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. The present work reports seasonal characteristics of small- and medium-scalegravity waves in the mesosphere and lower thermosphere (MLT) region. All-skyimages of the hydroxyl (NIR-OH) airglow emission layer over Sao Joao doCariri (7.4 ∘  S, 36.5 ∘  W; hereafter Cariri) were obtainedfrom September 2000 to December 2010, during a total of 1496 nights. Forinvestigation of the characteristics of small-scale gravity waves (SSGWs) andmedium-scale gravity waves (MSGWs), we employed the Fourier two-dimensional(2-D) spectrum and keogram fast Fourier transform (FFT) techniques,respectively. From the 11 years of data, we could observe 2343 SSGW and 537MSGW events. The horizontal wavelengths of the SSGWs were concentratedbetween 10 and 35  km , while those of the MSGWs ranged from 50 to 200  km . Theobserved periods for SSGWs were concentrated around 5 to 20  min , whereas theMSGWs ranged from 20 to 60  min . The observed horizontal phase speeds of SSGWswere distributed around 10 to 60  m s−1 , and the correspondingMSGWs were around 20 to 120  m s−1 . In summer, autumn, and winterboth SSGWs and MSGWs propagated preferentially northeastward andsoutheastward, while in spring the waves propagated in all directions. Thecritical level theory of atmospheric gravity waves (AGWs) was applied tostudy the effects of wind filtering on SSGW and MSGW propagationdirections. The SSGWs were more susceptible to wind filtering effects thanMSGWs. The average of daily mean outgoing longwave radiation (OLR) was alsoused to investigate the possible wave source region in the troposphere. Theresults showed that in summer and autumn, deep convective regions were thepossible source mechanism of the AGWs. However, in spring and winter the deepconvective regions did not play an important role in the waves observed atCariri, because they were too far away from the observatory. Therefore, weconcluded that the horizontal propagation directions of SSGWs and MSGWs showclear seasonal variations based on the influence of the wind filteringprocess and wave source location. Keywords. Atmospheric composition and structure (airglow and aurora) – electromagnetics (wave propagation) – history of geophysics (atmospheric sciences)

Seasonal characteristics of small- and medium-scale gravity waves in the mesosphere and lower thermosphere over the Brazilian equatorial region