| Sumario: | Two unusually clear mesospheric gravity wave events were observed by the Boston University all-sky imager and the Argentine airglow spectrometer on two consecutive nights at the El Leoncito Observatory, Argentina (31.8°S, 69.3°W), during August 2001. Both events exhibited brightness amplitudes an order of magnitude above typical values. The first event had the appearance of a large-amplitude (>60% in OH(6-2), 22% in O 2 , and 37% in O( 1 S) emissions (peak to peak), compared to 1-5% typically), upward propagating gravity wave with pronounced nonlinear behavior. The waves also showed noticeably curved wavefronts, indicating that they had originated from a relatively small source region within ∼180 km of El Leoncito. Estimates of the vertical flux of horizontal energy and momentum for the wave event in the OH and O 2 emissions were very large compared to typical values, and they also indicated a high degree of flux divergence over the nominal 8 km altitude between the two layers. The second event occurred on the following night and exhibited a frontal morphology and behavior consistent with an internal ducted gravity wave showing nonlinear behavior akin to a mesospheric bore. The disturbance also had several interesting characteristics; in particular, the propagation speed decreased during its passage across the sky. In addition, a strong vertical temperature gradient due to the semidiurnal tide appeared to influence the occurrence of the bore event. Copyright 2006 by the American Geophysical Union.
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