ON THE ORIGIN OF RIPPLE-TYPE WAVE STRUCTURE IN THE OH NIGHTGLOW EMISSION

The origin of small-scale mesospheric wave structure with horizontal wavelengths of only 5-15 km and lifetimes of less than 45 min, termed "ripples", has been investigated using video observations of the near infra-red OH nightglow emission taken over a period of several years. Previous studies in the literature indicate that ripples are associated with lower lunar transit, lunar high tide and with geomagnetic activity (as measured by Kp). Examination of our data indicated a peak in the occurrence of ripples around lower lunar transit. However, the distribution of observing time was also strongly peaked towards lower lunar transit since this occurred at night during New Moon periods, when most of the observations were made. Statistical analysis of these data revealed no significant correlation with lower lunar transit and no evidence was found to support the theory that ripples arise from the breakdown of lunar tides in the upper atmosphere. The comparison of ripple occurrence with Kp phase showed a strong correlation, with ripples tending to occur when Kp was falling. However, in an analysis using the AE magnetic index, no evidence was found to support the hypothesis that ripples are associated with the breakdown at mid-latitudes of travelling ionospheric disturbances (TID) generated at polar latitudes by auroral processes. It appears more likely that ripples are the result of short-lived velocity shears generated in situ by the chance combination of wind and wave motions. © 1990.