R. GUNN has recently suggested (Physical Review, July 1928) that the daily variation of the earth's magnetic field is due to the diamagnetism of the outermost layer of the atmosphere, where the ions and electrons can freely spiral many times round the lines of the earth's magnetic field between collisions; their circular motion renders them equivalent to small magnets directed opposite to the field. The magnetic effect is, however, far outweighed by that of a drift acquired by the charges under the joint action of the magnetic field, gravity, and the vertical electrostatic field which prevents the light electrons from spreading upwards much farther than the heavier ions.
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CHAPMAN, S. The Daily Terrestrial Magnetic Variations; and the Sun's Magnetic Field. Nature 122, 572 (1928). https://doi.org/10.1038/122572b0