THE KEY OBJECTIVE OF THIS STUDY IS TO BETTER UNDERSTAND THE SOURCE AND CAUSE OF HIGH-ENERGY ELECTRONS OBSERVED BY THE MMS IN THE DAYSIDE MAGNETOSHEATH. THE MAGNETOSPHERIC MULTI-SCALE (MMS) MISSION IS A FOUR-SPACECRAFT CONSTELLATION ORBITING IN FORMATION AROUND EARTH WITH A MAIN GOAL TO STUDY THE MICROPHYSICS OF MAGNETIC RECONNECTION AT THE DAYSIDE MAGNETOPAUSE. RECENT MMS OBSERVATIONS SHOWED HIGH ENERGY (40 KEV) ELECTRONS LEAKING INTO THE MAGNETOSHEATH (COHEN ET AL. JGR 2017). HOWEVER THE DOMINANT LEAKING MECHANISM HAS NOT BEEN FULLY UNDERSTOOD. GLOBAL LYON-FEDDER-MOBARRY (LFM) WITH TEST PARTICLE SIMULATIONS SUGGEST THAT LOW LATITUDE RECONNECTION AND THE NONLINEAR KELVIN-HELMHOLTZ (KH) INSTABILITY CAN CAUSE THE LEAK OF HIGH ENERGY ELECTRONS INTO THE MAGNETOSHEATH. BUT IT IS IMPORTANT TO NOTICE THAT MANY OF THE ELECTRONS LEAKING EVENTS WERE OBSERVED CLOSE TO FALL EQUINOX WHEN THE MMS ORBIT HAS A SIGNIFICANT Y-COMPONENT AND THE Z_GSM COORDINATE CAN BE SUBSTANTIAL (UP TO ~7 R_E). THEREFORE MMS HIGHENERGY ELECTRON EVENTS MAY HAVE A HIGH-LATITUDE SOURCE. FOR INSTANCE IT IS WELL DEMONSTRATED THAT MAGNETIC RECONNECTION BETWEEN THE INTERPLANETARY MAGNETIC FIELD (IMF) AND EARTH'S MAGNETIC FIELD SURROUNDING THE CUSPS CAN LEAD TO THE FORMATION OF CUSP DIAMAGNETIC CAVITIES (NYKYRI ET AL. JGR 2011A B; ADAMSON ET AL. ANGEO 2011) EXTENDED REGIONS OF DECREASED MAGNETIC FIELD WHICH CAN BE FILLED WITH HIGHER ENERGY (>30 KEV) ELECTRONS PROTONS AND O+ IONS. CLUSTER OBSERVATIONS REVEALED 90-DEGREE PITCH ANGLE ELECTRONS IN THE CAVITY STRONGLY SUGGESTIVE OF A LOCAL ACCELERATION MECHANISM (WALSH ANGEO 2010; NYKYRI ET AL JASTP 2012). TEST PARTICLE SIMULATIONS IN A HIGH-RESOLUTION 3D CUSP MODEL UNCOVERED THAT TRAPPED PARTICLES IN THE DIAMAGNETIC CAVITIES CAN BE ACCELERATED WHEN THEIR DRIFT PATHS GO THROUGH REGIONS OF "RECONNECTION QUASI-POTENTIAL" (NYKYRI ET AL JASTP 2012). ONCE THE IMF ORIENTATION CHANGES IT IS POSSIBLE FOR TRAPPED PARTICLES IN THE CAVITY TO END UP INTO THE LOSS CONE AND "LEAK OUT" OF THE CAVITY. A SYSTEMATIC APPROACH TO OUR SCIENCE OBJECTIVE ADDRESSES THE FOLLOWING COMPELLING SCIENCE QUESTIONS BY SYNERGY USING MMS OBSERVATIONAL DATA AND NUMERICAL SIMULATION. GOALS 1. WHAT IS THE DOMINANT ORIGIN OF HIGH ENERGY ELECTRONS OBSERVED IN MAGNETOSHEATH BY THE MMS? 2. HOW EFFICIENT IS THE LOCAL ELECTRON ACCELERATION IN DIAMAGNETIC CAVITIES UNDER DIFFERENT IMF/SW CONDITIONS? 3. HOW EFFICIENT IS THE LOCAL ELECTRON ACCELERATION IN LOW-LATITUDE RECONNECTION AND NONLINEAR KH INSTABILITY UNDER DIFFERENT IMF/SW CONDITIONS? METHODOLOGY OUR METHODOLOGY CONSISTS OF MMS DATA ANALYSIS AS WELL AS GLOBAL AND LOCAL MHD SIMULATIONS WITH TEST PARTICLES: 1. GENERATION OF LFM GLOBAL SIMULATIONS OF SELECT MMS HIGH-ENERGY ELECTRON EVENTS CLOSE TO FALL EQUINOX WHEN MMS ORBIT PASSES CLOSE TO DIAMAGNETIC CAVITIES. TEST PARTICLES WILL BE PLACED IN LFM SIMULATIONS TO STUDY THE POSSIBLE LEAKAGE DUE TO KH-INSTABILITY AND RECONNECTION CLOSE TO DIAMAGNETIC CAVITIES. 2. GENERATION OF HIGH-RESOLUTION LOCAL 3D CUSP SIMULATIONS WITH TEST PARTICLES THAT ARE GENERATED USING LFM PLASMA AND FIELD PROFILES AS AN INPUT. 3. HOW EFFICIENT IS THE LOCAL ELECTRON ACCELERATION IN LOW-LATITUDE RECONNECTION AND NONLINEAR KH INSTABILITY UNDER DIFFERENT IMF/SW CONDITIONS? DATA MMS OBSERVATIONAL DATA ARE AVAILABLE FROM NASA'S SPACE PHYSICS DATA FACILITY (SPDF). WE HAVE EXPERIENCE USING SPACE PHYSICS ENVIRONMENT DATA ANALYSIS SOFTWARE (SPEDAS) FOR MMS DATA ANALYSIS. ALL NUMERICAL SIMULATION CODES AND DIAGNOSTIC TOOLS HAVE BEEN DEVELOPED AND HAVE BEEN APPLIED TO OTHER STUDIES. RELEVANCE THIS PROPOSAL IS TARGETED AT THE DECADAL SURVEY KEY SCIENCE GOAL: "DETERMINE THE DYNAMICS AND COUPLING OF EARTH'S MAGNETOSPHERE [...] AND THEIR RESPONSE TO SOLAR AND TERRESTRIAL INPUTS." IT ALSO SUPPORTS THE SCIENCE PLAN FOR NASA'S SCIENCE MISSION DIRECTORATE (2007-2016)'S OBJECTIVE TO "UNDERSTAND THE PLASMA PROCESSES THAT ACCELERATE AND TRANSPORT PARTICLES" (SECTION 6.2.1).