SCIENCE GOALS AND OBJECTIVES: THE OBJECTIVE OF THE PROPOSAL IS TO STUDY THE DETAILED PHYSICS AND GLOBAL CONSEQUENCES OF MAGNETIC RECONNECTION IN THE MAGNETOTAIL REGIONS OF MARS AND VENUS USING A GLOBAL MHD WITH EMBEDDED PARTICLE IN CELL (MHD-EPIC) CODE. MAGNETIC RECONNECTION IS A FUNDAMENTAL PHYSICAL PROCESS IN SPACE PLASMAS THAT CHANGES THE MAGNETIC TOPOLOGY AND CONVERTS MAGNETIC ENERGY INTO BOTH KINETIC ENERGY AND THERMAL ENERGY. MAGNETIC RECONNECTION HAS BEEN DETECTED IN THE PLASMA TAIL REGION AT BOTH VENUS AND MARS [ZHANG ET AL. 2012 HARADA ET AL. 2015] AND IT IS EXPECTED TO PLAY A CRUCIAL ROLE IN CONTROLLING THE PARTICLE ACCELERATION AND ATMOSPHERIC LOSS PROCESSES AT UNMAGNETIZED PLANETS [KRYMSKII ET AL. 2002; BRAIN 2006; BRAIN ET AL. 2010; HALEKAS ET AL. 2011]. HERE WE USE A COUPLED APPROACH TO NUMERICALLY STUDY THE DETAILED PHYSICAL PROCESS AND THE GLOBAL CONSEQUENCES OF THE MAGNETIC RECONNECTION IN THE MAGNETOTAIL REGION. SUCH A COUPLED APPROACH SOLVES THE SOLAR WIND-PLANET PLASMA INTERACTION USING A GLOBAL MHD MODEL THAT IS FULLY COUPLED WITH AN EMBEDDED KINETIC MODEL (PARTICLE-IN-CELL). THE MAIN SCIENCE QUESTIONS TO BE ADDRESSED BY THE PROPOSAL ARE: 1. HOW IMPORTANT IS THE MAGNETIC RECONNECTION PROCESS IN CONTROLLING THE DYNAMICS OF THE PLASMA TAIL AT MARS UNDER TYPICAL SOLAR WIND CONDITIONS? 2. HOW IMPORTANT IS THE MAGNETIC RECONNECTION PROCESS IN CONTROLLING THE DYNAMICS OF THE PLASMA TAIL AT VENUS UNDER TYPICAL SOLAR WIND CONDITIONS? 3. HOW WILL THE CRUSTAL MAGNETIC FIELD AFFECT THE TAIL RECONNECTION PROCESS AT MARS? 4. HOW DIFFERENT IS THE MAGNETIC RECONNECTION PROCESS IN DRIVING THE ION LOSS FROM VENUS AND MARS? METHODOLOGY AN INNOVATIVE TWO-WAY COUPLED APPROACH IS TO BE USED IN THE STUDY WHICH COUPLES THE GLOBAL MHD MODELS OF MARS AND VENUS WITH AN EMBEDDED PIC CODE. SUCH A MODELING CAPABILITY HAS BEEN DEVELOPED RECENTLY IN THE SPACE WEATHER MODELING FRAMEWORK AT UNIVERSITY OF MICHIGAN [DALDORFF ET AL. 2014] TO COUPLE THE BATSRUS MHD MODEL WITH THE LOCAL PARTICLE-IN-CELL (IPIC3D) MODEL [MARKIDIS ET AL. 2010]. IT HAS BEEN SUCCESSFULLY APPLIED TO GANYMEDE [TOTH ET AL. 2016] EARTH [CHEN ET AL. 2017] AND MARS [MA ET AL. 2018 UNDER REVIEW] AND WILL BE ADAPTED TO VENUS. THIS APPROACH TAKES ADVANTAGES OF BOTH MODELS: THE EFFICIENCY OF THE GLOBAL MHD MODEL AND THE ABILITY TO INCLUDE KINETIC EFFECTS BY THE PIC MODEL AND MAKE IT FEASIBLE TO PERFORM KINETIC SIMULATION OF MAGNETIC RECONNECTION UNDER REALISTIC SOLAR WIND CONDITIONS. RELEVANCE: THE GOALS OF THIS PROPOSED WORK ARE DIRECTLY RELEVANT TO THE OBJECTIVES OF THE SOLAR SYSTEM WORKINGS (SSW) PROGRAM AS THE PROPOSED STUDY WILL USE AN INNOVATIVE APPROACH TO UNDERSTAND THE MAGNETIC RECONNECTION (A FUNDAMENTAL PHYSICAL) PROCESS AT BOTH MARS AND VENUS. IN PARTICULAR THE PROPOSAL ADDRESSES THE ROLE OF A FUNDAMENTAL PLASMA PROCESS UNDER "PLASMA ENVIRONMENTS" AS LISTED IN THE SCOPE OF THE PROGRAM.