Polar Audio Tape: VLF Plasma Waves

Auroral Kilometric Radiation (AKR)

Auroral Kilometric Radiation (AKR) is an intense radio emission escaping outward from the earth's auroral regions at frequencies above the local electron plasma frequency. AKR usually consists of a very intense band of emission in the frequency range of about 50-500 kHz. The AKR intensity is usually highly variable, often changing by as much as 60-80 dB on time scales of ten minutes or less. The periods of high intensity tend to occur in storms lasting from a fraction of an hour to days and are closely correlated with global auroral displays, particularly with discrete auroral arcs in the evening sector. The occurrence of intense bursts of AKR is closely associated with the occurrence of inverted-V electron precipitation events. Direction-finding measurements have shown that the most intense bursts of AKR come from a source region on the nightside auroral field lines at radial distances ranging from 2-4 RE. Dayside sources are also observed and are associated with the dayside cusp region. These dayside sources are typically less intense than the nightside sources. AKR is observed in both hemispheres and has been found to propagate in both the right- and left-hand polarization R-X and L-O free space modes.

High-resolution wideband AKR spectrograms consist of many narrowband emissions with rapidly varying center frequencies. This spectral structure is responsible for the rapid combinations of rising and falling tones that are heard on the audio tape.

The two consecutive wideband spectrograms for May 10, 1996 are taken from a pass through the nightside auroral zone in the southern hemisphere. The wideband receiver is configured to obtain data in the frequency range of 250-340 kHz and is connected to the electric Eu antenna. The multiple discrete spectral features are predominantly rising tones of varying frequency dispersions between 270 kHz and 340 kHz.