Rocket Observation of Sub-Relativistic Electrons in the Quiet Dayside Auroral Ionosphere

An energy spectrum of electrons from 180 to 550 keV precipitating into the dayside polar ionosphere was observed under a geomagnetically quiet condition (AE <= 100 nT, Kp = 1-). The observation was carried out at 73-184 km altitudes by the HEP instrument onboard the RockSat-XN sounding rocket that has been launched from Andoya, Norway. The observed energy spectrum of precipitating electrons follows a power law of -4.9 +/- 0.4 and the electron flux does not vary much over the observation period (similar to 274.4 s). A nearby ground-based VLF receiver observation at Lovozero, Russia shows the presence of whistler-mode wave activities during the rocket observation. A few minutes before the RockSat-XN observation, POES18/MEPED observed precipitating electrons, which also suggest whistler-mode chorus wave activities at the location close to the rocket trajectory. A test-particle simulation for wave-particle interactions was carried out using the data of the Arase satellite as the initial condition which was located on the duskside. The result of the simulation shows that whistler-mode waves can resonate with sub-relativistic electrons at high latitudes. These results suggest that the precipitation observed by RockSat-XN is likely to be caused by the wave-particle interactions between whistler-mode waves and sub-relativistic electrons. Plain Language Summary Sub-relativistic electrons precipitating into the Earth's dayside polar ionosphere are observed by a sounding rocket under geomagnetically quiet conditions. An energy spectrum of these electrons in an energy range from 180 to 550 keV is reported at the rocket altitude. A possible mechanism for generating this precipitation is the resonance scattering of electrons by whistler-mode waves, which we conducted a test-particle simulation based on the ground and satellite observations.