THIS PROPOSAL WILL INVESTIGATE THE MISSING SOLAR CYCLE RESPONSE IN MESOSPHERIC TEMPERATURE WATER VAPOR AND ICE DURING THE RECENT 15 YEARS. IT HAS LONG BEEN CONSIDERED THAT THE 11-YEAR SOLAR CYCLE WILL IMPACT THE UPPER ATMOSPHERE BY ALTERING PHOTOCHEMISTRY HEATING AND THE CIRCULATION. PRIOR TO NASA'S UARS MISSION (1991-2005) THE PRINCIPAL INDICATION OF SOLAR CYCLE EFFECTS IN THE UPPER MESOSPHERE WAS THE DECADAL OSCILLATION OBSERVED IN POLAR MESOSPHERIC CLOUDS (PMC) [DELAND AND THOMAS 2015]. THESE ICE CLOUDS NEAR 83 KM ALTITUDE ARE EXCELLENT MONITORS OF MESOSPHERIC VARIABILITY DUE TO THEIR EXTREME SENSITIVITY TO TEMPERATURE AND WATER VAPOR. PMCS ARE EXPECTED TO BE REDUCED (ENHANCED) DURING SOLAR MAXIMUM (MINIMUM) DUE TO INCREASED (DECREASED) HEATING AND WATER VAPOR PHOTOLYSIS. HOWEVER FROM ~2000 TO PRESENT THE SOLAR CYCLE RESPONSE OF THE POLAR UPPER MESOSPHERE APPEARS TO BE MISSING AS EVIDENT IN MEASUREMENTS OF PMCS [E.G. FIEDLER ET AL. 2011; SISKIND ET AL. 2013; HERVIG ET AL. 2016]. THE RELATIVELY QUIESCENT MAXIMUM ASSOCIATED WITH CYCLE 24 MAY BE A FACTOR; HOWEVER THE EXPECTED LARGE SURGE IN PMC OCCURRENCE DURING THE PREVIOUS DEEP SOLAR MINIMUM FROM 2007-2010 WAS NOT EVIDENT. IT HAS BEEN SUGGESTED THAT THE OBSERVED DECADAL OSCILLATIONS IN PMCS DURING 1979-2000 WERE AMPLIFIED BY MECHANISMS OTHER THAN SOLAR FORCING. A RECENT INVESTIGATION PROPOSED THAT SOLAR CYCLE EFFECTS DURING THE 1980'S AND 90'S WERE AMPLIFIED BY VOLCANIC ERUPTIONS THAT WERE COINCIDENT WITH SOLAR MAXIMA (EL CHICHON IN 1982 AND PINATUBO IN 1991) [HERVIG ET AL. 2016] ALTHOUGH THIS ASSERTION REMAINS UNTESTED. ANOTHER STUDY NOTED THE COINCIDENCE OF THE 1990S SOLAR MINIMUM TO LOW TEMPERATURES THROUGHOUT THE STRATOSPHERE AND MESOSPHERE [LUBKEN ET AL. 2009] AND ATTRIBUTED IT TO A REVERSAL OF COOLING TRENDS ASSOCIATED WITH OZONE RECOVERY BEGINNING IN THE MID-1990S. OUR SCIENTIFIC OBJECTIVE IS TO INVESTIGATE THE RESPONSE OF THE MESOSPHERE TO SOLAR CYCLE VARIABILITY AND THE MENTIONED FORCING MECHANISMS USING AN ARRAY OF OBSERVATIONS FROM NASA SATELLITES AND FOCUSED MODELING EXPERIMENTS. THIS PROPOSAL ADDRESSES TWO PRINCIPAL SCIENTIFIC QUESTIONS: 1. WHAT IS THE SOLAR CYCLE RESPONSE IN THE POLAR SUMMER MESOSPHERE AND WHY IS IT MISSING DURING THE RECENT 15 YEARS? 2. HAVE FACTORS SUCH AS CHANGING OZONE VOLCANIC ERUPTIONS OR INCREASING GREENHOUSE GASES CONTRIBUTED TO OR SUBTRACTED FROM SOLAR CYCLE EFFECTS? OUR SCIENCE OBJECTIVE IS DIRECTLY RELEVANT TO THE HELIOPHYSICS DECADAL SURVEY GOAL TO DETERMINE THE DYNAMICS AND COUPLING OF EARTH'S MAGNETOSPHERE IONOSPHERE AND ATMOSPHERE AND THEIR RESPONSE TO SOLAR AND TERRESTRIAL INPUTS AND TO THE HELIOPHYSICS SUPPORTING RESEARCH (H-SR) GOAL TO ADDRESS SOLAR OUTPUT IONOSPHERE/ATMOSPHERE COUPLING. TO UNTANGLE THIS MYSTERY WE PROPOSE A COORDINATED EFFORT TO INVESTIGATE SUN-EARTH CONNECTIONS USING AN ARRAY OF SATELLITE OBSERVATIONS AND FOCUSED MODELING STUDIES. THIS PROPOSAL USES PMC OBSERVATIONS AS A MONITOR OF THE UPPER MESOSPHERE BECAUSE THE CONTINUOUS OBSERVATION RECORD FAR EXCEEDS THAT FOR T AND H2O AND BECAUSE PMC VARIATIONS ARE A PROVEN SURROGATE FOR MONITORING T AND H2O [HERVIG ET AL. 2015; 2016]. THE SBUV INSTRUMENTS COMPRISE THE LONGEST PMC MEASUREMENT RECORD AVAILABLE (1979 TO PRESENT) WHICH IS READILY COMPARABLE TO MODELS. IN ADDITION TO SBUV SATELLITE OBSERVATIONS OF PMC'S FROM HALOE AND AIM WILL BE USED TOGETHER WITH MEASUREMENTS OF MESOSPHERIC T AND H2O FROM HALOE SABER MLS AND AIM TO FORM A BASELINE TIME-SERIES OF OBSERVATIONS THAT WILL BE COMPARED WITH THE PROPOSED MODELING SIMULATIONS. THIS PROPOSAL WILL USE THE WHOLE ATMOSPHERE COMMUNITY CLIMATE MODEL WITH PMC MICROPHYSICS (WACCM-PMC) TO RUN A SERIES OF FOCUSED EXPERIMENTS TO QUANTIFY THE INFLUENCE OF VARIOUS FORCING MECHANISMS ON MESOSPHERIC CONDITIONS.