Observations of the Phase-Locked Two day Wave Over the Australian Sector Using Medium Frequency Radar and Airglow Data

While the quasi two day wave (QTDW), with a nominal mean period near 50 hours, is a significant feature of the 80 to 100 km altitude region in both hemispheres, it becomes particularly prominent in the summer southern hemisphere at mid latitudes. A short time after summer solstice the QTDW amplitude rapidly increases and its mean period is found to be approximately 48 hours, producing a phase locked in local time. These changes last for a few weeks. Here we report on observations of the winds and airglow over Adelaide Australia, and the airglow over Alice Springs Australia, for four years during the austral summers of 2003 to 2006. We focus on times when the large-amplitude phase-locked two-day wave (PLTDW) is present and show that during these times the diurnal tide greatly decreases. This is consistent with the Walterscheid and Vincent (1996) model in which the PLTDW derives its energy from a parametric excitation by the diurnal tide. The data also show that the diurnal tide is more suppressed, and the PLTDW amplitude is larger, in odd-numbered years, suggesting some biannual effect. The airglow data allow us to determine the phase difference between the winds and temperature for the PLTDW. When the PLTDW is present times airglow amplitudes can become quite large, a result dependent on the local time of the PLTDW maximum that can vary from year to year.