> Are there any guide lines so we can define the severity
> of Auroras?
A book could be written on that subject! :-)
To define the overall intensity of a geomagnetic disturbance
on a global basis, the Kp and Ap indices (planetary K and
A) would be a good starting point. The K (Kp) index ranges
from 0 to 9. Last night Kp was running 4 to 6 (active to
major storm). Previous disturbances this season have
produced periods of Kp 8 and 9 (very severe to extremely
severe storm). The K index is a measure of geomagnetic
activity over a 3 hour period; A index covers a 24 hour
period and is derived from the eight 3-hour K indices for
the 24 hours in question. Kp (and Ap) is derived from a
large network of magnetometers around the world to provide
an overall global representation of storm intensity.
Kp/Ap provide a good means of defining the overall global
intensity of a disturbance, but are not well suited to
defining the intensity of aurora for a particular region at
a particular time, which might be more appropriate to our
purposes if we are interested in a particular propagation
path. There are many variables involved there: local
substorming, time of day, geomagnetic latitude, etc.
Indices derived from one or more local or regional
magnetometers would be more revealing there. I haven't
looked up last night's data for specific regions of
interest. There are other useful indices and measurements
in addition to K and A. I'm not writing the book so I
won't get into that at the moment.
Generally speaking, Kp of 4 to 6 results in very intense
aurora at high geomagnetic latitudes, but far less
impressive aurora here in the northeast US.
The descriptions of storm levels vary somewhat depending on
just where you look up the information, but generally the K
index correlations are as follows:
0=Inactive
1=Very quiet
2=Quiet
3=Unsettled
4=Active
5=Minor storm
6=Major storm
7=Severe storm
8=Very severe storm
9=Extremely severe storm
Paul N1BUG
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