Launch Watch · Earth

Published 4 October 2026, 08:54 HST (UTC−10)

Kp and Ap rank two 2024 storm days differently: a 366-day audit

May 10 and October 8 have nearly equal mean Kp but daily Ap of 105 and 54. Reproduce all eight intervals and the full 2024 aggregation check.

Which day ranks higher: 10 May or 8 October 2024? Average the eight rounded Kp fields and October edges ahead, 4.833375 versus 4.791875. Read the published daily Ap index and May is far higher, 105 versus 54.

The reversal is reproducible in GFZ’s geomagnetic-index data. It comes from summarizing different quantities. Daily Ap is based on the eight three-hour ap values, whose relationship with Kp is nonlinear. Taking a mean of Kp first does not reproduce that operation.

The Kp means in this table are our descriptive calculations from the file’s rounded fields. They are not an official daily Kp product. The capitalized Ap column is the daily value published by GFZ.

The eight intervals explain the reversal

The two days distribute their activity differently. On 10 May, the first five ap entries total 62; the last three are 179, 300 and 300, totaling 779. Those three intervals contribute about 92.6% of the day’s sum of ap values. On 8 October, the largest ap values occur early, at 111 and 132, and the later values are lower.

The daily calculation is visible without reading a chart: May’s eight ap values total 841, so their mean is 841 ÷ 8 = 105.125. October’s total is 436, giving 436 ÷ 8 = 54.5. The file records integer Ap values of 105 and 54 respectively. These sums are arithmetic over index values, not a calculation of energy released or a local physical exposure.

What Kp, ap and Ap represent

GFZ’s index documentation describes Kp as a three-hour planetary geomagnetic index derived from ground-observatory records. It is relevant to reading space-weather records, but is not measured from the satellite imagery discussed elsewhere on this site.

GFZ identifies ap as the corresponding linear index and Ap as its daily mean. The conversion from Kp to ap is nonlinear. The selected data make that visible: Kp 2.000 corresponds to ap 7, Kp 4.000 to ap 27, Kp 7.000 to ap 132 and Kp 8.667 to ap 300. Equal-size steps on one scale cannot be treated as equal-size steps on the other.

As a result, a short run of high Kp values can weigh strongly in the ap average. October’s mean of rounded Kp values exceeds May’s by only 0.0415, while May’s published Ap exceeds October’s by 51. A single Kp average hides the interval pattern that drives the difference.

This does not mean every use of averaged Kp is invalid. It means the average must be named correctly and used for its stated purpose. If the required quantity is daily Ap, average the corresponding ap values and retain the provider’s published Ap field instead of labeling a mean Kp as equivalent.

A full-year check around the example

To test the arithmetic beyond two selected dates, we extracted all 366 UT-day records for leap year 2024 from the frozen GFZ source. The audit includes 2,928 three-hour Kp/ap pairs, with no missing intervals and definitive Kp status throughout. There are 28 distinct Kp-to-ap value pairs in that subset.

For each day we averaged its eight ap entries and compared the result with the published Ap. The largest absolute difference is 0.5. All 366 published values match integer rounding with ties to even in our check, including October’s 54.5 becoming 54.

That last finding describes this retained snapshot. The file-format documentation says Ap is rounded to an integer; this year’s agreement does not establish an undocumented universal tie-breaking policy. For exact published historical values, the supplied Ap column remains the record to preserve.

Reproduce the result without changing its meaning

Start with the 2024 index-only CSV. Each row is one UT day, and columns 1 through 8 within each index run from 00:00–03:00 through 21:00–24:00. Select the two dates, add each group of eight values, and divide by eight. Keep “mean of rounded Kp,” “mean ap” and “published Ap” as separate labels.

Our independent re-extraction from the original source agrees numerically with the retained index-only subset for every field and day. Dates are continuous from 1 January through 31 December. The check validates this extraction and these calculations; it does not validate a forecast model or infer consequences for an individual spacecraft.

The comparison also cannot tell a reader whether aurora was visible at a specific location, whether a satellite was damaged, how much orbital drag occurred, or whether a launch should have been canceled. Those questions require observations and models beyond these two index summaries.

Data credit: GFZ Helmholtz Centre for Geosciences, Geomagnetic Observatory Niemegk; Matzka et al. (2021), Geomagnetic Kp index and The geomagnetic Kp index and derived indices of geomagnetic activity. Kp, ap and Ap are used under CC BY 4.0. Subsetting, calculations and charts are modifications by LaunchDetect; no endorsement is implied. Accessed 4 October 2026.

Only the Kp/ap/Ap fields and their date/status metadata are redistributed here. The combined provider file contains separately licensed sunspot data, which are excluded from every public data file and graphic in this article.