Mesospheric gravity wave imaging at a subauroral site: First results from Millstone Hill

Optical detections of short‐period quasi‐monochromatic gravity waves in the mesospheric nightglow emissions of OH (695–950 nm), O (557.7 nm), and Na (589.3 nm) have been obtained for the first time from Millstone Hill, Massachusetts (42.6°N, 71.5°W). Using a new all‐sky bare charge‐coupled device (CCD) imaging system at this subauroral site, we obtained 12 nights of observations during the period February 1 to July 21, 1998. Wave structure was observed during ∼55% of the total observing time, and 35 events were identified and analyzed. Two types of wave structure were observed: extensive, long‐lasting (∼1–5 hours) waves known as bands and small‐scale, short‐lived (∼5–40 min) features called ripples. The band events are believed to be due to freely propagating or ducted internal gravity waves. The mean measured parameter values of the band events were horizontal wavelength (λ h ) = 21±7 km, phase speed ( c ob ) = 47±20 m s −1 , and observed period (τ ob ) = 8.5±4.6 min. The intrinsic wave parameters ( c in and τ in ) and the vertical wavelength (λ z ) of the band events were calculated using available meteor radar winds for nine of the events. Of the deduced λ z values, five were evanescent, and the remaining four ranged from 12 to 21 km, with a mean of 17±4 km. Calculated energy and momentum fluxes for two case study events yielded 4–14 × 10 −3 W m −2 and 5–30 m 2 s −2 , respectively. Possible sources of the waves were investigated using additional data obtained during 1999. The majority of the events exhibited poleward propagation, even during periods of visible auroral activity. This indicates that although the thermosphere above Millstone Hill exhibits subauroral behavior during enhanced levels of geomagnetic activity ( Kp >3), the propagation characteristics and frequency of occurrence of mesospheric gravity waves at this location differ very little from other middle‐ and low‐latitude locations.