RECENT WORK HAS SHOWN THAT ADIABATIC INWARD TRANSPORT OF THE HOT PLASMA SHEET (>5 KEV) POPULATION AT ~6 RE CONTRIBUTES THE DOMINANT PRESSURE THAT DRIVES THE RING CURRENT. THUS UNDERSTANDING THIS NEAR-EARTH HOT PLASMA SHEET POPULATION IS ONE OF THE KEYS TO PREDICTING THE STRENGTH OF THE RING CURRENT. COMPARISONS OF THE PLASMA SHEET COMPOSITION AT 15-20 RE HAVE SHOWN THAT THE O+/H + RATIO INCREASES WITH DECREASING RADIAL DISTANCE. THIS INDICATES THAT THERE IS SIGNIFICANT ENTRY OF O+ TO THE PLASMA SHEET INSIDE 15 RE. THERE ARE TWO SOURCES FOR DIRECT ENTRY OF O+ TO THE PLASMA SHEET THE CUSP IONS THAT MOVE OVER THE POLAR CAPS AND ENTER THROUGH THE LOBE AND IONS THAT ENTER DIRECTLY FROM THE NIGHTSIDE AURORA. WORK FROM VAN ALLEN PROBES AND FROM CLUSTER HAVE SHOWN THAT THE NIGHTSIDE AURORAL SOURCE IS DOMINANT INSIDE 6.6 RE AND THE CUSP SOURCE IS DOMINANT OUTSIDE 15 RE. SO THERE IS A TRANSITION BETWEEN 6.6 AND 15 RE. HOW MUCH OF THE O+ ENTRY IN THIS INTERMEDIATE REGION COMES FROM THE NIGHTSIDE AURORA IS NOT KNOWN. IN ADDITION IONS ARE ACCELERATED IN THIS REGION IN PARTICULAR DURING SUBSTORMS AND THIS ACCELERATION CAN ALSO PREFERENTIALLY ENHANCE THE O+ IN THE HOT PLASMA SHEET. THE SCIENTIFIC QUESTION THIS PROPOSAL ADDRESSES IS: WHAT IS THE DOMINANT PROCESS THAT BRINGS O+ INTO THE PLASMA SHEET FROM 6-12 RE AND WHAT IS THE EFFECT OF THESE IONS ON THE HOT PLASMA SHEET AND ENERGETIC PARTICLES? TO RESOLVE THIS QUESTION WE WILL IDENTIFY ALL TIME PERIODS WHEN TAILWARD STREAMING O+ INDICATING DIRECT ENTRY OF O+ IS OBSERVED. WE WILL FIND HOW THESE TIME PERIODS ARE ASSOCIATED WITH SUBSTORMS. WE WILL DETERMINE HOW THE HOT AND ENERGETIC POPULATIONS CHANGE AT SUBSTORM ONSET AND WHETHER THAT CHANGE IS ASSOCIATED WITH THE ENTERING POPULATIONS. FINALLY WE WILL TEST WHETHER THE ENHANCED O+ IN THIS REGION ARE SUFFICIENT TO EXPLAIN THE STORM-TIME RING CURRENT OBSERVATIONS. DATA TO BE USED ARE HPCA EIS AND MAGNETOMETER SURVEY DATA FROM MMS AND LANL GEOSYNCHRONOUS DATA. SUPPLEMENTARY DATA INCLUDE HOPE RBSPICE AND MAGNETOMETER DATA FROM THE VAN ALLEN PROBES AND CODIF FROM CLUSTER. THIS PROJECT DIRECTLY ADDRESSES THE DECADAL SURVEY BROAD GOAL TO DETERMINE THE DYNAMICS AND COUPLING OF EARTH S MAGNETOSPHERE IONOSPHERE AND ATMOSPHERE AND THEIR RESPONSE TO SOLAR AND TERRESTRIAL INPUT AND SPECIFICALLY ADDRESS THE SWMI SCIENCE GOAL 2 TO IDENTIFY THE CONTROLLING FACTORS THAT DETERMINE THE DOMINANT SOURCES OF MAGNETOSPHERIC PLASMA. IT DOES THIS BY DETERMINING HOW THE MAGNETOTAIL IS IN POPULATED WITH HEAVY IONS FROM THE IONOSPHERE.