2 UNDERSTANDING THE ROLE OF KINETIC BALLOONING INSTABILITY IN TRIGGERING SUBSTORM ONSET 1. BACKGROUND AND SIGNIFICANCE SUBSTORMS ARE ONE OF THE MOST DRAMATIC DISTURBANCES OF THE GLOBAL MAGNETOTAIL-IONOSPHERE COUPLING SYSTEM THAT RELEASE LARGE AMOUNTS OF SOLAR WIND ENERGY ACCUMULATED IN THE MAGNETOTAIL AND ARE ASSOCIATED WITH AURORAL ACTIVATIONS. A CRITICAL, LONG-STANDING PROBLEM IN SUBSTORM RESEARCH IS TO IDENTIFY WHAT PHYSICAL MECHANISM LEADS TO SUBSTORM AURORAL ONSET. RECENT SATELLITE AND GROUND-BASED AURORAL OBSERVATIONS HAVE REVEALED THAT SUBSTORM AURORAL ONSET OFTEN INVOLVES WAVY STRUCTURES (CALLED 'BEADS') ALONG A PRE-EXISTING GROWTH PHASE ARC, AND THAT PRE-ONSET WAVES WITH ~1-2 MIN PERIODICITY OCCUR IN THE CONJUGATE NEAR-EARTH PLASMA SHEET. THE PRE-ONSET WAVES HAVE BEEN SUGGESTED TO BE AN IMPORTANT FACET OF THE INSTABILITY (SUCH AS THE BALLOONING MODE) IN THE NEAR-EARTH REGION THAT LEADS TO SUBSTORM ONSET. IT IS THUS CRITICAL TO DETERMINE IF, AND IF SO HOW, PRE-ONSET WAVES IN THE NEAR-EARTH PLASMA SHEET EVOLVE TO SUBSTORM ONSET WAVES. IF COUPLING BETWEEN THEM IS COMMON, PROPERTIES OF THOSE WAVES SHOULD PROVIDE IMPORTANT CLUES FOR UNDERSTANDING THE SUBSTORM ONSET INSTABILITY. 2. SCIENCE GOALS AND OBJECTIVES THE OVERARCHING GOAL OF THIS PROPOSED STUDY IS TO DETERMINE THE ROLE OF KINETIC BALLOONING INSTABILITY IN TRIGGERING SUBSTORM ONSET. THREE SPECIFIC SCIENCE TASKS ARE PROPOSED: (1) DETERMINE PROPERTIES OF PRE-ONSET WAVES AND THEIR CONNECTION TO ONSET WAVES (2) DETERMINE PROPERTIES OF ONSET WAVES (3) DETERMINE THE ROLE OF KINETIC BALLOONING INSTABILITY FOR DRIVING PRE-ONSET AND ONSET WAVES THE PROPOSED STUDY IS NOW POSSIBLE DUE TO THE NASA-SUPPORTED THEMIS AND GEOTAIL SATELLITES, WHICH HAVE COMPLETED MANY MAGNETOTAIL SURVEY SEASONS, IN COMBINATION WITH RECENT ADVANCE OF 3-D FULLY ELECTROMAGNETIC PARTICLE-IN-CELL SIMULATIONS THAT CAN SIMULATE PARTICLE KINETICS AND FIELDS SELF-CONSISTENTLY. 3. METHODOLOGY WE WILL USE NASA-SUPPORTED THEMIS AND GEOTAIL SATELLITE DATA TO EXAMINE THE SPATIAL (RADIAL DISTANCE, BETA, AND MLT) DEPENDENCE OF THE WAVE OCCURRENCE PRIOR TO AND DURING SUBSTORM ONSET. PHASE RELATIONS AMONG THE MAGNETIC FIELD, ELECTRIC FIELD AND PLASMA PRESSURE WILL BE DETERMINED TO CHARACTERIZE THOSE WAVES. SUBSTORM ONSET IS IDENTIFIED AS INITIAL AURORAL BRIGHTENING ALONG A QUIET ARC FOLLOWED BY POLEWARD EXPANSION. IMAGERS WILL ALSO BE USED TO EXAMINE WHETHER THOSE WAVES EVOLVE TO ONSET BEADS (CAUSALLY RELATED) OR ONSET BEADS EMERGE AT DIFFERENT LOCATIONS OR WAVELENGTHS (NOT CAUSALLY RELATED). THE SPATIAL DEPENDENCE AND PHASE RELATIONS OF PRE-ONSET AND ONSET WAVES