A New X-line Model: Comparison to MHD Magnetic Separator

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Abstract
Predicting the location of magnetopause reconnection remains a major challenge. Existing models often fail to predict the location of the reconnection line seen in global MHD simulations, particularly under northward IMF. This work presents a new X-line model that identifies a dominant reconnection line by maximizing the reconnection rate on both local and global scales. First, it determines the orientations at the magnetopause that locally maximize the rate and then finds the global path with the highest integrated rate. Across four global MHD simulations with diverse dipole tilts and IMF orientations, the new model was consistently more accurate than both the maximum magnetic shear and the global rate maximization models in predicting the location of the magnetic separator in between the magnetospheric cusps. Crucially, it succeeds for a challenging northward IMF case where previous models have failed. This model suggests the X-line's location is determined by a fundamental principle of maximizing the conversion of magnetic to plasma energy.