ELECTROMAGNETIC ION CYCLOTRON (EMIC) WAVES ARE KNOWN TO BE AN IMPORTANT INSTABILITY IN THE MAGNETOSPHERE REGULATING THE RING CURRENT TEMPERATURE ANISOTROPY PRECIPITATING RING CURRENT IONS TO THE IONOSPHERE AND CAUSING PROMPT LOSS OF THE OUTER ELECTRON RADIATION BELTS THROUGH RESONANT WAVE-PARTICLE INTERACTIONS. THE WAVES ALSO HEAT COLD IONOSPHERIC HE+ AND O+ CONTRIBUTING TO TRANSPORT OF IONOSPHERIC PLASMA TO THE OUTER MAGNETOSPHERE. WHILE EMIC WAVES HAVE BEEN WIDELY DOCUMENTED PRIOR THEORETICAL RESEARCH HAS FOCUSED PRIMARILY ON PREDICTING THEIR GENERATION AND CONDITIONS NEEDED FOR GROWTH. THE UNDERLYING FUNDAMENTAL PHYSICS OF WAVE PROPAGATION HOWEVER HAS YET TO BE FULLY UNDERSTOOD. THE WAVE PROPERTIES PROPAGATION CHARACTERISTICS AND INTERACTIONS WITH RING CURRENT PLASMA ENERGETIC ELECTRONS AND COLD IONS ARE GOVERNED BY WAVE DISPERSION THAT RELATES THE WAVE FREQUENCY W AND WAVE VECTOR K TO THE AMBIENT MAGNETIC FIELD AND PLASMA. TO DATE WE STILL HAVE NOT CONFIRMED THE BASIC WAVE PROPERTIES AND PROPAGATION CHARACTERISTICS OF EMIC WAVES BECAUSE K IS PROFOUNDLY DIFFICULT TO DETERMINE OBSERVATIONALLY. NEW OBSERVATIONS FROM MMS MADE WITH HIGH ACCURACY IN MAGNETIC FIELDS AND UNPRECEDENTED CLOSE SPACING OF THE FOUR MMS OBSERVATORIES FINALLY ALLOW ACCURATE DETERMINATION OF K VIA THE CURL OPERATOR. MORE EXTENSIVE TESTS OF WAVE DISPERSION AND PROPAGATION ARE ALSO POSSIBLE USING THE DIVERGENCE AND CURL OF FULLY THREE-DIMENSIONAL ELECTRIC FIELD MEASUREMENTS. EMIC WAVES ARE PREVALENT IN MMS OBSERVATIONS AND SPAN A WIDE RANGE OF MAGNETIC LATITUDES INCLUDING THE LATITUDES WHERE THE CRITICAL BI-ION RESONANCES OCCUR. THE COMBINATION OF THIS FUNDAMENTALLY NEW OBSERVATIONAL CAPABILITY AND THE ORBITAL COVERAGE ALLOWS THE FIRST EVER DEFINITIVE DETERMINATION OF THE EMIC WAVE PROPERTIES AND THEIR EVOLUTION WITH LATITUDE AS THEY PROPAGATE TO REGIONS OF HIGHER MAGNETIC FIELD STRENGTH. THIS RESEARCH IS DIRECTLY RELEVANT TO THE HELIOPHYSICS DECADAL SURVEY GOAL (2) DETERMINE THE DYNAMICS AND COUPLING OF THE EARTH S MAGNETOSPHERE IONOSPHERE AND ATMOSPHERE AND THEIR RESPONSE TO SOLAR AND TERRESTRIAL INPUTS. AND GOAL (4) DISCOVER AND CHARACTERIZE FUNDAMENTAL PROCESSES THAT OCCUR BOTH WITHIN THE HELIOSPHERE AND THROUGHOUT THE UNIVERSE . WE PROPOSE TO DETERMINE THE FUNDAMENTAL PROPERTIES OF EMIC WAVE PROPAGATION IN THE MAGNETOSPHERE AND COMPARE THESE RESULTS AGAINST DISPERSION AND PROPAGATION THEORY TO UNDERSTAND THE BASIC PHYSICS OF THESE WAVES. SPECIFICALLY WE WILL ADDRESS THE FUNDAMENTAL QUESTION OF HOW THE EMIC WAVE VECTOR EVOLVES AS THE WAVE PROPAGATES BY INVESTIGATING (A) HOW THE EMIC WAVE VECTORS AND WAVE PROPERTIES VARY BY L SHELL AND LATITUDE; (B) WHERE IN THE DISPERSION RELATION THE WAVES FALL AT DIFFERENT POINTS DURING PROPAGATION AND AS PLASMA CONDITIONS CHANGE; AND (C) HOW CHANGES IN THE WAVE VECTOR DURING PROPAGATION AFFECT THE WAVE CHARACTERISTICS AND RESONANCE CONDITIONS. WE WILL USE THE MMS OBSERVATIONS TOGETHER WITH THEORY AND MODELING OF THE DISPERSION RELATION AND PROPAGATION TO UNDERSTAND THE FUNDAMENTAL PHYSICS OF EMIC WAVES. IN ADDITION OBSERVATIONS OF EMIC WAVES WITH MMS DURING CLOSE CONJUNCTIONS WITH THE VAN ALLEN PROBES AND THEMIS WILL BE USED TO CONFIRM THEIR PROPAGATION EVOLUTION AT MULTIPLE LOCATIONS ON THE SAME FIELD LINES. THE RESULTS WILL YIELD A DEEPER UNDERSTANDING OF HOW THE WAVES RESONATE WITH PLASMA POPULATIONS LEADING TO THE LOSS OF RING CURRENT AND RADIATION BELT PARTICLES THEREBY ADVANCING OUR PREDICTIVE UNDERSTANDING OF THESE KEY WAVE-PARTICLE INTERACTIONS.