How did sunspots exert their influence? The first clue came on September 1, 1859, in an unexpected observation by the distinguished British astronomer Richard Carrington [Meadows, 1970, p. 181]. Carrington was in the middle of an 8-year study of sunspots and was observing a large sunspot group when "two patches of intensely bright and white light broke out. . . the brilliancy was fully equal to that of direct sunlight" Noting that the spot was rapidly brightening, Carrington rushed off to find a witness, but coming back only 60 seconds later he found the spot of light "much changed and enfeebled" and soon afterward it faded altogether [Carrington, 1860].
As luck had it, the astronomer Hodgson  (see Meadows [1970, p. 187]) observed the same event from another part of England. An unusually intense magnetic storm followed 17 hours afterward, accompanied by polar aurora that could be seen far from the polar regions (another such storm had occurred a few days earlier, probably from the same sunspot group). Carrington noted the coincidence but added "one swallow does not make a summer."
We now know that Carrington had seen a solar flare, a rapid release of energy probably drawn from the sunspot's magnetic field, capable of accelerating electrons and ions to high energies. Flares rank among the most rapid of the Sun's observed phenomena: they can extend over tens of thousands of kilometers, and their fastest features have time scales of seconds, though the whole sequence usually lasts tens of minutes to an hour.
Only rarely do flares emit intense white light, as Carrington's did, but they are readily observable through filters which isolate the red Hα brightenings near sunspots, and in 1892 George Ellery Hale [Wright, 1966] devised the spectroheliograph, which produced images of entire areas on the Sun using only a single spectral wavelength. On July 15 of that year, Hale produced a series of photographs documenting the evolution of a large flare, which was followed 19 hours later by a large magnetic storm [Hale, 1892].
More such correlations soon followed, leaving no doubt that something was propagating from the Sun to the Earth at about 1000 km/s (or faster, as in the two events cited here), causing a magnetic disturbance upon its arrival [Fitzgerald, 1892].
Electron Beams from the Sun?
What was it? One clue seemed to come from discharges in low-pressure gases and from beams of "cathode rays" propagating between electrodes in evacuated vessels. Laboratory studies showed that these "rays" consisted of electrically charged particles whose properties were measured by J. J. Thomson and which were eventually named electrons [Thomson, 1967; Shamos, 1959]. Electron beams propagated at great speed, which led to the plausible suggestion that the source of observed disturbances was streams of electrons emitted from sunspot regions.
The first serious study of this phenomenon was performed by the Norwegian Kristian Birkeland [Birkeland, 1901, 1908; Egeland, 1984,1986; Devik, 1968; Boström, 1968]. In 1896 Birkeland aimed cathode rays at a magnet and found that the magnet apparently "sucked in" cathode rays: he suggested that the Earth's field did the same to beams from the Sun. He communicated his findings to his former teacher, the French mathematical physicist Henri Poincaré, who showed that rather than being attracted, charged particles were guided by magnetic field lines [Poincaré, 1896]. Poincaré calculated the motion of an electron in the field of a magnetic monopole, a completely soluble problem, and found that the electron spiraled around a cone bounded by field lines, gradually losing headway until at a certain distance it was reflected backward [Rossi and Olbert, 1970, section 2.5; Mitchell and Burns, 1968].