Ap* is defined as the earliest occurring maximum 24-hour value obtained by computing an 8-point running average of successive 3-hour ap indices during a geomagnetic storm event without regard to the starting and ending times of the UT-day. It is uniquely associated with the storm event. Over many years, values of Ap* provide a maximum disturbance measure useful to identify major geomagnetic storms chronologically (by date and start time) and by amplitude from largest to the smallest. The earliest possible values are for 1932, because that is the first year for which the standard Kp and ap indices were produced. As NGDC staff or others have time to update Ap*, tables of values are available from 1932 to recent times. They are available by FTP transfer from the NGDC website.

For more information on the Ap* Index, see The Ap* Index of Maximum 24-Hour Disturbance for Storm Events: An index description and personal reminiscence by its author, J.H.Allen 18 January 2004 (pdf).