I n t r o d u c t i o n 295 1 * 3* D y n a m o t h e o r y f o r a n a r b i t r a r y R e f e r e n c e s 305 Assuming semi-diurnal tidal air flow a solution is made of the atmospheric dynamo problem, taking account both of the direct and transverse conductivities of the ionosphere. The spherical sheet ionosphere is divided into three regions, a narrow equatorial zone, and two wide polar caps, taking appropriate constant conductivities in each region. The current system is similar in shape and

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... phase to that derived on the assumption of uniform (direct) isotropic conductivity, but is considerably more intense than that which would be obtained without the existence of transverse (Hall) conductivity. The electric field system is very different, however, from that derived on this (isotropic) assumption. An abnormally large east-west current is found at the equator, which appears to provide the explanation of the anomalous magnetic variation in this region. Curves are given showing the distribution of the field and current components. V ol. 246. A. 39