Polar Cap Absorption Events

Abstract

In addition to the simultaneous effects, there are delayed ionospheric effects following a flare of two distinct kinds (Figure 1, Chapter 1):

• (i) Polar Cap Absorption Events (PCAs) first observed with Riometers in the polar regions, and caused by MeV solar cosmic ray particles spiralling into the polar regions along the Earth’s magnetic field lines some 20 min to 20 h after a flare.

• (ii) Ionospheric (and magnetic) storms, the auroras, and the auroral absorption effects occurring between 20–50 h after a flare caused by lower energy particles (mostly protons and electrons). These are really clouds of solar gas blown out from the Sun, consisting of equal number of positive ions and electrons.