3.1 Some historical remarks

Major optical flares occur only rarely, depending on the phase of the activity cycle, and they last only a few minutes. So it was by pure luck that Mr. R.C. Carrington on September 1, 1859 at 11:18 GMT, while doing routine sunspot observations, could witness such a unique event for the first time (see the worthwhile original report by Carrington, 1860, reprinted in Meadows, 1970Jump To The Next Citation Point). The brilliancy of the flash equaled that of direct Sunlight, and at first he attributed it to a failure of his telescope. Within the 60 seconds it took him to call a witness, the flash had already much changed and enfeebled. Fortunately enough, Mr. R. Hodgson, another observer at a different location, had also seen the flash and confirmed its existence (Hodgson, 1860, also reprinted in Meadows, 1970). Carrington, in his report to the Royal Society, mentioned the potential connection of this strange solar event with the strong geomagnetic storm that occurred only 17 hours and 40 minutes later. He was honest and careful enough not to overvalue this connection (“One swallow does not make a summer.”). However, this discovery can be considered not only as a landmark in modern solar astronomy but also the beginning of space weather research.

This remarkable event was recently revisited by Tsurutani et al. (2005Jump To The Next Citation Point). They used newly reduced and calibrated ground-based magnetometer data and derived characteristic data that can be compared with those from other events in the past 145 years. It turned out that the September 1, 1859 event was among the fastest and most energetic flares ever since (Cliver and Svalgaard, 2004). Of course, there is not much information recoverable about its radiation effects, besides the visual data given by Carrington and Hodgson. But the geomagnetic effects must have been extreme. For example, aurora were sighted to geomagnetic latitudes as low as 20 (Honolulu), and fires were set by arcings from ground-induced currents (GICs) in telegraph wires, both in Europe and the U.S. (see, e.g., Tsurutani et al., 20032005, and references therein).

For quite some time after the discovery of this most spectacular type of event on the Sun, various expressions have been used to name it. The solar flare nomenclature was investigated by Cliver (1995). He revealed that the term flare can be found for the first time in Bartels (1932)’s famous M-region paper. The unofficial use of this new term can be traced to a paper by Richardson (1944) who stated: “The strongest reason for the adoption of ‘flare’ is that in one word are combined the most outstanding features of the phenomenon: its sudden appearance, great brilliancy, and rapid variations in intensity.”

The flare phenomenon has always attracted the solar physics community. Thousands of flares have been observed in all detail and described in hundreds of articles and books (the interested reader is referred, e.g., to the books “Solar Flares” by Švestka, 1976 and “High Energy Solar Physics” by Ramaty et al., 1996, and to the article by Švestka, 1981Jump To The Next Citation Point, with some 500 references on flare observations, and further to the article in Living Reviews in Solar Physics “Flare Observations” by Benz, 2008).UpdateJump To The Next Update Information Because of their significance for solar-terrestrial relations (the term space weather has been coined not before the 1980s), flares were regularly observed by “flare patrols” at several observatories around the globe, and agencies like NOAA listed them in their Solar Geophysical Reports for many years. With the advent of regular X-ray observations from space with the SOLRAD satellite in 1968 and the GOES satellites in 1975 a new and more objective classification of flares in terms of X-ray brightness was established (for historical records see External Linkhttp://www.ngdc.noaa.gov/stp/SOLAR/ftpsolarflares.html#cf).

Theoreticians and modelers have been busy in finding consistent explanations of the flare phenomenon (see, e.g., the textbook Physics of the Solar Corona by Aschwanden (2004) or the upcoming article “Physics of Solar Flares” in Living Reviews in Solar Physics). However, without going into detail here I dare to state: there is no unique, consistent, and generally accepted explanation yet.

The total energy released in the course of flares can differ by several orders of magnitude: from some 1019 J for the smallest events that are barely recognizable as “events” up to some 1025 J for the most energetic ones. Significant fractions of that energy go into radiation, the rest goes into heating and acceleration of particles, the partition depending on the type of flare.

The different kinds of radiation come from different parts of the flare site and are released at different times of the flare process. In the following sections, I will describe the various emissions organized in a timely order as they appear rather than by their spectral properties.

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