FAST MAGNETOSONIC (MS) WAVES ARE ELECTROMAGNETIC EMISSION PRIMARILY CONFINED WITHIN A FEW DEGREES FROM THE GEOMAGNETIC EQUATOR (RUSSELL 1970). PREVIOUS STUDIES HAVE SUGGESTED THAT SUCH MS WAVES CAN BE EXCITED BY A RING DISTRIBUTION OF ENERGETIC PROTONS. THE RADIAL AND AZIMUTHAL PROPAGATION OF MS WAVES HOWEVER ENABLES WAVE OBSERVATIONS OFTEN AT REGIONS OTHER THAN THEIR REAL EXCITATION REGIONS WHICH MAKES UNDERSTANDING OF WAVE EXCITATION LESS CONCLUSIVE. WITH UNPRECEDENTEDLY ACCURATE WAVE AND PARTICLE MEASUREMENTS FROM VAN ALLEN PROBES WE PROPOSE TO INVESTIGATE MS WAVE EXCITATION IN THE HEART OF EXCITATION REGIONS FROM LINEAR AND NONLINEAR ASPECTS.MS EMISSIONS ARE SHOWN TO CONTRIBUTE TO RADIATION BELT DYNAMICS SPECIFICALLY LOCAL ELECTRON ACCELERATION (HORNE ET AL. 2007) NONRESONANT TRANSIT TIME SCATTERING (BORTNIK ET AL. 2010) AND REMOVAL OF THE EQUATORIALLY MIRRORING ELECTRONS (CHEN ET AL. 2015). THESE EFFECTS ARE HIGHLY SENSITIVE TO MS WAVE NORMAL ANGLES OVER A NARROW RANGE NEAR 90 DEGREES WHICH IS UNABLE TO BE RESOLVED BY WAVE MEASUREMENT. WE PROPOSE TO DISCOVER THE WAVE NORMAL ANGLE CHARACTERISTICS WITH THE AID OF OUR THEORETICAL ANALYSIS AND SIMULATION EFFORTS. IN SUMMARY OUR PROPOSAL WILL ADDRESS THE FOLLOWING SCIENTIFIC GOALS.SCIENTIFIC GOALS(1) TESTING LINEAR THEORY TO SEE WHETHER OBSERVED FREQUENCY SPECTRUM OF MS WAVES AT THE EXCITATION REGION CAN BE REPRODUCED.(2) COMPARING THE INSTABILITY DUE TO PROTON SHELL DISTRIBUTION WITH THAT DUE TO PROTON RING DISTRIBUTION.(2) UNDERSTANDING NONLINEAR EVOLUTIONS OF MS WAVE HARMONIC STRUCTURES.(3) CHARACTERIZING WAVE NORMAL ANGLE OF EXCITED MS WAVES.RELEVANCEWE PROPOSE TO STUDY MS WAVE EXCITATION IN THE HEART OF EXCITATION REGIONS WHICH IS A UNIQUENESS OF THIS STUDY AND WILL PROVIDE CONCLUSIVE KNOWLEDGE OF THE GENERATION MECHANISM. OUR PROPOSED STUDY WILL GREATLY ADVANCE THE UNDERSTANDING OF LINEAR EXCITATION AND NONLINEAR EVOLUTION OF MS WAVES IN DIRECT RELEVANCE TO A MAJOR SCIENCE GOAL OF HELIOPHYSICS DECADAL SURVEY "DISCOVER AND CHARACTERIZE FUNDAMENTAL PROCESSES THAT OCCUR BOTH WITHIN THE HELIOSPHERE AND THROUGHOUT THE UNIVERSE." THESE WAVES SERVE AS AN INTERMEDIARY TO TRANSFER ENERGY FROM RING CURRENT PROTONS TO RELATIVISTIC RADIATION BELT ELECTRONS. UNDERSTANDING THE EXCITATION CHARACTERISTICS OF THIS EMISSION IS RELEVANT TO HELIOPHYSICS DECADAL SURVEY'S GOAL TO UNDERSTAND DYNAMICS OF THE MAGNETOSPHERE. OUR PROPOSED STUDY IS TIMELY BECAUSE IT IS SIGNIFICANT TO THE OVERALL OBJECTIVE OF CURRENT VAN ALLEN PROBES MISSION SPECIFICALLY TO UNDERSTAND THE EFFECT OF RING CURRENT ON RADIATION BELT AND THUS THE DYNAMICS OF RELATIVISTIC ELECTRONS.METHODOLOGY AND WORK PLANYEAR 1: WE WILL IDENTIFY AND COLLECT MS WAVES EVENTS IN THE WAVE EXCITATION REGION BY ANALYZING WAVEFORMS OF VAN ALLEN PROBES' EMFISIS INSTRUMENT OVER THE ENTIRE MISSION PERIOD AVAILABLE. WE WILL USE H+ VELOCITY DISTRIBUTION FROM HOPE INSTRUMENT AND USE BACKGROUND MAGNETIC FIELD AND PLASMA DENSITY OBTAINED FROM EMFISIS INSTRUMENT IN ORDER TO PERFORM LINEAR INSTABILITY ANALYSIS ON MS WAVES AND MAKE COMPARISON BETWEEN LINEAR RESULTS AND OBSERVED WAVE CHARACTERISTICS. YEAR 2: WE WILL FOCUS ON NONLINEAR ASPECT OF MS WAVES USING AN EXISTING 2D PARTICLE-IN-CELL (PIC) CODE. THE PIC SIMULATION IN BOTH UNIFORM AND DIPOLE MAGNETIC FIELDS WILL APPLY TO THE MULTIPLE MS WAVE EVENTS IN THE EXCITATION REGIONS. YEAR 3: WE WILL PERFORM PIC SIMULATION AND USE LINEAR KINETIC INSTABILITY TO DISCOVER WAVE NORMAL ANGLE CHARACTERISTICS OF THE UNSTABLE MS WAVE IN THE EXCITATION REGION. WE WILL ALSO SET UP RAY TRACING TO SIMULATE WAVE NORMAL ANGLE CHARACTERISTICS OUTSIDE THE SOURCE REGION.