Characterization of the Semi-Annual Oscillation in Mesospheric Temperatures at Low-Latitudes

As part of the Maui-MALT initiative, long term measurements of the near IR mesospheric OH and O₂ emissions have been made from USAF AEOS facility at the summit at Haleakala Crater, Maui, Hawaii (20.8N, 156.2W). Since October 2001, measurements have been made routinely (typically 22 days/month) using the CEDAR Mesospheric Temperature Mapper (MTM). This imager sequentially measures the OH (6, 2) band intensity and temperature (peak altitude ∼ 87km) and the O₂ (0,1) Atmospheric band intensity and temperature (peak altitude ∼ 94km) with a precision of < 0.5% in 1 minute (intensity) and < 1-2 K in 3 minutes (temperature). To date, over 450 nights of quality data (of duration typically 4 -- 10 hours) have been obtained. Analysis of these data reveals large coherent tidal and gravity wave variability on a nocturnal, night-to-night, and seasonal scale. In addition to these variations the seasonal trend clearly shows a marked semi-annual oscillation (SAO) in intensity and temperature for both the OH and O₂ emission data. The peaks in temperature and intensity data occurred during the Spring and Fall in good agreement with the well established SAO signatures in the mesospheric wind field. SAO signatures in mesospheric temperatures have previously been investigated using rocket and satellite borne techniques, but, ground-based observations are exceptionally few. Our analysis agrees very well with the expected perturbation amplitudes, but also shows a marked asymmetry in the oscillations peaking during northern hemisphere spring-time. In this talk we will discuss the properties of our low-latitude SAO measurements in both airglow emissions and compare them with existing model results.