AstroLAB's Interesting Tables  
Classifcation of geomagnetic disturbances
Description K index A index
Quit 0 - 2 < 8
Unsettled 3 8 - 15
Active 4 16 - 29
Minor Storm 5 30 - 49
Major Storm 6 50 - 99
Severe Storm 7 - 9 100 - 400
     
Source: The Cambrdige Encyclopedia of the Sun (Kenneth R. Lang)
A index: a daily index of local geomagnetic activity; the average of the eight 3-hourly
a indices.
K index: a 3-hourly, quasi logarithmic local index of geomagnetic activity ranging from
0 to 9; the 3-hourly local a index of geomagnetic activity ranges from 0 to 400.