A polar cap absorption event results from the ionisation of the D-layer of the polar ionosphere by high energy protons. A PCA causes a HF radio blackout for trans polar circuits and can last several days. PCAs are almost always preceded by a major solar flare with the time between the flare event and the onset of the PCA ranging from few minutes to several hours. Increased absorption, can also occur with increased auroral oval activity, typically in association with geomagnetic storms. A radio was wave passing through auroral areas can experience high absorption.

A riometer (relative ionospheric opacity meter) is a radio receiver that measures the amount of absorption of galactic radio noise (a natural radio source) as this signal passes through the Earths ionosphere from space. During periods of increased auroral activity ionospheric absorption can increase. This increase in absorption, or opacity of the ionosphere, reduces the signal strength of the galactic radio signal. This reduction is signal strength is measured in dB. The Australian Government Antarctic Division (AGAD) operates riometers at its bases in Antarctica.