FAST OBSERVATIONS OF VLF WAVES IN THE AURORAL-ZONE - EVIDENCE OF VERY-LOW PLASMA DENSITIES

The Fast Auroral SnapshoT (FAST) explorer frequently observes the auro ral density cavity, which is the source region for Auroral Kilometric Radiation (AKR). An important factor in the generation of AKR is the r elative abundance of hot and cold electrons within the cavity, since h ot electrons introduce relativistic modifications to the wave dispersi on. VLF wave-form data acquired by FAST within the auroral density cav ity show clear signatures of whistler-mode waves propagating on the re sonance cone. This allows us to obtain the electron plasma frequency, and the cavity often has densities < 1 cm(-3). Moreover, the hot elect rons can be the dominant electron species, enabling AKR to be generate d below the cold electron gyro-frequency.