SCIENCE GOALS AND OBJECTIVES: FORESHOCK TRANSIENTS ASSOCIATED WITH AN IMF DIRECTIONAL DISCONTINUITY SUCH AS HOT FLOW ANOMALIES ARE GENERATED BY THE KINETIC INTERACTION OF THE DISCONTINUITY WITH THE QUASI-PARALLEL BOW SHOCK. THE STRONG DENSITY/DYNAMIC PRESSURE PERTURBATIONS WITHIN THE FORESHOCK TRANSIENTS RESULT IN SHARP MAGNETOPAUSE DISTORTION THUS GENERATING DISTURBANCES WITHIN THE MAGNETOSPHERE. OUR UNDERSTANDING OF SUCH GEOEFFECTIVE IMPACT ON THE DAYSIDE HAS GREATLY ADVANCED IN THE PAST FEW YEARS BUT LITTLE IS KNOWN ON HOW THEY IMPACT THE NIGHTSIDE GEOSPACE. OUR PRELIMINARY OBSERVATIONAL STUDY HAS SHOWN THESE FORESHOCK TRANSIENTS CAN PROPAGATE TO THE MIDTAIL MAGNETOSHEATH DISTORT THE TAIL MAGNETOPAUSE AND CAUSE DISTURBANCES ON THE NIGHTSIDE MAGNETOSPHERE AND IONOSPHERE. THEREFORE OUR GOAL IS TO CONDUCT OBSERVATIONAL EVENT STUDY AND SIMULATIONS TO ADDRESS THREE QUESTIONS KEY TO UNDERSTANDING THE NIGHTSIDE IMPACT: (1) HOW DO FORESHORE TRANSIENTS AND THEIR SPATIAL STRUCTURES EVOLVE AS THEY PROPAGATE FROM THE FORESHOCK TO THE TAIL MAGNETOSHEATH? (2) WHAT IS THE 3D STRUCTURE OF THE MAGNETOPAUSE DISTORTION CAUSED BY FORESHOCK TRANSIENTS? (3) WHAT IS THE SPATIAL EXTENT OF THE IMPACT ON THE NIGHTSIDE GEOSPACE? METHODOLOGY: OBSERVATION DATA: WE WILL USE SATELLITE DATA FROM ARTEMIS CLUSTER GEOTAIL MMS WIND AND ACE FROM 2010 TO 2020 TO INVESTIGATE THE TAILWARD PROPAGATION OF FORESHOCK TRANSIENTS. THE IMPACT WITHIN THE MAGNETOSPHERE WILL BE INVESTIGATED BY USING THEMIS AND MMS AND GROUND MAGNETOMETERS AND GEOMAGNETIC INDICES FROM SUPERMAG. SIMULATION: THE KINETIC INTERACTION OF AN IMF DISCONTINUITY AND THE BOW SHOCK CAN BE APPROPRIATELY SIMULATED BY A HYBRID CODE BUT NOT BY AN MHD CODE. THEREFORE WE WILL USE A 3D GLOBAL HYBRID CODE (ANGIE3D) DEVELOPED AT AUBURN UNIVERSITY. THIS IS THE ONLY GLOBAL HYBRID CODE THAT IS CAPABLE OF SIMULATING THE EARTH S NIGHTSIDE MAGNETOSPHERE. IT HAS BEEN USED TO STUDY THE DYNAMICS OF THE EARTH S MAGNETOTAIL. OBSERVATION TASKS: (1)WE WILL CONDUCT EVENT STUDIES OF FORESHOCK TRANSIENTS WITH SIMULTANEOUSLY MULTIPLE-SPACECRAFT OBSERVATIONS IN THE SOLAR WIND THE FORESHOCK AND THE TAIL MAGNETOSHEATH. WE WILL ANALYZE THE TAILWARD PROPAGATION AND EVOLUTION AND IDENTIFY FACTORS AFFECTING THESE PROCESSES. (2) FOR THESE EVENTS WE WILL CONSTRUCT THE SPATIAL STRUCTURES/SCALES OF FORESHOCK TRANSIENTS. (3) WE WILL COMPARE OUR OBSERVATION RESULTS WITH THE PROPOSED SIMULATION RESULTS TO EVALUATE OUR UNDERSTANDING OF THE PHYSICAL PROCESSES BYHIND THE EVOLUTION OF THE FORESHOCK TRANSIENTS AND IMPACTS. (2)SIMULATION TASKS: (1) WE WILL CONDUCT SEVERAL RUNS WITH DIFFERENT IMF DIRECTIONAL DISCONTINUITIES (FOR EXAMPLE DIFFERENT ORIENTATIONS FOR THE DISCONTINUITIES) EMBEDDED WITHIN SIMPLE AND IDEALIZED SOLAR WIND/IMF CONDITIONS TO INVESTIGATE GENERAL FEATURES OF FORESHOCK TRANSIENTS THEIR PROPAGATION AND EVOLUTION WITHIN THE TAIL MAGNETOSHEATH AND IMPACT ON THE MAGNETOPAUSE MAGNETOSPHERE AND IONOSPHERE. (2) WE WILL SELECT 4 GOOD EVENTS FROM OUR OBSERVATIONAL STUDIES AND CONDUCT EVENT SIMULATIONS WITH THE OBSERVED SOLAR WIND/IMF CONDITIONS. WE WILL USE THESE SIMULATION RESULTS TO GUIDE OUR INVESTIGATION OF THE OBSERVATIONS TO ADDRESS OUR SCIENCE QUESTIONS. RELEVANCE: FORESHOCK TRANSIENTS RESULTING FROM THE INTERACTION OF IMF DISCONTINUITIES WITH THE BOW SHOCK IS ONE OF THE FUNDAMENTAL PROCESSES IN HELIOSPHERE. OUR STUDY OF THEIR TAILWARD PROPAGATION AND IMPACT ON THE EARTH S NIGHTSIDE MAGNETOSPHERE DIRECTLY ADDRESSES TWO SCIENCE GOALS FROM HELIOPHYSICS DECADAL SURVEY DISCOVER AND CHARACTERIZE FUNDAMENTAL PROCESSES THAT OCCUR BOTH WITHIN THE HELIOSPHERE AND THROUGHOUT THE UNIVERSE AND DETERMINE THE DYNAMICS AND COUPLING OF EARTH S MAGNETOSPHERE IONOSPHERE AND ATMOSPHERE AND THEIR RESPONSE TO SOLAR AND TERRESTRIAL INPUTS.