Introduction

Abstract

The Sun acts as a formidable source of radiation during a solar flare, throwing out increased electromagnetic radiation in the X-rays and in the ultraviolet and a stream of particles of a wide spectrum of energies, from the solar cosmic rays (E~ 1 GeV) to sub-relativistic protons of energies between 1–1000 MeV that spiral along the Earth’s magnetic field lines into the polar regions within some 80 min to 4 h, down to the slower energy cloud of ions and electrons that envelope the Earth some 20–40 h later. In their interaction with the Earth’s atmosphere, several effects are produced. The increase in the XUV radiation produces immediate increase in the ionospheric ionization of varying degrees at different heights, together called the Sudden Ionospheric Disturbances or SIDs. These increases are also seen as a sudden brightening of the Hα radiation, for long the only way of monitoring such solar eruptions, and over the entire radio spectrum as radio bursts, much of which can penetrate to the ground and are monitored by radio astronomical observatories (Figure 1). The solar cosmic ray particles are recorded at the ground by neutron monitors as Ground Level Events (GLE); such events are relatively few. The sub-relativistic particles (mainly protons) entering the polar regions spiralling down the Earth’s field lines cause increased absorption in the polar cap region, called Polar Cap Absorption Events (PCAs). Slower particles causing magnetic storms also cause complex changes in the ionospheric ionization, principally in the F-region and above. These are the ionospheric storms; there are usually considerable structural changes in the ionization profile. In the higher latitudes there is, in addition, the generation of brilliant aurorae.