e-POP observations of suprathermal electron bursts in the ionospheric Alfvén resonator

Abstract:

Utilizing the suprathermal electron imager onboard the e-POP satellite, we performed a statistical study of repetitive suprathermal electron bursts (STEBs) with 52 events (each with $\geq$3 STEBs) during 21 e-POP orbits. The STEBs are observed on both the day side and night side, and within a wide range of geomagnetic latitudes, from 64$^{\circ}$ to 78$^{\circ}$. We suggest the repetition is due to the ionospheric Alfv{\'e}n resonator (IAR), since the statistical time separation of adjacent STEBs in the satellite frame is 0.5-1.5 s, which is consistent with the characteristic period of the IAR. Furthermore, the STEBs are associated with low-frequency magnetic field perturbations. The statistical occurrence of repetitive STEBs shows a preference towards post-noon and midnight sectors at high magnetic latitudes. We also present one example event of four successive STEBs associated with auroral rays in conjugate imagery. The repetition period of STEBs is similar to the oscillation period of magnetic fields observed by e-POP. One of the STEBs in this event exhibits ``normal'' dispersion (high energy particles with field-aligned pitch angle arriving first) and the adjacent STEB exhibits ``inverse'' dispersion (low energy particles with broad pitch angles arriving first). With a test particle simulation, we propose one possible explanation of inverse dispersion in terms of a suprathermal electron population and a downward-propagating parallel electric field.