Solar Energetic Particles

Abstract

The emission of energetic particles from the Sun is one of the major manifestations of high energy astrophysics and is related to a variety of astrophysical and geophysical phenomena in the heliosphere. An event of solar energetic particles (SEP) is seen as a sudden rise of the counting rate of the background galactic cosmic rays reaching a high value in a few minutes, as measured by ground level or spacecraft borne detectors. The counting rate i.e. the intensity reaches a maximum value which may be 10 to 100 times more than the background level in small and medium events and may be more than 1000 times more in large events. Following the maximum the intensity slowly decreases compared to the background during a few hours in an average event and in a day or so in case of a very large event. A day or two after the optical flare and the solar energetic particle (SEP) event, the plasma cloud emitted by the Sun reaches the earth and causes a magnetic storm and Forbush decrease in the galactic cosmic ray intensity (Chapter 3.5). In contrast to the solar wind and plasma clouds from the Sun which contain protons with energies of 1 to 10 keV, SEP events have protons with energies of 10 MeV to 100 MeV in average events and up to 1 to 10 GeV or more in large events. As these energies overlap the proton energies of galactic cosmic rays, these SEP events are also called solar cosmic ray (SCR) events by some authors.