What are Geomagnetic Storms?

A Geomagnetic Storm is a major temporary disturbance in the magnetosphere that can cuase auroras.

Major magnetic storms can disrupt HF propagation (3-30 MHz) by changing the distribution of free electrons in the ionosphere.

Illustration of Geonagnetic storms as seen from earth close to polar regions (public domain images):

What causes geomagnetic storms?

A CME is a shock-wave of Solar Energetic Particles emitted by the sun.

When a CME enters the magnetosphere, it causes a Geomagnetic Storm

Example: Alerting about G5 Storm, May 10 2024

Geomagnetic Storms from May 11 to May 13, 2024
Observed K index

Observed A index

Kp Index

References:
1. May 2024 Solar Storms ^Wikipedia
2. Geomagnetic Storms May 2024 ^Duckduckgo

_{The Understanding HF Propagation Project provides radio amateurs with a detailed overview and tutorials on several aspects of HF propagation.}