( ( To ( ( determine ( ( the sources ) ( of ( planetary ( scale perturbations ) ) ) ) ) ( in ( ( ( the mesosphere ) and ) ( lower thermosphere ) ) ) ) ) ( ( is ( one ( of ( ( the ( principal topics ) ) ( in ( ( ( the ( International ( Program PSMOS ) ) ) ( -LRB- ( ( Planetary ( Scale ( Mesopause ( Observing System ) ) ) ) -RRB- ) ) ) ( of ( ( the ( Scietific Commettee ) ) ( on ( ( Solar-Terrestrial Physics ) ( -LRB- ( SCOSTEP -RRB- ) ) ) ) ) ) ) ) ) ) ) ) . ) )
(ROOT (S (S (VP (TO To) (VP (VB determine) (NP (NP (DT the) (NNS sources)) (PP (IN of) (NP (JJ planetary) (NN scale) (NNS perturbations)))) (PP (IN in) (NP (NP (DT the) (NN mesosphere)) (CC and) (NP (JJR lower) (NN thermosphere))))))) (VP (VBZ is) (NP (NP (CD one)) (PP (IN of) (NP (NP (DT the) (JJ principal) (NNS topics)) (PP (IN in) (NP (NP (NP (DT the) (NNP International) (NNP Program) (NNP PSMOS)) (PRN (-LRB- -LRB-) (NP (NNP Planetary) (NNP Scale) (NNP Mesopause) (NNP Observing) (NNP System)) (-RRB- -RRB-))) (PP (IN of) (NP (NP (DT the) (NNP Scietific) (NNP Commettee)) (PP (IN on) (NP (NP (NNP Solar-Terrestrial) (NNP Physics)) (PRN (-LRB- -LRB-) (NP (NNP SCOSTEP)) (-RRB- -RRB-)))))))))))) (. .)))
To determine the sources of planetary scale perturbations in the mesosphere and lower thermosphere is one of the principal topics in the International Program PSMOS (Planetary Scale Mesopause Observing System) of the Scietific Commettee on Solar-Terrestrial Physics (SCOSTEP).
(ROOT (S (NP (DT The) (NN mesopause)) (VP (VBZ is) (PP (IN at) (NP (NP (DT the) (NN top)) (PP (IN of) (NP (DT the) (NN mesosphere)))))) (. .)))