Comparison of flow angle variations of E-region echo characteristics at VHF and HF

Abstract

In this study, characteristics of the auroral E-region echoes at two significantly different radar frequencies of 12 and 50 MHz are compared. Considered observations were performed at the Syowa Antarctic station in March of 1997 using two HF and one VHF radars at various angles with respect to the magnetic L shells. The diurnal variation of echo occurrence was found to be similar at two frequencies and consistent with previous studies. On the other hand, variation of echo occurrence with L-shell angle phi was shown to be significantly different at two frequencies. 50-MHz echoes were detected preferentially along the L shell (dominating direction of the electrojet flow) while 12-MHz echoes were detected in a broad range of azimuths with the maximum in echo occurrence at phi approximately equal to 40-50 deg. By plotting the Doppler velocity versus L-shell angle, it is demonstrated that 12-MHz echoes can be divided into two populations, the high- and low-velocity echoes. The high-velocity echoes were observed mostly along the L shells while the low-velocity echoes were observed at all directions. It is also shown that the echo populations exhibit different variation of the Doppler velocity with the L-shell angle. It is argued that while the 50-MHz echoes are related to the Farley-Buneman and gradient drift plasma instabilities, the 12-MHz echoes can have additional sources, such as the thermo-diffusion instability and/or neutral wind-related plasma instabilities.