Observed tidal variation in the lower thermosphere through the 20th century and the possible implication of ozone depletion

Climatological change in the tides in the lower thermosphere has been estimated at five different latitudes using wavelet analysis of geomagnetic data series extending back to the beginning of the 20th century. The tidal signature present in the geomagnetic data is a consequence of atmospheric tides in the dynamo region between approximately 120 and 140 km altitude. The diurnal and semidiurnal spectral power have been accumulated through each year. The effect of solar and geomagnetic drivers has been minimized through use of proxy indices (aa index, sunspot number, F 10.7 solar flux) to leave a residual which shows ∼20% reduction in both diurnal and semidiurnal amplitudes at midlatitudes since the middle of the century. This midlatitude reduction is qualitatively similar to the decrease observed using wind data from 1964 onward at approximately 90 km altitude over a range of latitudes by Bremer et al. (1997), but quantitatively it is about half the magnitude. At high latitude a similar decrease would be a statistically insignificant measurement because the estimation becomes sensitive to errors in the aa index. At low latitude there is no significant change throughout the century. The observed decrease at midlatitudes appears consistent with that predicted by Ross and Walterscheid (1991) for the dynamo region current as a consequence of global ozone depletion. Reduced tidal heating of the stratosphere during the latter half of the 20th century and the consequent reduction in tidal power propagating up to the lower thermosphere is implicated.