Daily variability of HF propagation is caused in large part by solar energetic particle radiation carried by the solar wind,
especially during the four years closest to solar minimum when x-ray and proton radiation from solar flares are less frequent and
The three-hourly Planetary Kp Index uses geomagnetic measurements from 13 observatories at mid-latitudes to indirectly measure solar
energetic particle radiation. The Planetary Kp Index is a proxy measurement of the energy input from the solar wind to Earth,
similar to the use of the 2800 MHz Solar Flux Index as a proxy for the Sun's ionizing ultraviolet radiation.
A three day -- updated every three hours -- forecast of the Planetary Kp Index is now available from the Helmholtz Centre Potsdam -
GFZ German Research Centre for Geosciences.