Rapid Periodic Fluctuations of the Geomagnetic Field. I

Part I of this paper contains the presentation of a new analysis of observational material, mainly from Eskdalemuir, and describes some unexpected properties of geomagnetic fluctuations. The chief of these are (a) that there is a change of type at sunset from a continuous flux of disturbance to a comparative quiet punctuated by short damped wave trains, and (b) that the daytime fluctuations have a spectrum with definite fine structure. Part II is a discussion of the probable origin of geomagnetic pulsations. By comparing the characteristics found in Part I with those reported by previous observers it is concluded that the effects have a common cause, and very general arguments are then put forward to show that their origin is terrestrial. A survey of possible physical models points, after elimination, to a magneto-hydrodynamical resonance in one or more of the ionospheric layers.