Geomagnetic K index and A index

Ap & Kp index plot for February 1986 (Source: NOAA)

We see in reports for geomagnetic disturbance transmitted by DK0WCY that they contain a K index for the Kiel Magnetometer. The reported K index (Kennziffer index) corresponds to the geomagnetic disturbances at a particular observatory during a three hour period, so it is always followed by the location and time.

The A index provides a daily average level for geomagnetic activity. 

Because of the non-linear relationship of the K-scale to magnetometer fluctuations, it is not meaningful to take the average of a set of K indices. What is done instead is to convert each K back into a linear scale called the "equivalent three hourly range" a-index (note the lower case ‘a’), according to the following table:

K indexa index
00
14
27
315
427
548
680
7132
8207
9400

Geomagnetic a-index vs K-index

The a indices can then be manipulated artithmetically using descriptive statistics methods. The mean, variance etc can be calculated as usual. The daily A index of a geomagnetic observatory can then be calculated as the average of eight a index values. 

The weekly propagation bulletin of ARRL (ARLP) lists daily planetary (Ap) and mid-latitude A indices for the past week. These are derived by averaging the A indices of selected observatories. The Ap index can be converted back to a planetary Kp index, which is reported by other sites, like SpaceWeather.
 
Further reading



The graph of a index vs K index was created using the following GNU Octave script:



% k_index_4 
% Create a plot of geomagnetic a index vs K index 
% based on NOAA data [1] using a cubical spline 
% interpolation in MATLAB or GNU Octave 

% References 
% ---------- 
% .. [1] Geomagnetic kp and ap Indices. 
% http://www.ngdc.noaa.gov/stp/GEOMAG/kp_ap.html

noaa_table = [0.0, 1.0, 2.0, 3.0, 4.0, 5.0, 6.0, 7.0, 8.0, 9.0; 
0.0, 4.0, 7.0, 15.0, 27.0, 48.0, 80.0, 132.0, 207.0, 400.0]; 

k_fit = linspace(0,9,133); 
pp = spline(noaa_table(1,:), noaa_table(2,:)); 
a_fit = ppval(pp, k_fit); 

gr = plot(noaa_table(1,:), noaa_table(2,:), 'ro', k_fit, a_fit, 'b'); 
axis([0, 10.0, 0.0, 500.0], 'tic'); 

title ('Geomagnetic a-index vs K-index'); 
xlabel ('K index'); 
ylabel ('a index'); 
grid('on'); 

% save the graph in SVG format 
print('gr', 'a-index-4.svg', '-dsvg');




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