Theories of Magnetic Irregularities and Variation

The inferences to be drawn from terrestrial magnetism as re gards the conductivity of the upper atmosphere have been sum marized under ELECTRICITY : ATMOSPHERIC.

Magnetic

of the first attempts to explain mag netic storms attributed them to the magnetic fields produced by streams of charged particles emanating from the sun ; but, as pointed out by Schuster, such theories are untenable because the natural repulsion of the parts of such a stream would prevent its arriving at the earth in sufficiently concentrated form. S. Chap has shown that the most characteristic features of mag netic storms may be explained by supposing that they are pro duced by atmospheric currents set up as a result of the cutting of the earth's horizontal component by a radial motion of an at mospheric conducting layer. The magnetic phenomena suggest a radial motion inwards at the commencement of the storm, fol lowed by an outward radial motion. In the original form of Chapman's theory the primary agency responsible for the storm was a stream of alpha particles from the sun, the outward radial motion resulting from the subsequent mutual repulsion of the positive charges. Lindemann") criticized the theory in this form on the grounds that a stream of alpha particles of the necessary strength would imply an unwarrantedly large radio activity in the sun, and that further, a stream of the necessary intensity could not hold itself together against self-repulsion in its journey from sun to earth. Lindemann himself suggested as the origin of these storms, groups of positive and negative particles shot out from the solar prominences under the influence of the pressure of light. As a matter of fact, actual observations of the rate of growth of solar prominences indicate that the gases in these cases attain velocities of the order of cms. per second, and this velocity is sufficient to make reasonable the assumption of their journey from sun to earth without serious recombination. The neutrality of the stream as a whole would provide an escape from the difficulties inherent in self-repulsion. Lindemann sup posed that the negative ions may be lighter than the positive, so that the former are stopped in the outermost regions of the at mosphere, while the positive ions penetrate deeper down and are responsible for auroral phenomena.

If, as is likely to be the case, the positive ions were hydrogen atoms or molecules, they would be lighter than the molecules to which the electrons attach themselves on their impact with the outer atmosphere. The subsequent attraction of the positive and negative ions would consequently result in an outward motion of the negative ions which would be more rapid than the inward motion of the positive ions. The net result would be that, follow ing the initial effects due to the influx of the stream, there would be the equivalent of an outward flow of gas, and this gas cutting across the horizontal component of the earth's field would give rise to the type of atmospheric currents necessary to account for the most characteristic features of magnetic storms").

Of course, the earth's magnetic field would act independently on the two oppositely charged portions of the neutral stream of particles, tending to separate them, but with a sufficiently in tense beam this tendency to separate would become almost im mediately compensated by the electrostatic field resulting from the separation, so that the beam as a whole would travel practi cally uninfluenced by the earth's magnetic field. The facts asso ciated with the aurora do demand a concentration of ionizing towards high latitudes, however, so that it would seem difficult to avoid the conclusion that there is an excess charge of one sign in the material radiations from the sun, unless, indeed, it is possible to formulate a condition, satisfying the mathematical requirements and in which the magnetic field of the earth bends the paths of both the positive and negative particles in such a manner as to result in a concentration in the auroral zones without there being necessarily a large excess density of one sign anywhere. In this connection it must be remembered that the path of a charged particle may be bent completely around the earth, so that it is not out of the realm of possibility that in spite of the fact that the positive particles are bent in opposite directions to the negative by the magnetic field, there may exist a general inter mixing and intersection of orbits of oppositely charged particles such as to reduce greatly the electrostatic fields which would other wise be set up in the absence of such conditions.