WHISTLER MODE WAVES ARE KNOWN TO PLAY AN IMPORTANT ROLE IN RADIATION BELT DYNAMICS. HOWEVER MANY ASPECTS OF WHISTLER MODE WAVE OCCURRENCE AND PROPAGATION ARE STILL NOT FULLY UNDERSTOOD AND NEW OBSERVATIONS ARE CHALLENGING LONG HELD ASSUMPTIONS. WE FOCUS ON THE REGION OUTSIDE OF THE PLASMAPAUSE WHERE CHORUS WAVES ARE DOMINANT AND HAVE A DUAL ROLE OF SCATTERING AND ACCELERATING RELATIVISTIC ELECTRONS. THE PROPAGATION CHARACTERISTICS OF THESE WAVES CAN BE STRONGLY INFLUENCED BY BACKGROUND PLASMA DENSITY STRUCTURES WHICH HAVE LONG BEEN THEORIZED AND RECENTLY BECOME OBSERVABLE IN HIGH RESOLUTION BUT HAVE YET TO BE TAKEN INTO ACCOUNT IN GLOBAL MODELS. DEPENDING ON THEIR PREVALENCE AND SIZE DENSITY STRUCTURES CAN STRONGLY INFLUENCE THE COHERENCE OF CHORUS WAVE PACKETS AND AFFECT WHETHER WAVE-PARTICLE INTERACTIONS WILL BE IN THE LINEAR OR NONLINEAR REGIME. THE PROJECT SEEKS TO ANSWER THREE SCIENCE QUESTIONS USING ANALYSIS OF SPACECRAFT OBSERVATIONS AND NUMERICAL MODELING 1) WHAT IS THE DISTRIBUTION AND PREVALENCE OF DENSITY STRUCTURES IN THE EQUATORIAL REGION? 2) HOW DO SUCH STRUCTURES AFFECT CHORUS WAVE PARAMETERS INCLUDING AMPLITUDE WAVE NORMAL ANGLE AND COHERENCE? 3) DOES OBSERVATIONAL EVIDENCE CONFIRM THAT DENSITY STRUCTURES ARE NECESSARY TO EXPLAIN CHORUS WAVES PROPERTIES AND THAT THESE STRUCTURES ENHANCE NONLINEAR WAVE PARTICLE INTERACTIONS? TO ADDRESS THE DISTRIBUTION AND PREVALENCE OF COLD PLASMA DENSITY STRUCTURES MMS AND CLUSTER MISSION OBSERVATIONS WILL BE ANALYZED. THE FOUR SPACECRAFT OF THE CLUSTER MISSION WERE CLOSELY SPACED (50 500 KM) DURING EQUATORIAL CROSSINGS AND THE FOUR MMS SPACECRAFT WERE SPACED AT ~20-50 KM APART ON AN EQUATORIAL ORBIT. THIS SPACING ALLOWS FOR DENSITY STRUCTURES OF ~50 KM SPATIAL SCALE TO BE OBSERVED AND QUANTIFIED. WE PLAN TO USE MEASUREMENTS OF DENSITY FLOATING POTENTIAL VLF WAVES AND THE GEOMAGNETIC FIELD TO INVESTIGATE THE PRESENCE OF COLD PLASMA DENSITY FLUCTUATIONS THEIR SIZE OCCURRENCE AND CAPABILITY OF GUIDING WAVES AT DIFFERENT LOCATIONS AND UNDER VARIABLE GEOMAGNETIC ACTIVITY. THIS WILL BE DONE BY CO-I AGAPITOV WHO HAS PIONEERED TECHNIQUES FOR SUCH IDENTIFICATION. DATA ON STRUCTURES DERIVED FROM OBSERVATIONS WILL BE USED IN CONJUNCTION WITH NUMERICAL RAYTRACING TO ADDRESS THE SECOND SCIENCE QUESTION ON THE IMPACT OF DENSITY STRUCTURES ON CHORUS WAVES. MOST PAST WORK ON MAGNETOSPHERIC RAYTRACING HAS NEGLECTED SMALL DENSITY STRUCTURES. HERE WAVE PROPAGATION IN THE PRESENCE OF THE IDENTIFIED STRUCTURES WILL BE PRECISELY MODELED. THE RAYTRACING MODEL WILL BE 3-DIMENSIONAL AND INCLUDE RECENTLY DEVELOPED WARM PLASMA CORRECTIONS ALLOWING ACCURATE SIMULATION EVEN AT HIGHLY OBLIQUE ANGLES. THE RAYTRACING EFFORT AND ALSO FINITE DIFFERENCE MODELING WILL INFORM ON THE WAVE AMPLITUDE COHERENCE AND WAVE NORMAL ANGLE TO BE EXPECTED AS A CONSEQUENCE OF THE OBSERVED DENSITY STRUCTURES. FINALLY THE MODELING PREDICTIONS WILL BE VALIDATED BY ANALYSIS OF WAVE AND PARTICLE DATA FROM THE VAN ALLEN PROBES MISSION. OBSERVATIONS OF WAVE NORMAL ANGLE WAVE AMPLITUDES AND ENERGETIC PARTICLE FLUXES WILL BE USED TO ASSESS LINEAR VS. NONLINEAR PARTICLE INTERACTION REGIMES INCLUDING MICROBURSTS. THIS WILL ANSWER THE QUESTION OF WHETHER EQUATORIAL DENSITY STRUCTURES PLAY A ROLE IN CHORUS WAVE DRIVEN RADIATION BELT DYNAMICS. THIS PROJECT ADDRESSES OUTSTANDING QUESTIONS ON WHISTLER MODE WAVES IN THE CONTEXT OF NASA DECADAL SURVEY GOALS #2 AND #4 WHICH SEEK TO DETERMINE THE ENERGY DYNAMICS OF THE EARTH S MAGNETOSPHERE AND IONOSPHERE AND CHARACTERIZE FUNDAMENTAL PROCESSES IN THE HELIOSPHERE. FURTHERMORE TWO OF THE VAN ALLEN PROBES MISSION'S GENERAL SCIENTIFIC OBJECTIVES: #1-DISCOVER WHICH PROCESSES ACCELERATE AND TRANSPORT THE PARTICLES IN THE RADIATION BELT AND UNDER WHAT CONDITIONS; #4-UNDERSTAND HOW THE RADIATION BELTS CHANGE IN THE CONTEXT OF GEOMAGNETIC STORMS WILL ALSO BE DIRECTLY ADDRESSED.