et al, ; Leblanc et al, ), we also only consider measurements performed when the average magnetic field between 200 and 350 km is less than 60 nT. For each set of solar wind parameters, we calculate the solar wind dynamic pressure, the norm of the solar wind motional electric field defined as where B IMF is the interplanetary magnetic field (IMF), the Alfvén Mach number M A = V SW / V A where V A is the Alfvén speed, the solar wind flux defined as F SW = n SW V sw and the pickup O + gyroradius in the solar wind , where m O is the mass of an O + ion, θ cone is the angle between the IMF direction and the Mars solar Origin (MSO) X axis, and q is the electron charge value.…”