MOTIVATION: THE ACCUMULATION TRANSPORT AND ENERGIZATION OF THE O+ IONS IN THE PLASMA SHEET AND THE INNER MAGNETOSPHERE IS A COMPELLING ISSUE OF MAGNETOSPHERIC PHYSICS: I) THE O+ IONS ARE AN IMPORTANT CONSTITUENT OF THE RING CURRENT POPULATION ESPECIALLY DURING INTENSE GEOMAGNETIC STORMS AS THEIR ENERGY DENSITY CAN BECOME COMPARABLE TO THAT OF THE H+ IONS II) THE PRESENCE OF O+ IONS AT LEAST PARTIALLY CONTROLS THE OVERALL MAGNETOTAIL DYNAMICS AND SOLAR WIND-MAGNETOSPHERE COUPLING BY SIGNIFICANTLY AFFECTING THE RECONNECTION RATES. DESPITE THE IMPORTANT ROLE OF O+ IONS IN MAGNETOSPHERIC DYNAMICS JUST HOW THEY POPULATE THE PLASMA SHEET AND INNER MAGNETOSPHERE AND HOW THE O+ ION DISTRIBUTION DEPENDS ON EXTERNAL (SOLAR WIND) AND INTERNAL (GEOMAGNETIC INDICES) CONDITIONS REMAIN COMPELLING OPEN QUESTIONS.SCIENCE GOALS AND OBJECTIVES: IN THE PRESENT PROJECT WE SEEK TO INVESTIGATE THROUGH SYNERGY BETWEEN DATA ANALYSIS AND STATE-OF-THEART NUMERICAL SIMULATIONS THE SPATIAL AND TEMPORAL EVOLUTION OF THE O+ ION PROPERTIES (E.G. TEMPERATURE NUMBER DENSITY AND ENERGY DENSITY) THROUGHOUT THE MAGNETOSPHERE FROM THE PLASMA SHEET TO WITHIN GEOSYNCHRONOUS ORBIT. MORE SPECIFICALLY WE WILL ADDRESSES THE FOLLOWING THREE SCIENCE QUESTIONS: SQ1. RESPONSE TIME DURING GEOMAGNETIC STORMS: HOW DO O+ ION PROPERTIES (AVERAGE ENERGY NUMBER DENSITY AND ENERGY DENSITY) EVOLVE DURING THE COURSE OF GEOMAGNETIC STORMS BOTH IN THE PLASMA SHEET AND INNER MAGNETOSPHERE? FOLLOWING THE SUDDEN COMMENCEMENT OF A GEOMAGNETIC STORM HOW DO THE RESPONSE TIMES OF THOSE O+ ION PROPERTIES DEPEND ON RADIAL DISTANCE FROM THE EARTH? SQ2. DEPENDENCE ON INTERNAL AND EXTERNAL CONDITIONS: HOW DOES THE SPATIAL DISTRIBUTION OF O+ IONS DEPEND ON SOLAR WIND CONDITIONS (EXTERNAL)? HOW DOES THE SPATIAL DISTRIBUTION OF O+ IONS DEPEND ON GEOMAGNETIC INDICES (INTERNAL)? SQ3. TRANSPORT FROM THE TAIL TO WITHIN GEOSYNCHRONOUS ORBIT: IN WHICH REGION DOES THE TRANSPORT OF O+ IONS TRANSITION FROM NONADIABATIC TO ADIABATIC? METHODOLOGY: FOR THE OBSERVATIONAL PART OF OUR PROJECT WE WILL USE GEOTAIL EPIC/STICS INSTRUMENT DATA TO INVESTIGATE O+ PROPERTIES IN THE PLASMA SHEET AND DATA FROM THE HOPE AND RBSPICE INSTRUMENTS ON BOARD VAN ALLEN PROBES TO INVESTIGATE O+ PROPERTIES IN THE INNER MAGNETOSPHERE. FOR THE MODELING PART OF OUR PROJECT WE WILL USE I) MULTI-FLUID LFM (MFLFM) GLOBAL MHD CODE THAT INCLUDES O+ IONS AS A FLUID TO INVESTIGATE O+ IONOSPHERIC OUTFLOW AND SUBSEQUENT TRANSPORT WITHIN THE MAGNETOSPHERE II) 3D TESTPARTICLE SIMULATIONS IN LFM-RCM SIMULATED ELECTRIC AND MAGNETIC FIELDS TO INVESTIGATE NON-ADIABATIC O+ TRANSPORT FROM THE PLASMA SHEET INTO THE INNER MAGNETOSPHERE. THE COMBINED DATA-MODEL APPROACH OF THIS PROJECT WILL ALLOW US TO COMPREHENSIVELY ADDRESS O+ DYNAMICS IN THE COUPLED MAGNETOSPHERE IONOSPHERE (M-I) SYSTEM. REVELANCE: THE PROPOSED INVESTIGATION WILL PROVIDE NEW INSIGHTS ON O+ ION DYNAMICS IN THE COUPLED M-I SYSTEM THEREFORE IT DIRECTLY ADDRESSES HIGH-LEVEL SCIENCE GOAL #2 FROM THE HELIOPHYSICS DECADAL SURVEY NAMELY: DETERMINE THE DYNAMICS AND COUPLING OF EARTH S MAGNETOSPHERE IONOSPHERE AND ATMOSPHERE AND THEIR RESPONSE TO SOLAR AND TERRESTRIAL INPUTS .