Solar wind electron precipitation into the dayside Martian upper atmosphere through the cusps of strong crustal fields

Abstract

Measurements made by the magnetometer/electron reflectometer on board the Mars Global Surveyor spacecraft have shown spatially localized enhancements in electron fluxes over the strong crustal fields on both the dayside and night, which are used to identify the cusps in between the closed magnetic fields. This paper provides a comprehensive statistical study on the occurrence rate of dayside solar wind/magnetosheath precipitation over the strong crustal fields. Also, the occurrence rate's dependence on the magnetic elevation angles and the solar zenith angle is presented. A seasonal variation of the precipitation is also expected and found, due to both the tilt and the orbital eccentricity of Mars. The maximum occurrence rate is 40%, when the solar zenith angles are small and the magnetic fields are nearly vertical. Finally, the energy flux deposition of the solar wind electrons is calculated as well, which is 0.1%–2% of solar EUV flux input.

Mars nightside electrons over strong crustal fields

“…The field of view spanned 360 ∘ × 14 ∘ and was divided into sixteen 22.5 ∘ sectors. "Modified pitch angles" were used similar to Xu et al [2014b] to identify if the electrons were moving toward or away from the planet. Important to note is the uneven sampling by the ER due to the orientation of the magnetic field with respect to the ER.…”

“…Xu et al [2014aXu et al [ , 2014b conducted the analogous studies for the dayside strong crustal field regions. All measurements with a solar zenith angle of less than 90 ∘ were excluded to limit this study to the nightside.…”

“…Xu et al [2014b] used the electron flux ratio of multiple energy pairs (26 eV/115 eV, 36 eV/115 eV, and 47 eV/115 eV) to identify the populations of photoelectrons and solar wind electrons. Doing so shrinks the range from the usual −90 ∘ to +90 ∘ to a reduced range of just 0 ∘ -90 ∘ .…”

“…On the nightside, Brain et al [2007] classified two-sided loss cones (trapped populations) and plasma voids (locations where the observations are at or near the instrument background level), as indicators of closed magnetic field lines corresponding to the Martian crustal fields. These cusps are ideal locations for solar wind/magnetosheath electrons to precipitate Liemohn et al, 2003;Safaeinili et al, 2007;Xu et al, 2014b]. In particular, radial crustal fields form magnetic cusps generally located between magnetic loop structures.…”

“…Several others have investigated auroral electron spectra from MGS [e.g., Brain et al, 2006] and Mars Express [e.g., Lundin et al, 2006aLundin et al, , 2006b. A new method of separating photoelectrons from solar wind electrons (classifying by energy spectra rather than PAD) was used by Xu et al [2014b] in their statistical study of dayside solar wind precipitation on the magnetic cusps. The accelerated electrons in these papers have enough energy to produce auroral emissions.…”