Location of the Currents Causing the Solar and Lunar Diurnal Magnetic Variations

Abstract

In 1882 Balfour Stewart1 put forward the view that the solar and lunar diurnal magnetic variations were produced by currents in the upper atmosphere. He suggested that these currents were produced by the ‘dynamo’ action of thermal or tidal motions of conducting air across the earth's main magnetic field. Schuster2 put these suggestions on a quantitative basis. It was later realized (Chapman3) that there were difficulties in assigning both the solar and lunar variations to a single ionized region. For example, the amplitude of the former is some 2·4 times larger in summer than winter, whereas the corresponding ratio for the latter is 4. Again, solar variations are some 1·6 times greater at sunspot maximum than at sunspot minimum, but lunar variations show little difference in amplitude between sunspot maximum and minimum. Since it is unlikely that the tidal motions change so markedly with season and sunspot activity, it seems probable that the conductivities of the regions responsible for the variations change in the above ratios: and since these ratios are different for solar and lunar variations it would seem that these variations must arise in different conducting regions of the atmosphere.