On the recurrence of sudden ionospheric disturbances

Abstract

Recurrence tendency of maximum sudden ionospheric disturbances (SIDs) has been investigated using power spectrum analysis. The power spectrum obtained shows that most significant power appears at the frequencies of 0.03703, 0.05555 and 0.07407 cycle day¯¹, indicating periods of 27.02, 18.0, 13.5 days respectively. This shows that at least four 'active longitudes' in two pairs exist on the solar disc. Each pair is separated by 60° in longitude, and corresponding active longitudes of a pair are separated by 180° in longitude. It is also inferred that the timescale of the evolution of magnetic configuration of active complex prior to flare build-up is in the vicinity of 18.0 days.