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1.2 Brief history of flare observations

A few years after Carrington and Hodgson, the Sun was studied extensively in the Hα 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 centimeter1 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, 1971Jump To The Next Citation Point). 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 γ-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 Hα 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 Figure 1Watch/download Movie).

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Figure 1: 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 Hα image. The movie starts in white light zooming into an active region. The color then changes to the Hα line of hydrogen, emitted in the chromosphere. Its brightening indicates the start of the flare. Visualization by RHESSI scientists.

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