Alternate Definitions for geomagnetic activity

Natural variations in the geomagnetic field classified quantitatively into quiet, unsettled, active, and geomagnetic storm levels according to the observed a index:
quiet
   0 - 7
unsettled  
  8 - 15
active
 16 - 29
minor storm
 30 - 49
major storm
 50 - 99
severe storm
100 - 400