Category

“A” index range

Typical “K” values

Amplitude (Nanotesla)

1. Quiet (I^o)

0 < A < 8

Usually No. > 3

0 - 20

2. Unsettled (II^o)

8 < A < 16

Usually No. > 3

21 - 40

3. Active (III^o)

16 < A < 30

Few indices of 4

41 - 70

4. Minor storm (IV^o)

30 < A < 50

Mostly 4 & 5

71 - 120

5. Major storm (IV^o)

50 < A < 100

Some indices 6

121 - 200

6. Severe storm (IV^o)

100 < A

Some indices 7

201 - >550