1.1 Detection and definition
Since September 1, 1859, when R.C. Carrington and R. Hodgson observed the first flare in the
continuum of white light, the localized, minute-long brightenings on the Sun have remained an enigma.
Local flaring of the Sun has been reported at all wavelengths accessible from the ground and
from space. Thus the word “flare” is used in solar physics today in a rather ill-defined way,
describing a syndrome of apparently related processes at various wavelengths. The problem
is even more acute in other languages, such as German and French, denoting flares with the
equivalent to “eruption”, which may be confused with Coronal Mass Ejections (CMEs) that often
happen simultaneously at the time of large flares. The general use of the term “flare” today often
alludes to a “sudden release of magnetic energy by reconnection”. However, one has to bear in
mind that such a definition represents a specific, although widely accepted, interpretation of
observations. It may be used as a guide for novices to distinguish flares from other plasma
physical phenomena in the solar atmosphere also associated with brightenings, such as the
expulsion of magnetic flux or dissipation of shock waves. Nevertheless, it is better to define the flare
phenomenon observationally as a brightening of any emission across the electromagnetic spectrum
occurring at a time scale of minutes. Most manifestations seem to be secondary responses to the
original energy release process, converting magnetic energy into particle energy, heat, waves, and
motion.