OBTAINED FROM THE SATELLITE OBSERVATIONS WILL BE COMPARED TO THOSE FROM 3-D FULL KINETIC SIMULATIONS FOR IDENTIFYING THE ROLE OF KINETIC INSTABILITY INCLUDING BALLOONING FOR DRIVING PRE-ONSET AND ONSET WAVES. SCIENCE CLOSURE WILL BE ACHIEVED WITH THIS DATA- MODEL COMPARISON BY DETERMINING THE ROLE OF KINETIC INSTABILITY IN TRIGGERING THE WAVES SEEN BY SPACECRAFT AND VIA AURORAL BEADING THAT APPEAR TO BE A SIGNATURE OF THE MAGNETOSPHERIC INSTABILITY DRIVING SUBSTORM ONSET. 4. RELEVANCE TO NASA H-SR PROGRAM THIS PROPOSED STUDY AIMS TO REVEAL FUNDAMENTAL PHYSICAL PROCESSES OF SUBSTORM ONSET, WHICH IS A CRITICAL ISSUE IN MAGNETOSPHERE- IONOSPHERE COUPLING PHYSICS AND MAGNETOTAIL PLASMA PHYSICS. THUS, THIS STUDY OF SUBSTORM ONSET DIRECTLY ADDRESSES HELIOPHYSICS DECADAL SURVEY GOAL #4 AND THE M-I COUPLING PORTION OF THE HELIOPHYSICS DECADAL SURVEY GOAL #2. WE WILL PRIMARILY USE IN-SITU OBSERVATIONS IN THE PLASMA SHEET FROM TWO NASA-SUPPORTED MISSIONS (THEMIS AND GEOTAIL SATELLITES) TO DETERMINE PROPERTIES OF PRE-ONSET AND ONSET WAVES, AND RESULTS FROM OBSERVATIONS WILL BE COMPARED TO KINETIC SIMULATIONS. THUS, THE PROPOSED DATA-MODEL COMPARISON STUDY MEETS ALL THREE ASPECTS OF A HIGH-PRIORITY INVESTIGATION FOR THIS H-SR PROGRAM. Funding Only Action ($143k) 6/1/17 Not listed UNDERSTANDING THE ROLE OF KINETIC BALLOONING INSTABILITY IN TRIGGERING SUBSTORM ONSET 1. BACKGROUND AND SIGNIFICANCE SUBSTORMS ARE ONE OF THE MOST DRAMATIC DISTURBANCES OF THE GLOBAL MAGNETOTAIL-IONOSPHERE COUPLING SYSTEM THAT RELEASE LARGE AMOUNTS OF SOLAR WIND ENERGY ACCUMULATED IN THE MAGNETOTAIL AND ARE ASSOCIATED WITH AURORAL ACTIVATIONS. A CRITICAL LONG-STANDING PROBLEM IN SUBSTORM RESEARCH IS TO IDENTIFY WHAT PHYSICAL MECHANISM LEADS TO SUBSTORM AURORAL ONSET. RECENT SATELLITE AND GROUND-BASED AURORAL OBSERVATIONS HAVE REVEALED THAT SUBSTORM AURORAL ONSET OFTEN INVOLVES WAVY STRUCTURES (CALLED 'BEADS') ALONG A PRE-EXISTING GROWTH PHASE ARC AND THAT PRE-ONSET WAVES WITH ~1-2 MIN PERIODICITY OCCUR IN THE CONJUGATE NEAR-EARTH PLASMA SHEET. THE PRE-ONSET WAVES HAVE BEEN SUGGESTED TO BE AN IMPORTANT FACET OF THE INSTABILITY (SUCH AS THE BALLOONING MODE) IN THE NEAR-EARTH REGION THAT LEADS TO SUBSTORM ONSET. IT IS THUS CRITICAL TO DETERMINE IF AND IF SO HOW PRE-ONSET WAVES IN THE NEAR-EARTH PLASMA SHEET EVOLVE TO SUBSTORM ONSET WAVES. IF COUPLING BETWEEN THEM IS COMMON PROPERTIES OF THOSE WAVES SHOULD PROVIDE IMPORTANT CLUES FOR UNDERSTANDING THE SUBSTORM ONSET INSTABILITY.2. SCIENCE GOALS AND OBJECTIVESTHE OVERARCHING GOAL OF THIS PROPOSED STUDY IS TO DETERMINE THE ROLE OF KINETIC BALLOONING INSTABILITY IN