Observations of the phase-locked 2 day wave over the Australian sector using medium-frequency radar and airglow data

The quasi 2 day wave, with a nominal mean period just above 50 h, is a significant feature of the 80–100 km altitude region in both hemispheres. It becomes particularly prominent in the Southern Hemisphere summer at midlatitudes where, a short time after summer solstice, its amplitude rapidly increases and its mean period is found to be approximately 48 h, producing an oscillation phase locked in local time. This lasts for a few weeks. Presented here are observations of the meridional winds and airglow over two sites in Australia, for 4 years during the austral summers of 2003–2006. We show that during those times when the large‐amplitude phase‐locked 2 day wave (PL‐TDW) is present the diurnal tide greatly decreases. This is consistent with the Walterscheid and Vincent (1996) model in which the PL‐TDW derives its energy from a parametric excitation by the diurnal tide. These data also show that the diurnal tide is more suppressed and the PL‐TDW amplitude is larger in odd‐numbered years, suggesting a biannual effect. The airglow data indicated that, for the PL‐TDW, the winds and temperature are nearly out of phase. When the PL‐TDW is present airglow amplitudes can become quite large, a result dependent on the local time of the PL‐TDW maximum. The airglow intensity response was, in general, much larger than what would be expected from the airglow temperature response, suggesting that the PL‐TDW is causing a significant composition change possibly due to minor constituent transport.