Planetary wave trends in the lower thermosphere—Evidence for 22-year solar modulation of the quasi 5-day wave

Geomagnetic data from five northern hemisphere observatories at latitudes ranging from 22°N to 67°N have been used to investigate long-term trends in planetary wave activity in the lower thermosphere. The data extends back over eight 11-year solar cycles. At quasi 10- and 16-day periods, there is no long-term change in planetary wave activity through the 20th century; decadal-scale variability reaching 25% peak-to-peak is observed. At the quasi 5-day period there is a steady decrease in activity totalling 15% through the 20th century. Additionally, there is a Hale ‘22-year’ solar cycle modulation of ∼20% peak-to-peak in the 5-day planetary wave activity at 22°N which decreases in amplitude with increasing latitude. It is not clear what causative mechanism is responsible for this modulation which shows signs of breaking down at the very end of the century.