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 index | a index |
0 | 0 |
1 | 4 |
2 | 7 |
3 | 15 |
4 | 27 |
5 | 48 |
6 | 80 |
7 | 132 |
8 | 207 |
9 | 400 |
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
- Ian Poole, G3YWX: "Understanding Solar Indices", QST, September 2002, pp. 38-40.
- NOAA/NCEI: "Geomagnetic kp and ap Indices"
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');
No comments:
Post a Comment