FINE SPECTRAL AND TEMPORAL STRUCTURE OF VLF-CHORUS OBSERVED BY CLUSTER

We analyze chorus VLF emissions measured by the four Cluster spacecraft (SC) Wideband Detectors (WBD) at close separations during geomagnetically disturbed periods. Data of electric field Cluster WBD antennas were used. Satellites space separation along magnetic field was 50-300 km, and cross field about 10-50 km. Direct waveform signal study revealed very fast variations of signal frequency and amplitude at the time scales about only several milliseconds without correlation for the different satellites. For chorus spectral analyses we used not only ordinary FFT, we mostly applied methods basing on autoregression (AR) models of signal, providing very high spectral and temporal resolution of spectrograms (better than 5-10 milliseconds and 10-20 Hz). All emissions (hiss, chorus, triggered and others) are structured at the scales less then 10 hertz and 10 milliseconds, and spectral details are rather different, even if satellites space separation is less than 100 km. Using correlation functions calculation for vertical and horizontal lines of AR-spectrogram matrix and constructing resulting two-dimensional pictures allowed measuring signals time shift for Cluster satellites, and so detecting the direction and velocity of emissions motion (and correspondingly frequency shifts). Cluster data for 27-11-2002 on chorus with decreasing frequency are shown. Wavelet spectral analysis demonstrates different interconnections and correlations between low (50-400 hertz) and high (3000-10000 hertz) components of spectra.