Are type III radio aurorae directly excited by electrostatic ion cyclotron waves

In 1981, a network of three 50-MHz radar transmitters and two receivers was operated in the CW mode on the Canadian prairies. The echoes obtained from coherent ionospheric backscatter were divided into segments of 205 ms such that their FFT spectra yielded a frequency resolution of 4.9 Hz. The spectra were subsequently averaged over 10 s. Type III spectra (narrow spectra with sub ion-acoustic Doppler shifts) were observed (often simultaneously) on radar links whose wave vector components perpendicular to the geomagnetic field were almost identical while their components parallel to the field were significantly different. From a statistical analysis of more than 300 type III spectra it is inferred that these are in general unlikely to arise from electrostatic ion cyclotron waves directly excited by an essentially linear process. Doppler shifts around 55 Hz were much more frequently observed than around 30 Hz, the occurrence of type III spectra increased with increasing magnetic aspect angle (deviation of the scatter wave vector from perpendicular to the geomagnetic field), and the mean Doppler shifts of type III spectra observed simultaneously on different radar links went through a minimum for aspect angles between 4° and 7° (depending on the assumed backscatter height). These three results disagree with theoretical expectations. The spectral width of the type III echoes decreased linearly with magnetic aspect by about 2 Hz/deg.