Polar cap absorption (PCA) events

Strong solar particle (SP) events (see Cosmic rays) produce intense ionization in the D-region (between 50 and 90 km altitude) ionosphere over the polar caps. These events are called polar cap absorption (PCA) events because the increased ionization absorbs radio waves in the HF and VHF bands. PCA events can be studied, e.g., by riometers.

See also:

• Space Physics Text Book of Oulu