Gravity wave coupling between the mesosphere and thermosphere over New Zealand

All-sky images obtained with the Boston University all-sky imaging system located at the Mount John University Observatory, New Zealand (43.98 degrees S, 170.42 degrees E) show clear evidence of dynamic coupling between the mesosphere and thermosphere. Gravity wave (GW) breaking events in the upper mesosphere at altitudes (z) of 80 to 100km were observed in the 557.7nm emission on the evening of 4 March 2009 from 08:40 to 13:50 UT. During this time, unusual oppositely propagating weak northwestward (NW-ward) and strong southeastward (SE-ward) GWs were observed in the all-sky images of the thermospheric atomic oxygen O(D-1) 630.0nm emission at 250km altitude. The waves appeared to originate from the same location over New Zealand, with phase fronts nearly parallel to the landmass axis of the South Island of New Zealand. Additionally, the southern portion of the wave train ends abruptly at the southern tip of New Zealand. The SE-ward GWs were stronger and appeared for similar to 5h, while the NW-ward GWs were weaker and only appeared for similar to 1.75h. We provide evidence that mountain waves were likely generated in the troposphere that evening. Momentum deposition from GW breaking excites secondary GWs. We model these secondary GWs and show that these GWs have a similar morphology and behavior as observed. Wind filtering in the thermosphere can account for the larger amplitudes and persistent appearance of the SE-ward GWs and the smaller amplitudes and less persistent appearance of the NW-ward GWs. Thus, the morphology and behavior of the 630.0nm GWs suggest that they were secondary GWs generated from mountain wave breaking in the upper mesosphere. We also show that similar SE-ward GWs have occurred in the 630.0nm emission on other occasions.