On the SuperDARN autocorrelation function observed in the ionospheric cusp

The 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 transmitted signal backscattered from the ionospheric irregularities, and derive parameters such as the Doppler velocity and the spectral width. The associated spectra have a specific behaviour inside the cusp, a strong temporal and spatial evolution of the velocity and spectral width, and a high value of the spectral width. Although no studies have explained these characteristics, they are routinely used to detect the cusp in the radar data and estimate the open/closed field line boundary. Both satellite and magnetometer data show in the same region broadband wave activity in the Pc1, Pc2 frequency band. In this study, we evaluate how such wave activity modifies the radar's ACF, and conclude that it explains the spectra seen in the cusp.