Introduction

Solar flares are remarkably diverse and complicated phenomena involving the transient heating of localized regions of the corona and underlying chromosphere within an active region. The sudden release of energy is accompanied by the emission of electromagnetic radiation over a very wide span of the spectrum, ranging – in extreme cases – from γ-rays to kilometric radio waves. In almost all cases flares seen in the chromospheric Hα line also produce an increase in the flux of soft X-rays. Moreover, the variation in the soft X-ray flux with time roughly follows that of the Hα intensity at the brightest point of the flare (Svestka, 1981, p. 74). Both curves are characterized by a rapid rise to a comparatively short-lived maximum, followed by a much slower decay. The intensity of the soft Xray emission generally increases with the optical importance of the flare, although individual flares may show marked deviations from this rule. For this reason it has become customary to assign both an optical (Hα) and an X-ray importance to each flare. Both the Hα and soft X-ray emission pertain to what is conventionally called the ‘thermaly’ or ‘quasi-thermal’ component of the flare, i.e. they originate in plasmas where the distribution of electron velocities is believed to be Maxwellian.

Flares are classified optically according to their area at the time of maximum Hα brightness into the four categories listed in Table 4.1, adapted from Svestka (1981).