Comparisons between strong HF backscatter and VHF radar aurora

Abstract

It is well known that various types of irregularity structures exist in the E and F regions of the auroral and subauroral ionosphere. In this study the location and occurrence probability of these structures have been compared with HF and VHF radars located in Finland and Germany. It has been found that E‐region radar aurora is observed at both HF and VHF frequencies and that its cross section approximately follows a λ3 law in this frequency range. The F‐region irregularities and the backscatter echoes caused by them are found to fall into auroral and nonauroral groups. The auroral group occurs near the L values of radar aurora and is often observed on the same magnetic field unes as the equatorward portion of the VHF radar aurora. Moreover, there is a second type of F‐region echo that appears to be anticorrelated with radar auroral activity. This latter type of F‐region echo occurs at lower latitudes (L value between 3 and 4) and appears to require low electron densities in the F region. HF and VHF radar auroras are known to be produced by electrostatic plasma streaming instabilities. The sources of F‐region HF backscatter are not as yet known; however, they may be due to either irregular structures in particle precipitation or to plasma instability mechanisms.