The magnetic field in the solar photosphere frequently carries strong electric currents, even though the global coronal configuration often resembles a potential field ringed by the heliospheric current sheet. To understand this, we compare TRACE EUV images of active-region coronae and potential-field source-surface extrapolations based on SOHO MDI magnetograms for 95 active regions. We conclude that significant nonpotentiality of the overall active-region coronal field occurs (1) when new flux has emerged within or very near a region within the last ~30 hr, resulting in complex polarity separation lines, or (2) when rapidly evolving, opposite-polarity concentrations are in contact at 4'' resolution. If these criteria are met by more than 15% of the region's flux, they correctly identify the (non) potentiality of active-region coronae in 88% of the cases. Flares are found to occur 2.4 times more frequently in active regions with nonpotential coronae than in near-potential regions, while their average X-ray peak flare brightness is 3.3 times higher. We suggest that the currents associated with coronal nonpotentiality have a characteristic growth and decay timescale of ~10-30 hr. We find that shear flows drive enhanced flaring or coronal nonpotentiality only if associated with complex and dynamic flux emergence within the above timescale. We discuss the implications of this finding for the modeling of the coronal-heliospheric coupling.
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