A storm is precisely defined as occurring when the daily Ap index exceeds 29. Of a geomagnetic storm, that period when the horizontal magnetic field at middle latitudes is generally decreasing, owing to the effects of an increasing westward-flowing magnetospheric ring current. The northward component can be depressed as much as several hundred nanoteslas in intense storms. The main phase can last for hours, but typically lasts less than 1 day. Of a geomagnetic storm, that period when the depressed northward field component returns to normal levels.
Recovery is typically complete in one to two days, but can take longer.
A rapid fluctuation of the geomagnetic field having periods from a fraction of a second to tens of minutes and lasting from minutes to hours. Pulsations occur at magnetically quiet as well as disturbed times. Do not seem to be physically related to Pc1 or Pc3. Are observed over a wide range of latitudes. Are sometimes called giant micropulsations.