TRIGGERING SUBSTORM ONSET. THREE SPECIFIC SCIENCE TASKS ARE PROPOSED:(1) DETERMINE PROPERTIES OF PRE-ONSET WAVES AND THEIR CONNECTION TO ONSET WAVES(2) DETERMINE PROPERTIES OF ONSET WAVES(3) DETERMINE THE ROLE OF KINETIC BALLOONING INSTABILITY FOR DRIVING PRE-ONSET AND ONSET WAVES THE PROPOSED STUDY IS NOW POSSIBLE DUE TO THE NASA-SUPPORTED THEMIS AND GEOTAIL SATELLITES WHICH HAVE COMPLETED MANY MAGNETOTAIL SURVEY SEASONS IN COMBINATION WITH RECENT ADVANCE OF 3-D FULLY ELECTROMAGNETIC PARTICLE-IN-CELL SIMULATIONS THAT CAN SIMULATE PARTICLE KINETICS AND FIELDS SELF-CONSISTENTLY.3. METHODOLOGYWE WILL USE NASA-SUPPORTED THEMIS AND GEOTAIL SATELLITE DATA TO EXAMINE THE SPATIAL (RADIAL DISTANCE BETA AND MLT) DEPENDENCE OF THE WAVE OCCURRENCE PRIOR TO AND DURING SUBSTORM ONSET. PHASE RELATIONS AMONG THE MAGNETIC FIELD ELECTRIC FIELD AND PLASMA PRESSURE WILL BE DETERMINED TO CHARACTERIZE THOSE WAVES. SUBSTORM ONSET IS IDENTIFIED AS INITIAL AURORAL BRIGHTENING ALONG A QUIET ARC FOLLOWEDBY POLEWARD EXPANSION. IMAGERS WILL ALSO BE USED TO EXAMINE WHETHER THOSE WAVES EVOLVE TO ONSET BEADS (CAUSALLY RELATED) OR ONSET BEADS EMERGE AT DIFFERENT LOCATIONS OR WAVELENGTHS (NOT CAUSALLY RELATED). THE SPATIAL DEPENDENCE AND PHASE RELATIONS OF PRE-ONSET AND ONSET WAVES OBTAINED FROM THE SATELLITE OBSERVATIONS WILL BE COMPARED TO THOSE FROM 3-D FULL KINETIC SIMULATIONS FOR IDENTIFYING THEROLE OF KINETIC INSTABILITY INCLUDING BALLOONING FOR DRIVING PRE-ONSET AND ONSET WAVES. SCIENCE CLOSURE WILL BE ACHIEVED WITH THIS DATA- MODEL COMPARISON BY DETERMINING THE ROLE OF KINETIC INSTABILITY IN TRIGGERING THE WAVES SEEN BY SPACECRAFT AND VIA AURORAL BEADING THAT APPEAR TO BE A SIGNATURE OF THE MAGNETOSPHERIC INSTABILITY DRIVING SUBSTORM ONSET.4. RELEVANCE TO NASA H-SR PROGRAMTHIS PROPOSED STUDY AIMS TO REVEAL FUNDAMENTAL PHYSICAL PROCESSES OF SUBSTORM ONSET WHICH IS A CRITICAL ISSUE IN MAGNETOSPHERE- IONOSPHERE COUPLING PHYSICS AND MAGNETOTAIL PLASMA PHYSICS. THUS THIS STUDY OF SUBSTORM ONSET DIRECTLY ADDRESSES HELIOPHYSICS DECADAL SURVEY GOAL #4 AND THE M-I COUPLING PORTION OF THE HELIOPHYSICS DECADAL SURVEY GOAL #2. WE WILL PRIMARILY USE IN-SITU OBSERVATIONS IN THE PLASMA SHEET FROM TWO NASA-SUPPORTED MISSIONS (THEMIS AND GEOTAIL SATELLITES) TO DETERMINE PROPERTIES OF PRE-ONSET AND ONSET WAVES AND RESULTS FROM OBSERVATIONS WILL BE COMPARED TO KINETIC SIMULATIONS. THUS THE PROPOSED DATA-MODEL COMPARISON STUDY MEETS ALL THREE ASPECTS OF A HIGH-PRIORITY INVESTIGATION FOR THIS H-SR PROGRAM. ($143k) 6/1/17 Not listed UNDERSTANDING THE ROLE OF KINETIC BALLOONING INSTABILITY IN TRIGGERING SUBSTORM ONSET 1. BACKGROUND