The Ap-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. Instead, every 3-hour K-value will be converted back into a linear scale called the a-index. The average from 8 daily a-values gives us the Ap-index of a certain day. The Ap-index is thus a geomagnetic activity index where days with high levels of geomagnetic activity have a higher daily Ap-value.
The daily Ap-value is obtained by averaging the eight 3-hour values of ap for each day. To get the Ap-value you first need to convert the 3-hour Kp-values to ap-values. Be aware that we use the official, finalized Kp which comes from the GFZ in Potsdam, Germany. This Kp-index works slightly different then the preliminary Kp-index. Read about this in our Kp-index help article. To make it a bit more clear on how you can determine the Ap for a certain day, we will work with an example: we take one day with the following measured Kp-values: 0+, 2-, 2o, 3o, 7-, 8o, 9- and 9o. The next step would be to convert these Kp-values to ap-values. The table at the bottom of this article will help you with this. When we are done converting we get these eight ap-values: 2, 6, 7, 15, 111, 207, 300 and 400. The average of these eight values will give you the Ap for that day. The day that we used in this example day would have an Ap-value of 131. The table below will let you convert the Kp-values to ap-values.
Kp | Kp in decimals | ap | G-scale |
---|---|---|---|
0o | 0,00 | 0 | G0 |
0+ | 0,33 | 2 | G0 |
1- | 0,67 | 3 | G0 |
1o | 1,00 | 4 | G0 |
1+ | 1,33 | 5 | G0 |
2- | 1,67 | 6 | G0 |
2o | 2,00 | 7 | G0 |
2+ | 2,33 | 9 | G0 |
3- | 2,67 | 12 | G0 |
3o | 3,00 | 15 | G0 |
3+ | 3,33 | 18 | G0 |
4- | 3,67 | 22 | G0 |
4o | 4,00 | 27 | G0 |
4+ | 4,33 | 32 | G0 |
5- | 4,67 | 39 | G1 |
5o | 5,00 | 48 | G1 |
5+ | 5,33 | 56 | G1 |
6- | 5,67 | 67 | G2 |
6o | 6,00 | 80 | G2 |
6+ | 6,33 | 94 | G2 |
7- | 6,67 | 111 | G3 |
7o | 7,00 | 132 | G3 |
7+ | 7,33 | 154 | G3 |
8- | 7,67 | 179 | G4 |
8o | 8,00 | 207 | G4 |
8+ | 8,33 | 236 | G4 |
9- | 8,67 | 300 | G4 |
9o | 9,00 | 400 | G5 |
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