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 groups of repetitive suprathermal electron bursts (STEBs) with 52 events (each with ≥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° to 78°. We interpret these data as temporally varying signatures; we suggest the repetition is due to the ionospheric Alfvén 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 toward postnoon 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 Enhanced Polar Outflow Probe (e‐POP). One of the STEBs in an example 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.

Small-Scale Dynamic Aurora

“…Continuous international effort is therefore needed for systematic monitoring observations of both global and local auroras at the same time, which would be necessary to build a firm empirical relationship among global-scale, mesoscale, and the small-scale dynamic auroras as reviewed in this article. Systematic surveys have also been conducted to identify the large-scale context of the small-scale structures, such as large-scale upward current or downward current (Liang et al 2019;Wu et al 2020aWu et al , 2020b. Next generation high-resolution global imaging of aurora from spacecraft would be a dream, but for the time being, rapidly increasing resolution of ground-based all-sky cameras, the networked distributions of imagery and small spacecraft, and timely data sharing combined with realistic simulations will renovate this aspect.…”

“…In a recent study using suprathermal (100-350 eV) electron measurements on ePOP in addition to field measurements from Swarm, Wu et al (2020b) carried out a statistical study of periodic electron bursts which recurred several times at periods consistent with the eigenfrequency of the ionospheric Alfvén resonator (IAR). These events are similar to those reported by Chaston et al (2002aChaston et al ( , 2002b using the FAST satellite in the nightside auroral region, and Tanaka et al (2005) using a sounding rocket in the dayside cusp, but extend the number of events to more than 50, and show that they are distributed over a wide range of magnetic local times (from pre-noon to post-midnight) and geomagnetic latitudes (64-78 degrees).…”

“…In at least one case, the electron bursts observed by ePOP were associated with auroral rays, a connection reported previously by Lynch et al (2012) based on sounding rocket observations, and indicating a possible coupling between rays and the IAR. Wu et al (2020b) were also able to reproduce detailed properties of electron energy and pitch-angle dispersion observed within individual bursts by tracing test particles within a downward-propagating Alfvén wave.…”