Trend of photospheric magnetic helicity flux in active regions generating halo coronal mass ejections^*

¹ Dipartimento di Fisica e Astronomia – Sezione Astrofisica, Università di Catania, via S. Sofia 78, 95123 Catania, Italy e-mail: emilia@oact.inaf.it
² Institute of Mathematics, University of St. Andrews, The North Haugh, St. Andrews, Fife KY 169SS, Scotland, UK
³ INAF – Osservatorio Astrofisico di Catania, via S. Sofia 78, 95123 Catania, Italy
⁴ Centre for Plasma Astrophysics, K. U. Leuven, Celestijnenlaan 200B, 3001 Leuven, Belgium

Received: 10 September 2009
Accepted: 22 April 2010

Abstract

Context. Coronal mass ejections (CMEs) are very energetic events (~ 10³² erg) initiated in the solar atmosphere, resulting in the expulsion of magnetized plasma clouds that propagate into interplanetary space. It has been proposed that CMEs can play an important role in shedding magnetic helicity, avoiding its endless accumulation in the corona.

Aims. The aim of this work is to investigate the behavior of magnetic helicity accumulation in sites where the initiation of CMEs occurred to determine whether and how changes in magnetic helicity accumulation are temporally correlated with CME occurrence.

Methods. We used MDI/SOHO line-of-sight magnetograms to calculate magnetic flux evolution and magnetic helicity injection in 10 active regions that gave rise to halo CMEs observed during the period 2000 February to 2003 June.

Results. The magnetic helicity injection does not have a unique trend in the events analyzed: in 40% of the cases it shows a large sudden and abrupt change that is temporally correlated with a CME occurrence, while in the other cases it shows a steady monotonic trend, with a slight change in magnetic helicity at CME occurrence.

Conclusions. The results obtained from the sample of events that we have analyzed indicate that major changes in magnetic helicity flux are observed in active regions characterized by emergence of new magnetic flux and/or generating halo CMEs associated with X-class flares or filament eruptions. In some of the analyzed cases the changes in magnetic helicity flux follow the CME events and can be attributed to a process of restoring a torque balance between the subphotospheric and the coronal domain of the flux tubes.