1. OBJECTIVE AND SCIENCE GOALS THE OBJECTIVE OF THIS PROPOSAL IS TO INVESTIGATE THE ORIGIN OF MAJOR ACTIVITY IN SOLAR ACTIVE REGIONS (ARS). ARS THAT EXHIBIT COMPACT POLARITY INVERSION LINES (PILS) ARE KNOWN TO BE VERY FLARE-PRODUCTIVE. HOWEVER THE PHYSICAL MECHANISMS BEHIND THIS STATISTICAL INFERENCE HAVE NOT BEEN DEMONSTRATED CONCLUSIVELY. OUR PRELIMINARY WORK ON THE SUBJECT (CHINTZOGLOU ET AL 2018; UNDER REVIEW) HAS SHOWN THAT SUCH PILS CAN OCCUR DUE TO THE COLLISION BETWEEN TWO EMERGING FLUX TUBES WITHIN THE SAME AR (COLLISIONAL-PIL; CPIL). THE PROPOSED WORK WILL DETERMINE AND QUANTIFY THE DOMINANT MECHANISMS RESPONSIBLE FOR MAJOR ACTIVITY AT CPILS BY ANSWERING THE FOLLOWING MAJOR SCIENCE QUESTIONS: (Q1) WHICH ARE THE MECHANISMS RESPONSIBLE FOR MAJOR ERUPTIVE AND CONFINED FLARES IN SEVERAL GENERIC BUT REALISTIC MAGNETIC CONFIGURATIONS (USING SIMULATIONS INSPIRED FROM REAL OBSERVATIONS)? (Q2) WHICH MECHANISM(S) LEAD TO THE PRECURSOR PHASE OBSERVED BEFORE ERUPTING FLARES? (Q3) WHAT IS THE IMPACT OF THE 180 AMBIGUITY RESOLUTION IN RECOVERING MFR SIGNATURES IN SYNTHETIC MAGNETOGRAMS? 2. RELEVANCE THE PROPOSAL IS HIGHLY RELEVANT TO ONE HELIOPHYSICS DECADAL SURVEY SOLAR AND SPACE SCIENCE GOAL: DETERMINE THE ORIGINS OF THE SUN S ACTIVITY AND PREDICT THE VARIATIONS IN THE SPACE ENVIRONMENT. 3. METHODOLOGY REALISTIC MHD SIMULATIONS PLAY A CENTRAL ROLE IN INTERPRETING OBSERVATIONS AND UNDERSTATING PHYSICAL PROCESSES IN THE CORONA. IN THIS CONTEXT REALISM IS A COMPROMISE BETWEEN REALISTIC PHYSICS AND REALISTIC SETUPS. SIMULATIONS NEED TO INCLUDE A SUFFICIENT AMOUNT OF PHYSICS TO ALLOW FOR THE DIRECT COMPARISON WITH OBSERVATIONS THROUGH FORWARD MODELING OF SYNTHETIC OBSERVABLES. AT THE SAME TIME SIMULATIONS NEED TO CAPTURE REALISTIC SETUPS IN TERMS OF THE LENGTH AND TIME-SCALES OF SOLAR ACTIVE REGION EVOLUTION WHICH REQUIRES COMPROMISES IN THE PHYSICS THAT ARE CAPTURED. FOR THE SCIENCE QUESTIONS ADDRESSED IN THE PROPOSAL THE BEST COMPROMISE ALONG THESE LINES IS GIVEN BY RADIATIVE MHD SIMULATIONS WITH CORONAL PHYSICS IN TERMS OF OPTICALLY-THIN RADIATIVE LOSS AND MAGNETIC FIELD ALIGNED HEAT CONDUCTION. SUCH MODELS CAPTURE THE DETAILS OF PHOTOSPHERIC AND SUB-PHOTOSPHERIC FLUX EMERGENCE AND COUPLE THAT DYNAMICALLY TO THE EVOLUTION OF THE OVERLYING CORONA. A DIRECT COMPARISON WITH OBSERVATIONS IN TERMS OF PHOTOSPHERIC VECTOR MAGNETOGRAMS (SDO/ HMI) AND THERMAL EMISSION FROM THE CORONA (SDO/AIA) CAN BE ACCOMPLISHED WITHIN THIS FRAMEWORK. WE FOCUS ON DATA-INSPIRED SETUPS IN WHICH WE USE A COMBINATION OF (A) THE INITIAL STATE PRE-EXISTING FLUX (B) NEW FLUX EMERGENCE AND (C) FOOTPOINT MOTIONS OF SPOTS TO MIMIC TO OBSERVED BEHAVIOR FROM A LIST OF WELL-OBSERVED SUPER-ACTIVE ARS. THE MURAM CODE HAS A FLEXIBLE BOTTOM BOUNDARY CONDITION THAT ALLOWS TO SPECIFY THE LOCATION OF FLUX EMERGENCE TOGETHER WITH PROPERTIES SUCH AS STRENGTH EXTENT AND TWIST NECESSARY FOR THE FORMATION OF COLLISIONAL PILS. THE PRODUCTION OF REALISTIC SIMULATIONS OF ARS WILL PERMIT THE IDENTIFICATION OF THE ERUPTION MECHANISMS IN CPILS FROM DIFFERENT CONFIGURATIONS WILL ELUCIDATE THE PHYSICS BEHIND THE SO-CALLED PRECURSOR PHASE IN CPILS BY STUDYING THE EVOLUTION BEFORE ERUPTIONS AND WILL ALLOW THE DEVELOPMENT OF A NOVEL MACHINE-LEARNING APPROACH (TRAINED FROM ENSEMBLE DERIVED FROM THE SIMULATED CASES) TO CORRECT FOR THE 180 AMBIGUITY IN MAGNETOGRAM OBSERVATIONS