[IRCA] A & K Indices

[Russ Edmunds <wb2bjh@xxxxxxxxx>]

Since there's been some recent discussion, it may be appropriate to
directly explain the relationship between the two.

The K index is measured every 3 hours from various sites.  What we see
on the reports posted to the list is the 'Mid-Latitude K Index'.
Mid-Latitude is defined as 20 = 50 degrees latitude, which includes
mpost if not all of the lower 48 states and adjacent Canada and Mexico.
Usually, these measurements are made from the station in Boulder, CO,
although on occasion reports from another US mainland monitoring
station may be used.

The A index is a mathematical construct which averages the prior 24
hours' worth of K index values according to the scale below:

K    0    1    2    3    4    5    6    7    8    9
a    0    3    7   15   27   48   80  140  240  400

Therefore because the A is an average of the past 24 hours' K indices,
it will naturally lage behind the K, which is a current value. This
becomes especially npoticeable when there is a sudden large change in
the value of K as we've seen this past week. Similarly, the A will lag
behind the K in the recovery phase which we're currently entering from
the recent disturbances.

Hope this answers the lingering questions.




Russ Edmunds
Blue Bell, PA  ( 360' ASL )
[15 mi NNW of Philadelphia]
40:08:45N; 75:16:04W, Grid FN20ID
<wb2bjh@xxxxxxxxx>
FM: Yamaha T-80 & APS9B @15'
AM: Hammarlund HQ-150 & 4' FET air core loop

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