Identification of the low-altitude cusp by Super Dual Auroral Radar Network radars:: A physical explanation for the empirically derived signature

The Super Dual Auroral Radar Network (SuperDARN) radars are proving to be a very powerful experimental tool for exploring solar wind-magnetosphere-ionosphere interactions. They measure the autocorrelation function (ACF) of the signal backscattered from ionospheric irregularities, and they derive parameters such as the Doppler velocity and the spectral width. The associated spectra have a specific behavior inside the cusp, a strong temporal and spatial evolution of the velocity and spectral width, and a high value of the spectral width. Until now, no studies have explained these characteristics, but they are routinely used to detect the cusp in the radar data, for example, to estimate the location of the open/closed field line boundary. Both satellite and ground-based magnetometer data from the cusp region show broadband wave activity in the Pc1 and Pc2 frequency band. In this study we evaluate how such wave activity modifies the radar's ACF and we conclude that it explains the spectra seen in the cusp. More specifically, we find that (1) even a monochromatic electric field variation can cause apparently turbulent behavior, including wide spectral widths and apparent multiple components, (2) even low-amplitude waves are capable of causing large spectral widths, if the frequency is sufficiently high, (3) for a fixed low-amplitude electric field variation the measured spectral width increases with wave frequency, displaying a sharp transition from low to high spectral width above an onset frequency, and (4) the determination of the background velocity field is not strongly affected by such conditions. While the wave activity is shown to have a major impact on the spectral width, it is found that the radar does accurately represent the large-scare plasma velocity.