High altitude occurrence of Auger electrons in the Martian upper atmosphere during solar flares

Abstract We study the high-altitude occurrence of Auger electrons in the Martian upper atmosphere during four solar flare events occurred on 19 October 2014, 24 March 2015, 04 May 2015, and 10 September 2017. The Auger electrons studied here were observed by the solar wind electron analyzer on the Mars Atmosphere Volatile EvolutioN (MAVEN) spacecraft. The high-altitude occurrence of the Auger electrons was observed during both the M- and X-class solar flares. These electrons were observed up to an altitude as high as 990 km near the terminator. The flux of Auger electrons was maximum for inbound orbital segments when the peak of the flare occurred before MAVEN reaches its periapsis. In addition, the high altitude Auger electrons were observed on the orbital segments that are closer to the planet than the ones further away from the planet. In regions near the terminator where high-altitude photoelectrons were observed, the magnetic field lines are pointing nearly horizontal with respect to the surface of Mars. Pitch angle distributions show that the electrons were field-aligned and were moving from the dayside to the nightside across the terminator. In addition, in regions of semi-vertical magnetic fields that point upward on the dayside, the Auger electrons were found to move away from the planet. It is proposed that the Auger electrons produced on the dayside ionosphere are transported towards nightside by the open or closed magnetic fields that straddle across the terminator. In both the cases, the foot points of the magnetic fields on the dayside are below the electron exobase so that the photoelectrons produced in the ionosphere are transported to high altitudes. The absence of Auger electrons in the high altitude nightside ionosphere indicates that either the electrons did not appear at the same location as that of the MAVEN spacecraft or the magnetic fields are closed on the nightside at lower altitudes. Precipitation of the Auger electrons in the nightside ionosphere is expected to produce extra ionization, in addition to the precipitation of the solar wind and 20–30 eV photoelectrons.