Geotail, Polar, and Wind Observations of Auroral Kilometric Radiation

Auroral kilometric radiation (AKR) is the plasma wave/radio phenomenon most clearly associated with substorms and increased geomagnetic activity. The GEOTAIL and POLAR Plasma Wave Instruments (PWI) both included sweep frequency receivers that had an upper frequency limit of 800 kHz and the WIND WAVES Thermal Noise Receiver (TNR) and Radio Receiver Band 1 (RAD1) went to 256 kHz and 1024 kHz, respectively. We have thus been able to observe the majority of the AKR spectrum in better detail than with earlier instrumentation and many important new discoveries have been made. Terrestrial low frequency (LF) bursts are a part of AKR observed during strong substorms. Although a limited portion of the LF burst spectrum is often detected on the dayside of the Earth and in the upstream solar wind, the complete spectrum is most frequently detected by spacecraft in the night side magnetosphere or geomagnetic tail. Frequently these observations show that the LF bursts have a tapered tail centered on the present or recent past solar wind plasma frequency. We have found that on the dayside and in the upstream solar wind the high frequency AKR is detected during LF burst events only if the path from the AKR source is not blocked by the earth or dense plasmasphere. POLAR observations from high over the AKR source region show that the AKR increases in intensity and its lower frequency limits decrease when LF bursts are observed indicating that the AKR source region is expanding to higher altitudes. Frequently the upper frequency limit also increases indicating that the source region is then also expanding to lower altitudes. Data from both satellite and ground-based experiments show that the LF bursts are well correlated with expansive phase onsets and occur during very geomagnetically-disturbed periods. High resolution (in both time and frequency) data from the POLAR Wide Band Receiver have yielded exciting data on the fine structure of AKR as well as details on the structure of LF bursts.