A periodic variation of τ, the semithickness of the F-layer, was found at ColIege, Alaska. Harmonic analysis of the variations of τ for the winter data of 1948–1949 reveals solar and lunar diurnal and semidiurnal waves. The solar 24-hour wave, considered to be mainly thermal in character, has an average amplitude of 11 km with maximum occurring at 1200 LMT. The solar semidiurnal wave, considered to be due chiefly to the gravitational attraction of the sun, has a mean amplitude of 6 km with maximum occurring at 0400 (or 1600) LMT. Instead of the significant lunar semidiurnal wave usually found in geophysical phenomena, a lunar diurnal tide is more evident. The terms for the lunar waves for the 6 months' mean variation are where t′ is the lunar hour angle, 2.3 and 0.3 are the amplitudes in km, and 70′ and 135′ the phase angles of the diurnal and semidiurnal waves respectively.

The appearance of a significant lunar diurnal wave is explained by simple tidal theory which shows that tides are distributed over the earth according to latitude. At a high latitude (such as College), the semidiurnal wave becomes insignificant as compared with the diurnal wave. Furthermore, in accordance with simple theory, the sign of the lunar diurnal oscillation corresponds to the sign of the moon's declination.

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