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Astron. Astrophys. 364, 845-852 (2000)

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

L. van Driel-Gesztelyi 1,2, J.-M. Malherbe 1 and P. Démoulin 1

1 Observatoire de Paris, DASOP, 92195 Meudon Cedex, France (Lidia.vanDriel@obspm.fr; Jean-Marie.Malherbe@obspm.fr; Pascal.Demoulin@obspm.fr)
2 Konkoly Observatory of the Hungarian Academy of Sciences, 1525 Budapest, Hungary

Received 4 July 2000 / Accepted 21 August 2000


Using SOHO/MDI magnetic maps we present the first direct observational evidence for the emergence of a U-loop in the solar photosphere. We show that two active regions (ARs), i.e. two adjacent [FORMULA]-loops, which were emerging 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 toroidal flux strand, and we bring five independent arguments to prove this assertion. The opposite polarity legs of the two [FORMULA]-loops were connected below the photosphere by a U-shaped loop. Following the emergence of the [FORMULA]-loops, the U-loop started emerging, manifested by the fast proper motion of the leading spots of the eastern (smaller) active region, which, after forming an elongated channel, collided with the following spots of the westerly 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 the 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 and discuss the implications of the confirmed existence of U-loops for the solution of such puzzles as the in-situ disappearance of magnetic flux from active regions, active nests and the formation of inter-AR filaments.

Key words: Sun: activity – Sun: magnetic fields

Send offprint requests to: Lidia van Driel-Gesztelyi

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© European Southern Observatory (ESO) 2000

Online publication: January 29, 2001