One remarkable characteristic of on-off switching (or pulsating) aurora in the dawn sector during the postbreakup phase is shown to be the quasi-periodic recurrence of nearly the same pattern. Such a remarkable recurrence of the same pattern can be likened to the storage of 'memory' in a computer and its recurrence as a result of 'calls' for the stored memory. Here the magnetosphere takes the role of the computer. Persistent irregularities in the background thermal plasma of the magnetosphere are shown to account for the memory of the pattern. The electron cyclotron instability accompanying growth and trapping of whistler mode waves in an irregularity column of the plasma, owing to energetic electrons overtaking the slowly drifting plasma, can be regarded as the call-up mechanism. The recurrence consequently can be understood in terms of the rates of supply and loss of energetic electrons into a certain column of thermal plasma. |