GeoMagnetism and Solar Physics

The subjects of geomagnetism and solar physics are closely related under the general heading of solar-terrestrial relations, and have been studied with increasing intensity over the past hundred years. A most interesting review of the very early historical development, from the time of Gilbert in 1600, may be found in the small book by Chapman, Solar Plasma, Geomagnetism, and Aurora. Chapman's own contributions to the subject of solar-terrestrial relations can be traced back to his interest in the kinetic theory of gases, beginning about 1916. The ideas developed by Chapman and Enskog for treating the transport properties of gases through a perturbation expansion of the solution of the Boltzmann equation have become the basis for most of the present theoretical understanding of the viscosity, electrical conductivity, thermal conductivity, and diffusion coefficients for gases. The best known summary of this work is in The Mathematical Theory of Non-Uniform Gases, by Chapman and Cowling. The numerical values derived from the Chapman-Enskog method form the basis for understanding the kinetic properties of the gases at the sun, the solar corona, the terrestrial ionosphere, etc. The brief summary of some of the results in the article by Chapman in the Astrophysical Journal (1954.5) is particularly useful for such problems.

Chapman's interest in the kinetic theory of gases has continued. For instance, he pointed out only ten years ago that the most highly ionized species in a mixture of fully ionized gases tend to accumulate in the regions of higher temperature. The effect is now proving to be of interest for understanding the anomalous abundances of heavy elements in the solar corona and solar wind.

In the twenties Chapman entered into studies and analyses of geomagnetic variations and auroral phenomena. His first contribution in these fields was his recognition of the relevant physical parameters, such as diurnal variations, storm time, etc., on which to base the statistical studies of the great mass of observational data available. The analysis, and consequent recognition and separation of the various physical effects, is now available in the work, Geomagnetism, by Chapman and Bartels (see also Chapman, The Earth's Magnetism). Present-day analysis of geomagnetic and auroral data uses methods which are, to a large extent, a direct outgrowth of the original methods introduced by Chapman.

Chapman's interest in geomagnetic, ionospheric, and auroral observations was only part of his interest in understanding their origin. He was also instrumental in developing the dynamical theory of the origin of the various phases of the geomagnetic storm. In 1882, Balfour Stewart pointed out that the daily variation of the geomagnetic field at the surface of Earth must be caused by electric currents in the upper atmosphere. It was recognized by Stormer that Maxwell's equations require that the observed magnetic storm variations be associated with electric currents. In 1924 Schmidt pointed out that the main phase of a magnetic storm was equivalent to the addition of a southward field of 0.5 x 10^-3 to 3.0 x l0^-3 gauss across Earth, and postulated that the current associated with this field was in the form of a ring with the ions circulating westward around Earth and the electrons eastward. There was, at the time, a great variety of ideas on the cause of the westward ring current, invoking ultraviolet effects, solar corpuscular emission, etc., in association with solar activity. In 1930, Chapman and Ferraro set up the specific line of thinking that has evolved into the present-day understanding of the geomagnetic storm. They pointed out that the delay of geomagnetic and auroral activity of one or more days following a solar outburst rules out the ideas based on electromagnetic radiation from the sun. Corpuscular emissions with velocities of the order of 10³ km/sec must be responsible. Adopting the point made by Lindemann that the solar corpuscular emission must be electrically neutral (because of the enormous electrostatic forces which arise when even very small numbers of charges of one sign are carried away from the sun), Chapman and Ferraro pointed out, and illustrated with a number of idealized examples, that the impact of a cloud or stream of electrons and ions against the geomagnetic field compresses the geomagnetic field. The compression produces an increase in the horizontal component at the surface of Earth. They pointed out that this must be the origin of the initial phase of the magnetic storm. They showed that the incident stream of particles and the geomagnetic field do not interpenetrate, the boundary layer being only 100 km or less in thickness. This picture was verified thirty years later by direct satellite observations of the magnetic field and the solar wind plasma. It is interesting to note, too, that the work of Chapman and Ferraro on the impact of charged particles against the boundary of a magnetic field has been taken up and vastly extended since 1950 in connection with the dynamics of laboratory plasma.

Chapman and Ferraro went on to suggest that the main phase of the storm was caused by differential circulation of ions and electrons in large concentric circles within the magnetic field around Earth. The idea has since been shown to be wrong in detail, but it called attention to the importance of looking directly at the dynamical properties of particles interacting with the geomagnetic field to account for the main phase. The present ideas are directly traceable through several stages of evolution to their original ideas.

Altogether, most of the present knowledge of solar-terrestrial relations can be traced back through earlier developments in which Chapman's work on kinetic and transport properties of gases, observational analysis, and particle-field dynamics played a central role.