Ion cyclotron waves as a possible source of resonant auroral radar echoes
Some auroral radar Doppler data which show the presence of spectra not previously discussed are described. The data were obtained during highly distributed magnetic conditions with a relatively small 50 MHz radar pointed northeast of Ithaca, New York. The data are characterized by strong discrete echoes and display spectral peaks which are even narrower than type 1 and are centered at a considerably smaller Doppler shift. These echoes were present simultaneously with and adjacent in range to the commonly observed auroral spectra, and were probably obtained from a height of about 140 km or higher, well above the center of the auroral electrojet. The results are compared with existing electrojet instability theories, and other plasma instabilities which might be important are discussed. The Doppler shift suggests that the narrow spectra might be caused by ion cyclotron waves generated by field-aligned currents, but the observations cannot be fully explained by any of the theories.