Overnight statistical variation of the north-south movements of radio auroral irregularities

The average overnight variation of the north-south mean movements of the auroral scatterers at 42 MHz are determined using mean Doppler shift chart recordings from 24 radio auroral events of different strength and duration. The results indicate, in the average, a well-defined overnight variation of the mean motions of the auroral ionization along the north-south direction. Positive mean frequency shifts (due to equatorward motion) are generally present before local midnight, while negative shifts (northward motions) dominate the morning sector of the events. The reversal of motion occurs during a transition period around local magnetic midnight. The results suggest that the meridional motion components of the electrojet current system in the evening and morning hours and the Harang discontinuity in the midnight-evening sector play an important part in the generation and motion of the scattering irregularities.