What does the K-index measure?

The K-index is a scale from 0 to 9 that characterizes the magnitude of geomagnetic storms, effectively measuring the short-term stability of Earth's geomagnetic field. It is updated every 3 hours based on measurements from magnetometers located around the globe. A low K-index (generally 0 to 2) indicates a quiet, stable geomagnetic field, which correlates with good HF propagation conditions. A high K-index indicates an unstable, active field or a geomagnetic storm, which generally degrades HF propagation by increasing signal absorption. It specifically measures Earth's field, not the Sun's magnetic field or solar flux.