A and K Indices
A Index
The A-Index describes the geomagnetic conditions of the last 24 hours. It can range from 0 to 400 but usually does not go above 75. The A-Index readings usually are between 4 and 50. Values below 10 are desirable for HF communications. Higher A-Index numbers can mean excessive absorption of radio waves in the ionosphere. The A-Index is an average calculated from eight daily 3-hour K-Indices.
K-Index
The Kp index is derived at the NOAA Space Weather Prediction Center using data from ground-based magnetometers and is used to predict ionospheric propagation of HF radio signals. Although geomagnetic storms, indicated by a Kp of 5 or higher, have no direct effect on propagation, they disturb the F-layer of the ionosphere causing an ionospheric storm. As a result, radio propagation is degraded mostly at middle and high geographical latitudes. The degradation is a reduction of the maximum usable frequency (MUF) by up to 50%. Sometimes the E-layer may be affected as well. In contrast with sudden ionospheric disturbances (SID) caused by solar flares, which affect high frequency radio paths near the Equator, the effects of ionospheric storms are more intense in the polar regions.
This chart is updated every 15 min. at 1, 16, 31, and 46 minutes past the hour.
This chart is updated every 15 min. at 1, 16, 31, and 46 minutes past the hour.
The Estimated 3-hour Planetary Kp-index, above, is derived at the NOAA Space Weather Prediction Center using data from ground-based magnetometers.
The K-index quantifies disturbances in the horizontal component of earth's magnetic field with an integer in the range 0-9 with 1 being calm and 5 or more indicating a geomagnetic storm. It is derived from the maximum fluctuations of horizontal components observed on a magnetometer during a three-hour interval. The label 'K' comes from the German word 'Kennziffer' meaning 'characteristic digit.' The K-index was introduced by Julius Bartels in 1938.
The official planetary Kp index is derived by calculating a weighted average of K-indices from a network of geomagnetic observatories. Since these observatories do not report their data in real-time, various operations centers around the globe estimate the index based on data available from their local network of observatories. The Kp-index was introduced by Bartels in 1949.
K-index Watches are issued when the highest predicted K-index for a day is K = 5, 6, 7, or >= 8 and is reported in terms of the NOAA G scale.
K-index Warnings are issued when NOAA K-indices of 4, 5, 6, and 7 or greater is expected.
K-index Alerts are issued when the NOAA K-index reaches 4, 5, 6, 7, 8, or 9 in a 3-hour period.
Artist's depiction of solar wind particles interacting with Earth's magnetosphere:
