Emergence of a U-loop - sub-photospheric link between solar active regions

Using SOHO/MDI magnetic maps we present the first direct observational evid ence for the emergence of a U-loop in the solar photosphere. We show that t wo active regions (ARs), i.e, two adjacent Omega -loops, which were emergin g at the same time at the same solar latitude, about 150000 km distance in longitude from each other, emerged from at least partially the same toroida l flux strand, and we bring five independent arguments to prove this assert ion. The opposite polarity legs of the two Omega -loops were connected belo w the-photosphere by a U-shaped loop. Following the emergence of the Omega -loops, the U-loop started emerging, manifested by the fast proper motion o f the leading spots of the eastern (smaller) active region, which, after fo rming an elongated channel, collided with the following spots of the wester ly AR and started cancelling with them. The full cancellation could not be followed because the ARs rotated out of sight. The total magnetic flux of t he two ARs was unequal, the flux in the smaller AR was a quarter of that of the larger one. We propose scenarios for the formation of such a U-loop an d discuss the implications of the confirmed existence of U-loops for the so lution of such puzzles as the in-situ disappearance of magnetic flux from a ctive regions, active nests and the formation of inter-AR filaments.