AND SIGNIFICANCE SUBSTORMS ARE ONE OF THE MOST $130.1k 5/19/16 1 UNDERSTANDING THE ROLE OF KINETIC BALLOONING INSTABILITY IN TRIGGERING SUBSTORM ONSET 1. BACKGROUND AND SIGNIFICANCE SUBSTORMS ARE ONE OF THE MOST DRAMATIC DISTURBANCES OF THE GLOBAL MAGNETOTAIL-IONOSPHERE COUPLING SYSTEM THAT RELEASE LARGE AMOUNTS OF SOLAR WIND ENERGY ACCUMULATED IN THE MAGNETOTAIL AND ARE ASSOCIATED WITH AURORAL ACTIVATIONS. A CRITICAL, LONG-STANDING PROBLEM IN SUBSTORM RESEARCH IS TO IDENTIFY WHAT PHYSICAL MECHANISM LEADS TO SUBSTORM AURORAL ONSET. RECENT SATELLITE AND GROUND-BASED AURORAL OBSERVATIONS HAVE REVEALED THAT SUBSTORM AURORAL ONSET OFTEN INVOLVES WAVY STRUCTURES (CALLED 'BEADS') ALONG A PRE-EXISTING GROWTH PHASE ARC, AND THAT PRE-ONSET WAVES WITH ~1-2 MIN PERIODICITY OCCUR IN THE CONJUGATE NEAR-EARTH PLASMA SHEET. THE PRE-ONSET WAVES HAVE BEEN SUGGESTED TO BE AN IMPORTANT FACET OF THE INSTABILITY (SUCH AS THE BALLOONING MODE) IN THE NEAR-EARTH REGION THAT LEADS TO SUBSTORM ONSET. IT IS THUS CRITICAL TO DETERMINE IF, AND IF SO HOW, PRE-ONSET WAVES IN THE NEAR-EARTH PLASMA SHEET EVOLVE TO SUBSTORM ONSET WAVES. IF COUPLING BETWEEN THEM IS COMMON, PROPERTIES OF THOSE WAVES SHOULD PROVIDE IMPORTANT CLUES FOR UNDERSTANDING THE SUBSTORM ONSET INSTABILITY. 2. SCIENCE GOALS AND OBJECTIVES THE OVERARCHING GOAL OF THIS PROPOSED STUDY IS TO DETERMINE THE ROLE OF KINETIC BALLOONING INSTABILITY IN TRIGGERING SUBSTORM ONSET. THREE SPECIFIC SCIENCE TASKS ARE PROPOSED: (1) DETERMINE PROPERTIES OF PRE-ONSET WAVES AND THEIR CONNECTION TO ONSET WAVES (2) DETERMINE PROPERTIES OF ONSET WAVES (3) DETERMINE THE ROLE OF KINETIC BALLOONING INSTABILITY FOR DRIVING PRE-ONSET AND ONSET WAVES THE PROPOSED STUDY IS NOW POSSIBLE DUE TO THE NASA-SUPPORTED THEMIS AND GEOTAIL SATELLITES, WHICH HAVE COMPLETED MANY MAGNETOTAIL SURVEY SEASONS, IN COMBINATION WITH RECENT ADVANCE OF 3-D FULLY ELECTROMAGNETIC PARTICLE-IN-CELL SIMULATIONS THAT CAN SIMULATE PARTICLE KINETICS AND FIELDS SELF-CONSISTENTLY. 3. METHODOLOGY WE WILL USE NASA-SUPPORTED THEMIS AND GEOTAIL SATELLITE DATA TO EXAMINE THE SPATIAL (RADIAL DISTANCE, BETA, AND MLT) DEPENDENCE OF THE WAVE OCCURRENCE PRIOR TO AND DURING SUBSTORM ONSET. PHASE RELATIONS AMONG THE MAGNETIC FIELD, ELECTRIC FIELD AND PLASMA PRESSURE WILL BE DETERMINED TO CHARACTERIZE THOSE WAVES. SUBSTORM ONSET IS IDENTIFIED AS INITIAL AURORAL BRIGHTENING ALONG A QUIET ARC FOLLOWED BY POLEWARD EXPANSION. IMAGERS WILL ALSO BE USED TO EXAMINE WHETHER THOSE WAVES EVOLVE TO ONSET BEADS (CAUSALLY RELATED) OR ONSET BEADS EMERGE AT DIFFERENT LOCATIONS OR WAVELENGTHS (NOT CAUSALLY RELATED). THE