Photospheric magnetic evolution of super active regions^*

¹ INAF – Osservatorio Astrofisico di Catania, via S. Sofia 78, 95123 Catania, Italy e-mail: prom@oact.inaf.it
² Dipartimento di Fisica e Astronomia – Sezione Astrofisica, Università di Catania, via S. Sofia 78, 95123 Catania, Italy

Received: 18 June 2007
Accepted: 11 August 2007

Abstract

Context.Solar flares are one of the most interesting phenomena of solar activity. Although many authors agree that their energy release is related to magnetic reconnection, the phases preceding the magnetic reconnection process are still unclear in several respects.

Aims.We studied the magnetic flux evolution in 26 super active regions observed on the solar disc from Jan. 1, 2000 to Dec. 31, 2006, in order to determine a physical relationship between flares and some properties of the photospheric magnetic fields.

Methods.This analysis is based on full disc line of sight MDI magnetograms. We measured the total magnetic flux, the magnetic flux imbalance, the horizontal magnetic gradient and the number of magnetic features characterized by a collection of pixels lying in the same bins of absolute strength. We also elaborated an algorithm for the identification and the length measurement of the magnetic inversion lines.

Results.We found a correlation between the evolution of the analyzed parameters and the flare occurrence. The most intense phases of activity are associated with phases of emergence of a magnetic field in 15 super active regions and with phases of flux cancellation in 5 super active regions.

Conclusions.Super active regions may be characterized by a different behavior depending on their morphology. Some of them show a continuous flare activity, while others present an alternation of active and quiet phases. The deviations from magnetic polarity balance and the length of the inversion lines characterized by a horizontal magnetic gradient greater than 0.3 G km^-1 seem to have a major role in producing a magnetic field topology able to trigger flares.