Seasonal variations in the correlation of mesospheric OH temperature and radiance at midlatitudes

Abstract

A correlative study of rotational temperatures and radiance of mesospheric hydroxyl (OH) was made using spectrographic measurements from the midlatitude site at Millstone Hill, Massachusetts (42.6°N, 71.5°W). The OH radiance and temperatures were strongly correlated, and the correlation showed a marked seasonal dependence. The dependence was also evident on smaller timescales ranging from days to months. Dynamical effects appeared to dominate the temperature‐radiance relationship compared to the photochemistry responsible for the hydroxyl emission rate. The effect was illustrated clearly when the measurements were temporally averaged, hence removing the dynamical influence. A strong summer maximum in OH radiance was observed contrary to a minimum in summer Mass Spectrometer Incoherent Scatter (MSIS) [O] volume mixing ratio values. An increase in mesospheric gravity wave activity during the summer, a feature not observed by the MSIS model, was the likely cause of the summer OH maximum.