K-index and Kp
The K-index is a number from 0 to 9 that describes the disturbance of the magnetic field at one observatory over a three-hour period. The scale is quasi-logarithmic, so each step is a much bigger disturbance than the previous one. The Kp index is the planetary average from a network of observatories and is the value normally quoted, often in thirds (for example 3+ or 4-).
When the solar wind carries a southward magnetic field (a negative Bz), energy enters the magnetosphere and Kp rises. A fast stream from a coronal hole or a coronal mass ejection (CME) are the usual causes.
A-index and ap
The ap index is a linear version of Kp, and the daily average of the eight three-hour values is the Ap or A-index. Because it is linear, big storms stand out. Conventional ranges: 0-7 quiet, 8-15 unsettled, 16-29 active, 30-49 minor storm, 50-99 major storm and 100 or more severe storm. Convert between the scales with the Kp to A-index converter.
What it means on the air
- Kp 0-2: quiet. Stable propagation, polar paths usable.
- Kp 3: unsettled. Mild fading on high-latitude paths.
- Kp 4: active. Auroral flutter, weaker signals on transpolar paths.
- Kp 5 and above: storm (NOAA G1 to G5). Lower MUF at mid and high latitudes, strong absorption on polar paths, possible aurora even at mid latitudes.
Low bands at night are affected first. At higher Kp the F layer thins out, so the upper bands lose the openings they would have had on that day.
Where to watch it
The live data page shows the current Kp and A-index with a three-day history, and the space weather forecast shows what NOAA expects for the next days.
