The Ap-index

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.

How do you determine the Ap-index?

The daily Ap-value is obtained by averaging the eight 3-hour values of Ap for each day. To get the these Ap-values 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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