Suprathermal Electron Acceleration Perpendicular to the Magnetic Field in the Topside Ionosphere

Abstract

We present the first direct observations of suprathermal (tens to hundreds of eV) electron acceleration perpendicular to the magnetic field in the topside (900–1,500 km) ionosphere. Based on measurements from the suprathermal electron imager (SEI) onboard the Enhanced Polar Outflow Probe satellite over several months, we identify 30 events (28 in the dayside cusp) of enhanced suprathermal electron fluxes peaking at ±90° pitch angles, with energies spanning from tens of eV to 325 eV, the upper limit of the SEI measurement. These events take place with a time duration of the order of 0.1 s, corresponding to a spatial scale of less than 1 km across the magnetic field. They are associated with parallel suprathermal electron bursts and upward currents. The correlation between perpendicular and parallel suprathermal electrons suggests a scenario in which downward bursts and the associated Alfvén waves that drive them provide a free energy source to destabilize plasma waves at electron frequencies, which in turn heat electrons in the perpendicular direction. This is a new energy dissipation process for cusp suprathermal electron precipitation in the topside ionosphere.

Potential Evidence of Low‐Energy Electron Scattering and Ionospheric Precipitation by Time Domain Structures

“…Panels (a)-(d) present electron images for typical (a) accelerated ionospheric electrons, (b) weak precipitating magnetospheric electrons, (c) superimposed accelerated ionospheric electrons and magnetospheric electrons, and (d) intense precipitating magnetospheric electrons. To improve the signal-to-noise ratio, we integrate 10 ms SEI electron images to a time resolution of 50 ms. Electron images are displayed in the same way as described in Shen and Knudsen (2020) where we black out halo electrons less than approximately 120 eV in all the electron images, in order to make suprathermal (120-325 eV in this paper) electron features stand out in the multicolored image. Electron energy increases with increasing distance from the image center.…”

“…Due to satellite blockage of electrons with a gyrodiameter comparable with the instrument boom length (Shen & Knudsen, 2020), electron fluxes are depleted at one perpendicular direction (−90° pitch angle). Therefore, electron images are artificially depleted in the area denoted by the black dashed circle in Figures 2b and 2d.…”

e‐POP Observations of Suprathermal Electron Bursts in the Ionospheric Alfvén Resonator

“…Figure 1a shows the field-aligned (within ±15° pitch angle) energy spectrogram of electrons in the energy range of 150-320 eV. Electrons with energy less than 150 eV are not included due to the large uncertainty associated with the SEI image gain correction in the lower energy range (Shen & Knudsen, 2020).…”

“…In contrast, STEB3 shows a normal dispersion with field‐aligned high‐energy electrons arriving first (UT 06:10:39.6) and followed by electrons with a broader pitch angle distribution (UT 06:10:39.8). Notice that the lack of signal around −90° pitch angle (where the sign distinguishes particles coming from opposite azimuthal angles in the plane perpendicular to B ) in Figure 1b and on the left bottom side of the images in Figures 1d and 1e is due to a satellite blockage effect, where electrons have gyro‐diameters comparable to the SEI boom length (Shen & Knudsen, 2020). A movie of SEI images for this event is provided in Movie .…”

“…Electron images are also affected by nonuniform gain variations gradually developed over e‐POP's operation period. All images presented have thus been corrected using a gain correction map, which has been detailed in Shen and Knudsen (2020). Electron image gain changes within a single orbital segment are negligible.…”