SPATIAL DEPENDENCE AND PHASE RELATIONS OF PRE-ONSET AND ONSET WAVES OBTAINED FROM THE SATELLITE OBSERVATIONS WILL BE COMPARED TO THOSE FROM 3-D FULL KINETIC SIMULATIONS FOR IDENTIFYING THE ROLE OF KINETIC INSTABILITY INCLUDING BALLOONING FOR DRIVING PRE-ONSET AND ONSET WAVES. SCIENCE CLOSURE WILL BE ACHIEVED WITH THIS DATA- MODEL COMPARISON BY DETERMINING THE ROLE OF KINETIC INSTABILITY IN TRIGGERING THE WAVES SEEN BY SPACECRAFT AND VIA AURORAL BEADING THAT APPEAR TO BE A SIGNATURE OF THE MAGNETOSPHERIC INSTABILITY DRIVING SUBSTORM ONSET. 4. RELEVANCE TO NASA H-SR PROGRAM THIS PROPOSED STUDY AIMS TO REVEAL FUNDAMENTAL PHYSICAL PROCESSES OF SUBSTORM ONSET, WHICH IS A CRITICAL ISSUE IN MAGNETOSPHERE- IONOSPHERE COUPLING PHYSICS AND MAGNETOTAIL PLASMA PHYSICS. THUS, THIS STUDY OF SUBSTORM ONSET DIRECTLY ADDRESSES HELIOPHYSICS DECADAL SURVEY GOAL #4 AND THE M-I COUPLING PORTION OF THE HELIOPHYSICS DECADAL SURVEY GOAL #2. WE WILL PRIMARILY USE IN-SITU OBSERVATIONS IN THE PLASMA SHEET FROM TWO NASA-SUPPORTED MISSIONS (THEMIS AND GEOTAIL SATELLITES) TO DETERMINE PROPERTIES OF PRE-ONSET AND ONSET WAVES, AND RESULTS FROM OBSERVATIONS WILL BE COMPARED TO KINETIC SIMULATIONS. THUS, THE PROPOSED DATA-MODEL COMPARISON STUDY MEETS ALL THREE ASPECTS OF A HIGH-PRIORITY INVESTIGATION FOR THIS H-SR PROGRAM. Funding Only Action $130.1k 5/19/16 Not listed UNDERSTANDING THE ROLE OF KINETIC BALLOONING INSTABILITY IN TRIGGERING SUBSTORM ONSET 1. BACKGROUND AND SIGNIFICANCE SUBSTORMS ARE ONE OF THE MOST DRAMATIC DISTURBANCES OF THE GLOBAL MAGNETOTAIL-IONOSPHERE COUPLING SYSTEM THAT RELEASE LARGE AMOUNTS OF SOLAR WIND ENERGY ACCUMULATED IN THE MAGNETOTAIL AND ARE ASSOCIATED WITH AURORAL ACTIVATIONS. A CRITICAL, LONG-STANDING PROBLEM IN SUBSTORM RESEARCH IS TO IDENTIFY WHAT PHYSICAL MECHANISM LEADS TO SUBSTORM AURORAL ONSET. RECENT SATELLITE AND GROUND-BASED AURORAL OBSERVATIONS HAVE REVEALED THAT SUBSTORM AURORAL ONSET OFTEN INVOLVES WAVY STRUCTURES (CALLED 'BEADS') ALONG A PRE-EXISTING GROWTH PHASE ARC, AND THAT PRE-ONSET WAVES WITH ~1-2 MIN PERIODICITY OCCUR IN THE CONJUGATE NEAR-EARTH PLASMA SHEET. THE PRE-ONSET WAVES HAVE BEEN SUGGESTED TO BE AN IMPORTANT FACET OF THE INSTABILITY (SUCH AS THE BALLOONING MODE) IN THE NEAR-EARTH REGION THAT LEADS TO SUBSTORM ONSET. IT IS THUS CRITICAL TO DETERMINE IF, AND IF SO HOW, PRE-ONSET WAVES IN THE NEAR-EARTH PLASMA SHEET EVOLVE TO SUBSTORM ONSET WAVES. IF COUPLING BETWEEN THEM IS COMMON, PROPERTIES OF THOSE WAVES SHOULD PROVIDE IMPORTANT CLUES FOR UNDERSTANDING THE SUBSTORM ONSET INSTABILITY. 