UNDER GLOBAL MAGNETOSPHERIC CONVECTION THE MAGNETOTAIL CURRENT SHEET THINS PROGRESSIVELY TO THE ION KINETIC SCALE. MAGNETIC RECONNECTION (WHICH IS RELATED TO THE GENERATION OF E.G. FLUX ROPES DIPOLARIZING FLUX BUNDLES FLOW BURSTS AND PARTICLE INJECTIONS) IS TRIGGERED BY PLASMA INSTABILITIES IN A THIN CURRENT SHEET (TCS) AND HAS GLOBAL CONSEQUENCES. OBSERVATIONS DURING THE PAST TWENTY YEARS HAVE SHOWN THAT MAGNETOTAIL RECONNECTION AND RELATED PHENOMENA EXHIBIT A PERVASIVE DUSKSIDE PREFERENCE. THIS PREFERENCE IS ALSO NATURALLY MANIFESTED IN OTHER GEOSPACE PHENOMENA (E.G. SUBSTORM EXPANSION). OUR RECENT STUDY SUGGESTS THAT THIS DAWNDUSK ASYMMETRY COULD BE DUE TO AN ASYMMETRIC EVOLUTION OF PLASMA PARAMETERS AND PARTICLE DYNAMICS DURING THE CURRENT SHEET THINNING PROCESS. TO BETTER UNDERSTAND ION AND ELECTRON KINETICS AND THE RELATED EVOLUTION OF PLASMA PROPERTIES IN TCSS WE WILL COMPARE OBSERVATIONS WITH A THREE-DIMENSIONAL (3-D) PARTICLE-IN-CELL (PIC) MODEL. WE WILL USE LARGE DATABASES OF TCS OBSERVATIONS BY MULTI-SPACECRAFT THEMIS AND CLUSTER MISSIONS IN THE NEAR-EARTH MAGNETOTAIL TO EVALUATE THE PROPOSED TCS FORMATION/THINNING SCENARIOS AND TEST OUR THEORETICAL UNDERSTANDING AGAINST SUCH MODELS. OUR OBSERVATIONS WILL PROVIDE AN EXTENSIVE SURVEY OF ASYMMETRIC MAGNETOTAIL TCSS MAGNETIC RECONNECTION AND RELATED PHENOMENA WHICH WILL GUIDE OUR SIMULATIONS AND OUR PHYSICAL INTUITION. SCIENCE GOALS AND OBJECTIVES: BY COMPREHENSIVE USE OF MODERN MULTI-SPACECRAFT DATASETS AND NUMERICAL SIMULATIONS WE WILL: (1) EXPLORE THE ASYMMETRIC NATURE OF THE CURRENT SHEET THINNING INCLUDING EFFECTS OF ELECTRON PRESSURE ANISOTROPY AND CURRENT DENSITY BIFURCATION AT THE DAWN AND DUSK MAGNETOTAIL. (2) DETERMINE THE LOCAL PHYSICS RESULTING IN THE DAWN-DUSK ASYMMETRY OF THE TCS. FOLLOWING OUR EARLY WORK WE WILL PARTICULARLY EXAMINE THE ROLE OF THE HALL ELECTRIC FIELD AND THE CROSS-FIELD DRIFT IN CURRENT DENSITY REDISTRIBUTION AT THE DAWN AND DUSK FLANKS. (3) ESTABLISH WHETHER THE ASYMMETRIC PROPERTIES OF MAGNETOTAIL RECONNECTION (E.G. RECONNECTION RATE ENERGY CONVERSION EFFICIENCY MAGNITUDE OF HALL FIELDS) ORIGINATED FROM THE ASYMMETRIC TCS PROPERTIES. RELEVANCE TO ONE OR MORE OF THE FOUR DECADAL SURVEY GOALS: THIS PROPOSED RESEARCH CONTRIBUTES TO THE 2ND AND 4TH OF THE FOUR HIGH LEVEL SCIENCE GOALS FROM THE HELIOPHYSICS DECADAL SURVEY. METHODOLOGY: (1) WE WILL USE SPACECRAFT OBSERVATIONS FOR EXPLORING THE CURRENT SHEET CONFIGURATION AND FOR MOTIVATING OUR SIMULATIONS AND COMPARING WITH MODEL RESULTS. THREE DATABASES OF CURRENT SHEET CROSSINGS IN THE NEAR-EARTH MAGNETOTAIL (<30RE DOWNTAIL) WILL BE USED: (A) ~300 TCS CROSSINGS BY THE FIVE THEMIS SPACECRAFT IN 2008-2009; (B) ~20 EVENTS WHEN THREE THEMIS SPACECRAFT SIMULTANEOUSLY OBSERVED CURRENT SHEET THINNING DURING GROWTH PHASE; (C) ~100 TCS CROSSINGS BY FOUR CLUSTER SPACECRAFT IN 2001-2004. WE WILL PERFORM STATISTICAL MULTI-CASE AND CASE STUDIES OF THESE DATASETS TO MOTIVATE MODEL RUNS AND TO COMPARE SIMULATION RESULTS AND THEORETICAL UNDERSTANDING WITH OBSERVATIONAL TRENDS. (2) WE WILL USE A 3-D PIC MODEL TO SIMULATE THE MAGNETOTAIL TCS AND ITS DAWN-DUSK ASYMMETRY. THE TIME ADVANCEMENT OF THE FIELDS EMPLOYS AN EXPLICIT LEAPFROG SCHEME BY SOLVING MAXWELL S EQUATIONS. THE PARTICLES ARE ADVANCED RELATIVISTICALLY IN THE ELECTRIC AND MAGNETIC FIELDS. THE SIMULATIONS EMPLOY A LEMB GE-PELLAT CURRENT SHEET AS THE INITIAL CONFIGURATION. TO SIMULATE THE DRIVEN NATURE OF THE MAGNETOTAIL TCS FORMATION/THINNING AND RECONNECTION A PLASMA INFLOW THROUGH Z BOUNDARIES WILL BE USED AS THE EXTERNAL DRIVING. THE INITIAL CONFIGURATION AND EXTERNAL DRIVING IS DAWN-DUSK SYMMETRIC AND THE ASYMMETRY IS EXPECTED TO DEVELOP FROM THE SYMMETRIC CONDITIONS. SUFFICIENT COMPUTATIONAL RESOURCES EXIST TO SIMULATE A WIDE SECTION OF THE MAGNETOTAIL (10RE) TO CAPTURE THE ASYMMETRY.