Spectral characteristics of UHF radar aurora

Abstract

Radar observations at 440 (Millstone Hill) and 933 MHz (European Incoherent Scatter (EISCAT)) and at both small and large magnetic aspect angle have been used to study the spectral characteristics of backscatter from the auroral E region. The predominantly single‐peaked power spectra were found to have very similar shapes at both frequencies and at both small and large aspect angle. Spectral moments were estimated directly from the autocorrelation functions (ACFs) and were also characterized in terms of a correlation time (related to spectral width) and a decay exponent which quite accurately described the observations. For the most part, spectral shapes were found to be intermediate between Gaussian and Lorentzian, at both large and small magnetic aspect angles. Similar ACF shapes have been observed at much larger wavelengths at HF [Villain et al., 1996], suggesting that this may be a widespread and possibly fundamental characteristic of radar auroral backscatter. In addition to confirming and extending a number of previous observations on UHF spectral characteristics, correlation times were found to decrease sharply near an aspect angle of 3°, having a nearly constant value at all larger aspect angles. This behavior is consistent with a simplified picture of the energy balance for these large aspect angle waves.