A few years after Carrington and Hodgson, the Sun was studied extensively in the H line originating
in the chromosphere, and the reports of flares became much more frequent, but also bewilderingly complex.
Variations of source size, ejections of plasma blobs into interplanetary space, and blast waves
(Moreton, 1964) were noted. Meter wave radio emissions, detected serendipitously in 1942 during military
radar operations, revealed the presence of non-thermal electrons in the corona (Hey, 1983). During a radio
burst, the total solar luminosity in radio waves may increase by several orders of magnitude. At about the
same time, S.E. Forbush noticed ground-level cosmic ray enhancements associated with major solar flares.
These discoveries could only mean that the flare phenomena is not confined to thermal plasma, but includes
high-energy particles and involves the corona. In the late 1950s it became possible to observe the
Sun in hard X-rays ( 10 keV) by balloons and rockets. Peterson and Winckler (1959)
discovered the first hard X-ray emission during a flare in 1958. Later, the enhancements observed in
radio and hard X-ray emissions have led to the surprising suggestion that the radiating energetic particles
may contain a sizeable fraction of the initial flare energy release (Brown, 1971). Hard X-rays are
caused by the bremsstrahlung of colliding electrons. The photon distribution in energy exhibits a
non-thermal shape, close to a power-law. Some of the broadband radio emissions from about 1 GHz
to beyond 100 GHz result from the gyration of mildly relativistic electrons in the magnetic
field (termed gyrosynchrotron emission). In 1972 the -ray line emission of heavy nuclei was
discovered, excited by MeV protons (Chupp et al., 1973). Finally, millimeter, EUV, and soft
X-ray ( 10 keV) emissions have shown that the flare energy heats the plasma of coronal
loops to temperatures from 1.5 MK to beyond 30 MK. Within minutes, some active-region
loops become brilliant soft X-ray emitters, outshining the rest of the corona. Such temperatures
and the existence of non-thermal particles requiring low collision rates suggest that the flare is
originally a coronal phenomenon. As a consequence, the discovery observations in the white
light and later in H turned out to have stumbled upon secondary phenomena. This review
will show that present observations get much closer to the heart of flares, but still miss the
primary process. On the other hand, this review will also demonstrate that flares are not purely
coronal. They couple to the chromosphere in a substantial way and must be understood as
processes in which corona and chromosphere form an interactive system (as exemplified in
mpeg-Movie (14976 KB)
Soft X-rays (red), hard X-rays (blue) and gamma-rays (purple) observed by the RHESSI
satellite are overlaid on an optical H image. The movie starts in white light zooming into an
active region. The color then changes to the H line of hydrogen, emitted in the chromosphere.
Its brightening indicates the start of the flare. Visualization by RHESSI scientists.