WE PROPOSE AN OBSERVATIONALLY DRIVEN STUDY TO EXAMINE THE ORIGIN AND EVOLUTION OF ENERGETIC PARTICLE POPULATIONS IN THE MAGNETOSPHERE-IONOSPHERE SYSTEM WITH FOCUS ON RAPID SPACE-TIME SCALES ASSOCIATED WITH SUBSTORMS. OUR APPROACH INVOLVES THE COORDINATED ANALYSIS OF IN-SITU MEASUREMENTS (THEMIS CLUSTER GEOTAIL AND VAN ALLEN PROBES (VAP)) AND MAGNETICALLY CONJUGATE MEASUREMENTS BY THE ELECTRONICALLY STEERABLE POKER FLAT INCOHERENT SCATTER RADAR (PFISR COMMISSIONED MARCH 2007). TECHNIQUES DEVELOPED IN PRIOR STUDIES WILL BE USED TO CREATE DETAILED SPATIALLY-RESOLVED MAPS OF REGIONAL IONOSPHERIC PARAMETERS (PLASMA DENSITY ELECTRON AND ION TEMPERATURE IONOSPHERIC TRANSPORT) AT<30-S CADENCE. THESE RESULTS WILL BE ANALYZED USING FIRST-PRINCIPLES MODELING OF THE ENERGY DEPOSITION PROCESS TO PROVIDE QUANTITATIVE ESTIMATES OF PRECIPITATING PARTICLE CHARACTERISTICS JOULE DISSIPATION RATES PLASMA FLOW FIELDS AND REGIONAL MAPS OF DYNAMIC MAGNETOSPHERIC BOUNDARIES. THE PREMISE OF THIS RESEARCH IS THAT SUBSTANTIAL PROGRESS IN OUR UNDERSTANDING OF THE COUPLING BETWEEN MAGNETOTAIL AND AURORAL DYNAMICS CAN BE REALIZED BY ESTABLISHING THE RELATIVE POSITION AND TIMING OF ACCELERATION MECHANISMS. WE WILL BEGIN BY USING THE LARGELY UNTAPPED DATABASE OF THEMIS-PFISR CONJUNCTIONS SUPPORTED BY ANCILLARY INSTRUMENTS TO INVESTIGATE RELATIONSHIPS BETWEEN PLASMA SHEET DYNAMICS AND CONJUGATE LOSS CONE POPULATIONS. OUR PRELIMINARY CASE STUDY [SIVADAS ET AL. 2017] HAS REVEALED THE SIMULTANEOUS PRESENCE OF SUB-RELATIVISTIC ELECTRONS (100 TO 500 KEV) NEAR-EARTH ACCELERATED ELECTRONS (10 TO 30 KEV) AND THE INTRIGUING SUGGESTION OF AN ISOLATED CHANNEL OF RELATIVISTIC (~1 MEV) ELECTRONS ALL WITHIN A SINGLE 200X200-KM REGION AT THE IONOSPHERIC FOOTPRINT OF THEMIS-D. WE WILL USE THE FULL COMPLEMENT OF SPACE-TIME INFORMATION IN SUCH DATA TO ADDRESS THREE QUESTIONS: (1) WHAT IS THE SOURCE OF THE ENERGETIC ELECTRON POPULATION IN THE>100 KEV ENERGY RANGE OBSERVED DURING SUBSTORM ONSET AND EXPANSION? (2) IS THIS POPULATION PRESENT IN ALL SUBSTORMS AND AT THE SAME LOCATION RELATIVE TO AAR POPULATIONS? AND (3) WHAT ROLE DOES THE AAR PLAY IN THE PRECIPITATION AND POSSIBLE MODULATION OF>100 KEV PRIMARY ELECTRONS? THE SUGGESTION OF A RELATIONSHIP BETWEEN GEOMAGNETIC SUBSTORMS AND RELATIVISTIC ELECTRON PRECIPITATION CALLS FOR A PARALLEL INVESTIGATION INTO VAP-PFISR CONJUNCTIONS. THE SPATIALLY RESOLVED MEASUREMENTS BY PFISR ENABLE THE POTENTIAL IDENTIFICATION OF LOCALIZED LOW ALTITUDE IONIZATION CHANNELS SUCH AS THOSE PRODUCED BY RELATIVISTIC ELECTRON MICROBURSTS FROM THE OUTER RADIATION BELTS. THIS ENIGMATIC PHENOMENON APPEARS BE BE CONNECTED TO TAIL RECONFIGURATIONS DURING SUBSTORM RECOVERY. BOTH THEMIS AND VAP WERE CONCEIVED AS MULTI-SPACECRAFT MISSIONS IN AN EFFORT TO ADDRESS THE INHERENT SPACE-TIME AMBIGUITY OF POINT MEASUREMENTS. WE SEEK TO ADD ADDITIONAL VALUE TO THE THEMIS AND VAP PROGRAMS THROUGH THE USE OF THE THEMIS ALL-SKY IMAGERS AND A NEW FORM OF RADAR IMAGING DIAGNOSTIC WHICH PROVIDES ACCESS TO FOUR-DIMENSIONAL (SPACE-TIME) MAPS OF PLASMA PARAMETERS AT THE IONOSPHERIC FOOTPRINTS. AS PART OF OUR PRELIMINARY EFFORT WE HAVE IDENTIFIED A LARGE NUMBER OF THEMIS-PFISR AND VAP-PFISR CONJUNCTIONS COVERING A RICH VARIETY GEOMAGNETIC CONTEXTS. WE WILL EXTEND THIS EFFORT TO THE CLUSTER AND GEOTAIL DATABASES AS WELL. A COMPLETE LIST OF STUDY INTERVALS AND THEIR CONNECTIONS WITH OUR SCIENCE OBJECTIVES WILL BE INCLUDED IN THE FULL PROPOSAL. EARTH S PLASMA ENVIRONMENT PRESENTS AN EFFECTIVE LABORATORY FOR STUDYING THE UNIVERSAL PROCESS OF PLASMA ACCELERATION AS HIGHLIGHTED IN THE DECADAL SURVEY. THIS PROPOSED STUDY AIMS TO UNDERSTAND FUNDAMENTAL PROCESSES IN ENERGETIC PARTICLE DYNAMICS IN THE MAGNETOSPHERE AND THUS THIS STUDY WILL ADDRESS DECADAL SURVEY KEY SCIENCE GOALS #2 AND#4.