Introduction

A solar flare is narrowly defined as a sudden atmospheric brightening, traditionally in chromospheric Hα emission but more practically now as a coronal soft X-ray source. The physical processes resulting in a flare include restructurings of the magnetic field, non-thermal particle acceleration, and plasma flows. Flares have intimate relationships with other observable phenomena such as filament eruptions, jets, and coronal mass ejections (CMEs). Chapter 6 discusses our current theoretical understanding, and in this chapter we review the observational aspects of these phenomena.

The phenomena associated with the term “solar flare” dominate our thinking about energy conversion from magnetic storage to other forms in the solar corona on time scales below a few minutes. The distinction between a gas dominated by hydrodynamic forces and a magnetized plasma becomes obvious in the solar atmosphere and in the solar wind. At first glance we do not need plasma physics to explain the basic (interior) structure of a star; hydrodyamics, nuclear physics, and the theory of radiative transfer seem to do quite well. Nevertheless, this apparently simple medium drives the currents that result in the violent and beautiful phenomena we see so readily above its surface (see Vol. III). We need plasma physics to describe them.

Understanding the flaring solar atmosphere (photosphere, chromosphere, and corona; see Chapter 8 in Vol. I for descriptions of these regions), since it involves electrodynamics, requires a strong overlap with magnetospheric physics as well as with astronomical techniques useful for studying stellar atmospheres.