FAST satellite wave observations in the AKR source region

The Fast Auroral SnapshoT (FAST) satellite has made observations in the Auroral Kilometric Radiation (AKR) source region with unprecedented frequency and time resolution. We confirm the AKR source is in a density depleted cavity and present examples in which cold electrons appeared to have been nearly evacuated ( n hot > n cold ). Electron distributions were depleted at low‐energies and up‐going ion beams were always present. Source region amplitudes were far greater than previously reported, reaching 2×10 −4 (V/m)²/Hz (300 mV/m) in short bursts with bandwidths generally <1 kHz. Intense emissions were often at the edge of the density cavity. Emissions were near or below the cold plasma electron cyclotron frequency in the source region, and were almost entirely electromagnetic. The | E |/| B | ratio was constant as a function of frequency and rarely displayed any features that would identify a cold plasma cutoff or resonance.