OUTER RADIATION BELT FLUX "DROPOUT" EVENTS DRASTIC DECREASES IN RELATIVISTIC ELECTRON FLUX OVER BROAD RANGES OF ENERGY PITCH ANGLE AND RADIAL DISTANCE IN ONLY A FEW HOURS OCCUR COMMONLY. WHILE BOTH ADIABATIC EFFECTS AND TRUE LOSSES (TO THE ATMOSPHERE AND/OR MAGNETOPAUSE) HAVE BEEN SUGGESTED THE DOMINANT MECHANISM REMAINS UNCLEAR. A SIMILAR DEBATE REMAINS REGARDING THE ULTIMATE MECHANISM(S) OF MAGNETOSPHERIC ENERGETIC PARTICLE ESCAPE AND THEIR SOURCE(S). THE PRIMARY MECHANISMS CONSIDERED ARE MAGNETIC RECONNECTION AND "LEAKAGE" DUE TO FINITE GYRORADIUS EFFECTS WHILE THE MAGNETOSPHERIC SOURCES REMAIN AMBIGUOUS. HOWEVER THE ASYMMETRIC PITCH ANGLE DISTRIBUTIONS (FIELD-ALIGNED STREAMING) SIGNATURE FEATURED IN THE STATISTICAL STUDY OF ENERGETIC ELECTRON ESCAPE BY COHEN ET AL. [2017] WAS OBSERVED UP TO ~200 KEV WHICH THE AUTHORS SUGGEST COULD RESULT FROM THE LOSS OF RELATIVISTIC OUTER BELT ELECTRONS BASED ON SIMPLE CONSIDERATION OF THE CONSERVATION OF THE FIRST ADIABATIC INVARIANT. THE GOAL OF THIS INVESTIGATION IS TO UNDERSTAND THE NATURE OF OUTER RADIATION BELT ELECTRON LOSSES TO THE MAGNETOPAUSE. TO ADDRESS THIS THE INVESTIGATION WILL TARGET THE FOLLOWING SCIENCE QUESTIONS: 1. IS LOSS DUE TO OUTER TRANSPORT SUFFICIENT ALONE TO EXPLAIN RADIATION BELT DROPOUT EVENTS? 2. OVER WHAT ENERGIES AND L-SHELLS ARE THESE LOSSES RELEVANT? 3. DO MAGNETOPAUSE ENERGETIC ELECTRON ESCAPE EVENTS CORRELATE WITH DROPOUT EVENTS AND WHAT ARE THE ESTIMATED LOSS RATES? THIS INVESTIGATION WILL PRIMARILY USE MMS ELECTRON (FEEPS EIS AND FPI) DATA FROM PHASES 1A THROUGH 2B TO ALLOW FOR SIGNIFICANT L-SHELL COVERAGE OF PHASE SPACE DENSITY (PSD) PROFILES. OBSERVATIONS FROM THE VAN ALLEN PROBES WILL COMPLEMENT THE MMS DATASET ALLOWING THE STUDY TO EXTEND TO>600 KEV ELECTRONS IN THE RADIATION BELTS. SUPPLEMENTARY MEASUREMENTS FROM THE ESA AND SST INSTRUMENTS ON THEMIS WILL BE USED TO IMPROVE STATISTICS WHERE NECESSARY. THIS INVESTIGATION COMPRISES A COMPARATIVE CASE STUDY OF 1-2 EVENTS INCLUDING EVENT SIMULATIONS TO ADDRESS Q1-2 AND A STATISTICAL ANALYSIS OF OBSERVATIONS TO ADDRESS Q2-3. TO IDENTIFY EVENTS FOR THE CASE STUDY ELECTRON DATA FROM THE VAN ALLEN PROBES MAGEIS AND REPT INSTRUMENTS DURING MMS PHASES 1A B WILL BE SURVEYED TO IDENTIFY SIGNIFICANT DROPOUT EVENTS BASED ON SELECTION CRITERIA FROM TURNER ET AL. [2012]. THEN BOTH VA PROBES AND MMS ELECTRON DATA WILL BE REVIEWED FROM TWO DAYS (T-2 DAYS) PRIOR TO ONE DAY (T+1 DAY) AFTER EACH EVENT TO GENERATE PROFILES OF PSD VERSUS L-SHELL. THESE PROFILES WILL THEN BE COMPARED BETWEEN THE CANDIDATE EVENTS AND WITH SIMULATION RESULTS UNDER APPROPRIATE BOUNDARY CONDITIONS FROM THE COUPLED MHD-RC (LFM-RCM) AND TEST-PARTICLE (CHIMP) SIMULATIONS [SORATHIA ET AL. 2017] TO INVESTIGATE ENERGY AND L-SHELL DEPENDENCE OF THE MAGNETOPAUSE LOSSES ACROSS THE RADIATION BELTS AND GENERALIZE THE WORK OF TURNER ET AL. [2014]. THE STATISTICAL ANALYSIS WILL BE CONDUCTED ON AN EVENT DATABASE COMPILED BY EXPANDING THE INITIAL SURVEY OF VA PROBES DROPOUT EVENTS TO INCLUDE LOWER INTENSITY EVENTS INCLUDING DATA FROM PHASES 1X AND 2B WHERE RELEVANT. THE ENERGETIC ELECTRON ESCAPE EVENT DATABASE FROM COHEN ET AL. [2017] WILL ALSO BE EXPANDED TO INCLUDE PHASES 1B AND 3A. EVENTS FROM THESE DATABASES WILL BE ANALYZED FOLLOWING A SIMILAR METHODOLOGY OUTLINED FOR THE CASE STUDY ABOVE AIMING TO STATISTICALLY INVESTIGATE THE ENERGY AND L-SHELL DISTRIBUTIONS OF FLUX DROPOUT EVENTS AND THEIR CORRELATION TO MAGNETOPAUSE ENERGETIC ELECTRON ESCAPE EVENTS. OTHER PARAMETERS TO BE INVESTIGATED INCLUDE CONTENT RATIOS AND TIMING; CORRELATION TO SOLAR WIND AND GEOMAGNETIC ACTIVITY; AND RADIAL GRADIENTS OF PSD VS. L-SHELL. THE TOPIC OF THIS INVESTIGATION RADIATION BELT ELECTRON LOSS AT THE MAGNETOPAUSE AND THE ULTIMATE SOURCE OF THESE RADIATION BELT ELECTRONS IS ESPECIALLY RELEVANT TO THE DECADAL SURVEY SWMI SCIENCE GOAL #4 WHICH CONTRIBUTES TO DECADAL KEY SCIENCE GOALS #1 #2 AND #4.