2. SCIENCE GOALS AND OBJECTIVES THE OVERARCHING GOAL OF THIS PROPOSED STUDY IS TO DETERMINE THE ROLE OF KINETIC BALLOONING INSTABILITY IN TRIGGERING SUBSTORM ONSET. THREE SPECIFIC SCIENCE TASKS ARE PROPOSED: (1) DETERMINE PROPERTIES OF PRE-ONSET WAVES AND THEIR CONNECTION TO ONSET WAVES (2) DETERMINE PROPERTIES OF ONSET WAVES (3) DETERMINE THE ROLE OF KINETIC BALLOONING INSTABILITY FOR DRIVING PRE-ONSET AND ONSET WAVES THE PROPOSED STUDY IS NOW POSSIBLE DUE TO THE NASA-SUPPORTED THEMIS AND GEOTAIL SATELLITES, WHICH HAVE COMPLETED MANY MAGNETOTAIL SURVEY SEASONS, IN COMBINATION WITH RECENT ADVANCE OF 3-D FULLY ELECTROMAGNETIC PARTICLE-IN-CELL SIMULATIONS THAT CAN SIMULATE PARTICLE KINETICS AND FIELDS SELF-CONSISTENTLY. 3. METHODOLOGY WE WILL USE NASA-SUPPORTED THEMIS AND GEOTAIL SATELLITE DATA TO EXAMINE THE SPATIAL (RADIAL DISTANCE, BETA, AND MLT) DEPENDENCE OF THE WAVE OCCURRENCE PRIOR TO AND DURING SUBSTORM ONSET. PHASE RELATIONS AMONG THE MAGNETIC FIELD, ELECTRIC FIELD AND PLASMA PRESSURE WILL BE DETERMINED TO CHARACTERIZE THOSE WAVES. SUBSTORM ONSET IS IDENTIFIED AS INITIAL AURORAL BRIGHTENING ALONG A QUIET ARC FOLLOWED BY POLEWARD EXPANSION. IMAGERS WILL ALSO BE USED TO EXAMINE WHETHER THOSE WAVES EVOLVE TO ONSET BEADS (CAUSALLY RELATED) OR ONSET BEADS EMERGE AT DIFFERENT LOCATIONS OR WAVELENGTHS (NOT CAUSALLY RELATED). THE SPATIAL DEPENDENCE AND PHASE RELATIONS OF PRE-ONSET AND ONSET WAVES OBTAINED FROM THE SATELLITE OBSERVATIONS WILL BE COMPARED TO THOSE FROM 3-D FULL KINETIC SIMULATIONS FOR IDENTIFYING THE ROLE OF KINETIC INSTABILITY INCLUDING BALLOONING FOR DRIVING PRE-ONSET AND ONSET WAVES. SCIENCE CLOSURE WILL BE ACHIEVED WITH THIS DATA- MODEL COMPARISON BY DETERMINING THE ROLE OF KINETIC INSTABILITY IN TRIGGERING THE WAVES SEEN BY SPACECRAFT AND VIA AURORAL BEADING THAT APPEAR TO BE A SIGNATURE OF THE MAGNETOSPHERIC INSTABILITY DRIVING SUBSTORM ONSET. 4. RELEVANCE TO NASA H-SR PROGRAM THIS PROPOSED STUDY AIMS TO REVEAL FUNDAMENTAL PHYSICAL PROCESSES OF SUBSTORM ONSET, WHICH IS A CRITICAL ISSUE IN MAGNETOSPHERE- IONOSPHERE COUPLING PHYSICS AND MAGNETOTAIL PLASMA PHYSICS. THUS, THIS STUDY OF SUBSTORM ONSET DIRECTLY ADDRESSES HELIOPHYSICS DECADAL SURVEY GOAL #4 AND THE M-I COUPLING PORTION OF THE HELIOPHYSICS DECADAL SURVEY GOAL #2. WE WILL PRIMARILY USE IN-SITU OBSERVATIONS IN THE PLASMA SHEET FROM TWO NASA-SUPPORTED MISSIONS (THEMIS AND GEOTAIL SATELLITES) TO DETERMINE PROPERTIES OF PRE-ONSET AND ONSET WAVES, AND RESULTS FROM OBSERVATIONS WILL BE COMPARED TO KINETIC SIMULATIONS. THUS, THE PROPOSED DATA-MODEL COMPARISON STUDY MEETS ALL THREE ASPECTS OF A HIGH-PRIORITY INVESTIGATION FOR THIS H-SR PROGRAM. Not listed $127.3k 5/4/15