Role of highly oblique whistler-mode waves in bursty precipitation of sub-relativistic electrons

Intense and bursty precipitation of energetic electrons to the Earth's atmosphere, termed as microbursts, are often attributed to the nonlinear interaction between whistler-mode chorus waves. Apart from quasi-parallel whistler mode chorus waves which can resonate with electrons through cyclotron resonance, oblique whistler waves with strong parallel electric field can resonate with electrons through Landau resonance trapping process. Being trapped by oblique whistler waves near equatorial regions, electrons can be transported by the wave to higher latitudes, leading to the decrease of pitch angle towards the loss cone and the acceleration of electrons to energies up to 100-200keV. Using observations from THEMIS satellites and observations from ELFIN Cubesats, we collected statistics of oblique whistler mode waves near the equatorial regions and statistics of intense sub-relativistic (<200keV) electron precipitation events observed in the ionosphere. Combing the statistics of these observations, we demonstrate that both oblique whistler mode waves and intense sub-relativistic electron precipitations are frequently observed at the radial distances of L = 6~10 and in the dayside magnetosphere around MLT = 06~16. Our work will emphasize the important role of oblique whistler mode waves in sub-relativistic electron acceleration and transport into the loss cone through nonlinear Landau resonance. The population of very oblique whistler-mode waves may significantly alter sub-relativistic electron flux dynamics and should be included into radiation belt models.