Magnetic aspect angle effects in radar aurora at 48.5 MHz, corrected for refraction

Abstract

It is now widely recognized that at lower VHF frequencies, refraction by electron density structures in the auroral E region can greatly affect the character of radio auroral backscatter, making the quantitative study of magnetic aspect angle effects difficult or impossible. In the present study of data taken with the 48.5‐MHz Bistatic Auroral Radar System (BARS) in central Canada, these difficulties have been minimized by using only data obtained from spatially and temporally uniform events. After correcting for effects of refraction, it has been found that true large aspect angle backscatter occurs for magnetic aspect angles exceeding 6°. Doppler velocities from the smaller aspect angle Red Lake radar behaved as if proportional to the line‐of‐sight component of the convection drift velocity; the resulting flow directions were found to be in good agreement with estimates obtained from magnetometers located in the BARS field of view. The constant of proportionality between the radar velocity and the drift velocity component appears to decrease by a factor of approximately 2 as the aspect angle increases from 3° to 6°. At 3° aspect angle, the backscatter power decreased with increasing aspect angle by about 13 dB/deg (the aspect sensitivity), similar to what has been found in other studies at UHF. In common with those other studies this aspect sensitivity is found to decrease with increasing aspect angle, but more rapidly, falling to less than 4 dB/deg at aspect angles of 5°. In contrast to previous studies at small aspect angle, there is no evidence of a dependence of backscattered power on the flow angle, the angle between the radar line of sight and the direction of E × B drift.