By the end of the 19th century at least some disturbances of the Earth's magnetic field could be traced to the Sun. Some seemed to follow flares, others showed their solar origin by tending to recur at intervals of 27 days, the Sun's rotation period.

  But how did the Sun exert its influence?

Early ideas that the Sun sent out streams of electrons (which also produced the aurora) were given up when it was realized that the unbalanced negative electric charge of such electrons would completely disrupt the process.

The Chapman-Ferraro Cavity



   Then in 1930 a different idea was proposed by Sidney Chapman in England and by his younger associate Vincent Ferraro: that the Sun sent out huge clouds of electrically neutral plasma, and that magnetic storms arose when those clouds enveloped the Earth. Many magnetic storms were observed to begin with a "sudden commencement," a small step-like jump in the magnetic field observed all over the world, taking just a minute or so. Chapman and Ferraro proposed that such jumps marked the cloud's arrival.

   The theory of the "Chapman-Ferraro Cavity" proved to be prophetic, except for one important detail: the flow of plasma from the Sun was not confined to isolated clouds, but went on all the time, in the form of the solar wind. Denser and faster clouds, such as arise from coronal mass ejections (see corona), were later identified as the real cause of sudden commencements.

Explorer 12

   The magnetopause itself was first observed in 1961 by NASA's Explorer 12. Many satellites have since then studied and probed it--notably, Europe's HEOS 1 and 2 (1975-6) above the poles, as well as NASA's ISEE-3 (1983) and Japan's Geotail (1992-4), extended missions which, during the marked years, probed the distant tail region, up to distances

      Further reading:

• Ferraro, Vincent C.A., "The Birth of a Theory" (an account of the beginnings of the Chapman-Ferraro theory), p. 14-18 in Sidney Chapman, Eighty, by S.-I. Akasofu, B. Fogle and B. Haurwitz, U. of Colorado Press, Boulder, 1969.

• Cahill, Lawrence J., Jr., "Early study of the boundary of the geomagnetic field" (an account of the Explorer 12 observations), in Physics of the Magnetopause, B.U.Ö.Sonnerup and P. Song, editors, Amer. Geophys. Union, Washington, D.C. 1995.