GRAVITY WAVE FORCING NEAR THE MESOPAUSE DRIVES THE WELL-KNOWN MESOSPHERIC SUMMER-TO-WINTER RESIDUAL CIRCULATION AND A REVERSED LOWER THERMOSPHERIC WINTER-TO-SUMMER RESIDUAL CIRCULATION. THE MESOSPHERIC RESIDUAL CIRCULATION HAS BEEN WELL STUDIED BUT THE LOWER THERMOSPHERIC RESIDUAL CIRCULATION HAS ONLY COME TO OUR ATTENTION RECENTLY DUE TO ITS IMPACT ON TRACE SPECIES DISTRIBUTIONS IN THE MESOSPHERE AND LOWER THERMOSPHERE REGION. RECENT STUDIES FOUND THAT THE LOWER THERMOSPHERIC RESIDUAL CIRCULATION ACTS LIKE A LARGE EDDY THAT MIXES THE LOWER THERMOSPHERE SIGNIFICANTLY IMPACTING THE SUMMER-TO-WINTER LATITUDINAL GRADIENT OF THERMOSPHERIC COMPOSITION AND THUS AFFECTING THE IONOSPHERE WINTER ANOMALY AS WELL AS ANNUAL/SEMIANNUAL VARIATIONS IN THE THERMOSPHERE AND IONOSPHERE (T-I) SYSTEM. HOWEVER THESE RESIDUAL CIRCULATIONS ARE DIFFICULT TO OBSERVE AND CHARACTERIZE. ALSO THE GRAVITY WAVE FORCING THAT DRIVES THEM IS PARAMETERIZED SOMEWHAT ARBITRARILY IN GENERAL CIRCULATION MODELS. IT IS NOT CLEAR WHAT THE CHARACTERISTICS OF THE LOWER THERMOSPHERIC RESIDUAL CIRCULATION ARE. IT IS ALSO NOT CLEAR HOW THESE FEATURES CHANGE FROM THE SOLSTICES TO THE EQUINOXES. THIS UNDERMINES OUR UNDERSTANDING TO THE VARIABILITY OF COMPOSITION IN THE REGION AND THUS LIMITS OUR UNDERSTANDING OF THE T-I SYSTEM AND OUR PREDICTIVE CAPABILITY FOR THE SYSTEM. ALTHOUGH THE RESIDUAL CIRCULATIONS ARE DIFFICULT TO OBSERVE AND CHARACTERIZE THEY LEAVE CLEAR SIGNATURES IN TRACE SPECIES DISTRIBUTION. FOR EXAMPLE AT HIGH LATITUDES IN SUMMER THE UPWELLING ASSOCIATED WITH THE MESOSPHERIC RESIDUAL CIRCULATION INCREASES CO2 VOLUME MIXING RATIO (VMR) WHEREAS THE DOWNWELLING OF THE LOWER THERMOSPHERIC RESIDUAL CIRCULATION DECREASES IT. THIS PRODUCES A MAXIMUM CO2 VMR VERTICAL GRADIENT AT THE TRANSITION ALTITUDE BETWEEN THE TWO CIRCULATIONS AND THE MAGNITUDE OF THE CO2 GRADIENT REFLECTS THE CONVERGENCE INTENSITY. THUS WE PROPOSE TO DETERMINE THE SEASONAL VARIABILITY OF THE RESIDUAL CIRCULATIONS FROM THE SEASONAL VARIABILITY OF TRACER DISTRIBUTION. WE WILL ADDRESS THE FOLLOWING SPECIFIC SCIENCE QUESTIONS: 1) WHERE ARE THE TOP AND LOWER BOUNDARIES OF THE LOWER THERMOSPHERIC RESIDUAL CIRCULATION? HOW DO THEY VARY FROM THE SOLSTICES TO THE EQUINOXES? 2) HOW DOES THE CONVERGENCE INTENSITY AT THE BOUNDARIES OF THE LOWER THERMOSPHERIC RESIDUAL CIRCULATION VARY WITH SEASON? 3) DOES SEASONAL VARIABILITY OF THE LOWER THERMOSPHERIC RESIDUAL CIRCULATION CHANGE FROM YEAR TO YEAR? TO ADDRESS THESE QUESTIONS WE WILL USE TRACER DATASETS INCLUDING CO2 VMR DERIVED FROM TIMED-SABER (THERMOSPHERE IONOSPHERE MESOSPHERE ENERGETICS AND DYNAMICS-THE SOUNDING OF THE ATMOSPHERE USING BROADBAND EMISSION RADIOMETRY) OBSERVATIONS FROM 2002-2017 H2O VMR DERIVED FROM AIM-SOFIE (AERONOMY OF ICE IN THE MESOSPHERE-THE SOLAR OCCULTATION FOR ICE EXPERIMENT) OBSERVATIONS FROM 2007-2017 AS WELL AS NEUTRAL TEMPERATURE DERIVED FROM SABER AND SOFIE OBSERVATIONS. EXISTING CLIMATE SIMULATIONS BY THE NCAR WACCM (WHOLE ATMOSPHERE COMMUNITY CLIMATE MODEL) WILL BE USED TO INTERPRET THE DATA AND THE MODEL S CAPABILITY IN REPRESENTING THE CHARACTERISTICS AND SEASONAL VARIABILITY OF THE LOWER THERMOSPHERIC RESIDUAL CIRCULATION AND THEIR EFFECTS ON TRACERS WILL BE ASSESSED. THE PROPOSED STUDY USES DATASETS FROM THE ONGOING NASA HELIOPHYSICS MISSION TIMED AND AIM. IT RESPONDS TO THE NASA HGI PROGRAM TO MAXIMIZE THE SCIENTIFIC RETURN FROM OPERATING MISSIONS . THE PROPOSED INVESTIGATION ADDRESS GOAL #2 OF THE SOLAR AND SPACE PHYSICS DECADAL SURVEY: DETERMINE THE DYNAMICS AND COUPLING OF EARTH S MAGNETOSPHERE IONOSPHERE AND ATMOSPHERE AND THEIR RESPONSE TO SOLAR AND TERRESTRIAL INPUTS. THE PROPOSED WORK ALSO ADDRESS OBJECTIVE H3 OUTLINED IN THE NASA S HELIOPHYSICS SCIENCE AND TECHNOLOGY ROADMAP FOR 2014-2033: UNDERSTAND THE COUPLING OF THE EARTH S MAGNETOSPHERE-IONOSPHERE-ATMOSPHERE SYSTEM AND ITS RESPONSE TO EXTERNAL AND INTERNAL FORCING.