There is currently much interest in fluctuations of airglow emissions
caused by atmospheric gravity waves. The fluctuations of brightness te
nd to be found in phase (or occasionally in antiphase) with the fluctu
ations of measured temperature, whereas current theory tends to antici
pate substantial phase differences. We suggest here that the discrepan
cy results from failure of the common theoretical assumption that the
relevant gravity waves are dominated by a single upgoing component: th
at, instead, there is an accompanying downgoing component of comparabl
e magnitude, produced by reflection. In the case of total reflection,
simple, steady state chemistry and vertical viewing, the phase differe
nce is necessarily zero (or 180 degrees).