SEASONAL CHANGES ARE ONE OF THE MAIN DYNAMICAL FEATURES OF THE EARTHS UPPER ATMOSPHERE. THE STATE OF THE THERMOSPHERE AND IONOSPHERE AT THE WINTER SOLSTICE IS VERY DIFFERENT FROM THE STATE AT THE SUMMER SOLSTICE. TO GET FROM ONE STATE TO ANOTHER REQUIRES A TRANSITION NEAR THE EQUINOX. SURPRISINGLY WE KNOW VERY LITTLE ABOUT HOW THIS TRANSITION OCCURS YET THERE IS GROWING EVIDENCE THAT IT IS FAR FROM THE SIMPLE ANNUAL SINUSOID THAT IT IS OFTEN ASSUMED TO BE. THEREFORE WE PROPOSE TO UNDERTAKE A SERIES OF STUDIES TO ELUCIDATE THE NATURE OF THE EQUINOX TRANSITION USING TIMED-GUVI (THERMOSPHERE IONOSPHERE MESOSPHERE ENERGETICS AND DYNAMICS GLOBAL ULTRAVIOLET IMAGER) DATA TO DETERMINE THE BEHAVIOR OF O/N2 NEAR AT THE EQUINOXES. PRELIMINARY STUDIES USING GUVI DATA HAVE SHOWN THAT THE NUMBER OF DAYS THAT O/N2 SPENT IN AN EQUINOX STATE (WITH MAXIMUM O/N2 AT THE EQUATOR RATHER THAN IN THE MIDDLE LATITUDES) IS SHORT. SIMILARLY RAPID EQUINOX TRANSITIONS HAVE BEEN SEEN IN THE TOPSIDE IONOSPHERE WHICH IS CONNECTED TO O/N2 AS A RESULT OF THE F2 REGION IONOSPHERE BEING CREATED BY THE BALANCE BETWEEN THE IONIZATION OF O AND RECOMBINATION THROUGH CHEMICAL REACTIONS BETWEEN O+ AND N2 AND O2. TOPSIDE IONOSPHERE DATA ALSO SUGGEST THAT THERE ARE FUNDAMENTAL DIFFERENCES IN THE BEHAVIOR OF THE IONOSPHERE BETWEEN THE SEPTEMBER EQUINOX AND THE MARCH ONE. STUDYING THIS BEHAVIOR IS IMPORTANT FOR A NUMBER OF REASONS. FIRST BOTH DATA ANALYSIS AND MODELING STUDIES OFTEN LUMP THE 3 MONTHS AROUND THE EQUINOX TOGETHER PRODUCING TWO-PEAKED BEHAVIOR AROUND THE EQUATOR WHEN THE ACTUAL BEHAVIOR MAY BE MASKED OR CONTAMINATED BY TWO SOLSTICE SEASONS. SECOND THE RAPID TRANSITION HAS A DIRECT IMPACT ON OUR UNDERSTANDING OF IONOSPHERE-THERMOSPHERE PHENOMENA SUCH AS THE IONOSPHERIC WINTER ANOMALY BUT THIS IMPACT IS AS YET POORLY UNDERSTOOD. WE PROPOSE A SERIES OF STUDIES TO ENABLE US TO BETTER UNDERSTAND THE NATURE OF THE EQUINOX TRANSITION OF O/N2 AND THE CAUSES OF ITS VARIABILITY: 1. DOES THE LENGTH OF THE EQUINOX TRANSITIONS CHANGE FROM YEAR-TO-YEAR IN O/N2 DATA?2. ARE THE DIFFERENCES IN THE O/N2 EQUINOX TRANSITION BETWEEN MARCH AND SEPTEMBER CONSISTENT FROM YEAR TO YEAR? 3. WHY IS THERE AN EQUATORIAL PEAK IN O/N2 DURING THE MARCH EQUINOX? 4. HOW DOES THIS TRANSITION IN O/N2 NEAR THE EQUINOXES IMPACT THE IONOSPHERE? THESE QUESTIONS WILL BE ADDRESSED PRIMARILY BY USING THE CURRENT DATABASE OF GUVI MEASUREMENTS OF THE HEIGHT-INTEGRATED O/N2 RATIOS. A THEORETICAL BACKGROUND FOR THESE STUDIES WILL BE PROVIDED BY THE NCAR-TIEGCM (NATIONAL CENTER FOR ATMOSPHERIC RESEARCH THERMOSPHERE-IONOSPHERE-ELECTRODYNAMICS GENERAL CIRCULATION MODEL). ADDITIONAL OBSERVATIONAL DATA FROM GROUND BASED TOTAL ELECTRON CONTENT AND IONOSONDE OBSERVATIONS OF F PEAK DENSITIES WILL BE USED. THE PROPOSED WORK ADDRESSES PART OF THE PRIMARY GOAL OF THE CURRENT NASA TIMED FLIGHT MISSION AS WELL AS THE BASIC STRUCTURE (I.E. PRESSURE TEMPERATURE AND WINDS) THAT RESULTS FROM THE ENERGY TRANSFER INTO THE REGION (DYNAMICS) . IT ALSO RELATES TO TWO OF THE FOUR AREAS OF THE DECADAL SURVEY (2 AND 4). IT IS CENTRAL TO DETERMINING THE DYNAMICS OF THE EARTH S UPPER ATMOSPHERE TO UNDERSTAND HOW AND WHY COMPOSITION VARIES IN THIS HETEROGENEOUS REGION (2). THE WAY THAT THE THERMOSPHERIC CHANGES THROUGH THE EQUINOXES AND UNDERSTANDING THE CAUSES OF THIS CHANGE IS ALSO A FUNDAMENTAL PROCESS THAT IS AS YET POORLY UNDERSTOOD (4). THE PROPOSED WORK WILL ALSO ENABLE US TO BETTER UNDERSTAND THE CAUSES OF COMPOSITION CHANGES AND HELP TO PROVIDE A CONTEXT FOR THE FORTHCOMING NASA GOLD (GLOBAL-SCALE OBSERVATIONS OF THE LIMB AND DISK) AND ICON (IONOSPHERIC CONNECTION EXPLORER) FLIGHT MISSIONS.