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

Contarino

Fárník

et al. 2011

A&A

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Context. During complex and long duration solar flares, several filament destabilizations or eruptions can occur that are often related to coronal mass ejections (CMEs). Aims. We describe the study of an X3.8 long duration event (LDE) that occurred in NOAA 10720 on 17 January 2005 and was characterized by three filament destabilizations and two CMEs. Methods. Using multi-wavelength data provided by both ground-based instruments and satellites, in addition to MDI magnetograms, we investigated the morphological and magnetic evolution of the active region before and during the LDE. Results. Our analysis of H α and 1600 Å images showed that initially a two-ribbon structure developed in the central part of the active region, where a filament was previously observed. At a later time, two bright ribbons (in the most eastern side) and a strong brightness increase (at the western outskirt of the active region) were simultaneously observed. In a subsequent time interval, a new pair of ribbons was observed in the western side of the active region. Moreover, a linear force-free field extrapolation helped identify a null point in the central part of the active region. Conclusions. The initial filament destabilization that occurred in the central part of NOAA 10720 was probably due to magnetic flux emergence and photospheric shearing motions, which caused a slow tether-cutting process beneath the filament. The rearrangement of the magnetic field configuration, occurring in the same area as the location of the null point, changed the magnetic field connectivity in the active region, triggering two filament eruptions in the eastern and western part of the active region and two halo CMEs, in a kind of domino effect.

Section: Magnetic Field Evolutionsupporting

confidence: 91%

“…We note that the change in the magnetic helicity flux observed after the flare, was probably due to the restoring of the torque balance between the coronal field and the subphotospheric portion of the flux tubes, once the two CMEs were ejected (see Smyrli et al 2010).…”

Section: Discussionmentioning

confidence: 94%

Filament destabilization and CME release during a long duration flare

Contarino

Fárník

et al. 2011

A&A

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“…Owing to the difficulties encountered when directly measuring the magnetic helicity in the solar atmosphere, usually the authors measure the magnetic helicity transported across the photosphere by the emergence of new magnetic flux or by the shuffling horizontal motion of field lines (see Démoulin 2007, for a review). Several authors have tried to find a temporal correlation between the magnetic helicity variations and eruptive events (Moon et al 2002;Nindos et al 2003;Romano et al 2005;Zhang et al 2008;Smyrli et al 2010). In this context, we recall Hartkorn & Wang (2004) pointing out that the impulsive input of helicity observed during flares might be only an artificial effect of the impact of particle beams, which modifies the spectral line used by the magnetographs.…”

Section: Introductionmentioning

confidence: 93%

A solar eruption triggered by the interaction between two magnetic flux systems with opposite magnetic helicity

Romano

Pariat

Sicari

et al. 2010

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Context. In recent years the accumulation of magnetic helicity via emergence of new magnetic flux and/or shearing photospheric motions has been considered to play an important role in the destabilization processes that lead to eruptive phenomena occurring in the solar atmosphere. Aims. In this paper we want to highlight a specific aspect of magnetic helicity accumulation, providing new observational evidence of the role played by the interaction of magnetic fields characterized by opposite magnetic helicity signs in triggering solar eruption. Methods. We used 171 Å TRACE data to describe a filament eruption on 2001 Nov. 1 in active region NOAA 9682 and MDI full disk line-of-sight magnetograms to measure the accumulation of magnetic helicity in corona before the event. We used the local correlation tracking (LCT) and the differential affine velocity estimator (DAVE) techniques to determine the horizontal velocities and two methods for estimating the magnetic helicity flux. Results. The chirality signatures of the filament involved in the eruption were ambiguous, and the overlying arcade visible during the main phase of the event was characterized by a mixing of helicity signs. However, the measures of the magnetic helicity flux allowed us to deduce that the magnetic helicity was positive in the whole active region where the event took place, while it was negative near the magnetic inversion line where the filament footpoints were located. Conclusions. These results suggest that the filament eruption may be caused by magnetic reconnection between two magnetic field systems characterized by opposite signs of magnetic helicity. We also find that only the DAVE method allowed us to obtain the crucial information on the horizontal velocity field near the magnetic inversion line.

“…Recently, many authors have studied the magnetic helicity variations in active regions where solar eruptions took place (Moon et al 2002;Nindos et al 2003;Romano et al 2005;Zhang et al 2009;Smyrli et al 2010). Indeed, the magnetic helicity provides a measure of the global complexity of the magnetic field in an active region, i.e., a measure of the writhe and twist of the magnetic flux tubes, whose interaction favors processes of magnetic reconnection and the consequent release of free magnetic energy.…”

Section: Introductionmentioning

confidence: 99%

“…To understand the above mentioned relationship between the solar eruptions and the magnetic helicity variations, Smyrli et al (2010) applied this method to MDI/SOHO line-of-sight magnetogram data to calculate the magnetic helicity trend in 10 active regions that gave rise to halo coronal mass ejections (CMEs). The results obtained from the sample of analyzed events indicate that the changes in magnetic helicity flux following the CMEs could be attributed to a process of restoring a torque balance between the subphotospheric and the coronal domain of the flux tubes.…”

Section: Introductionmentioning

confidence: 99%

Magnetic helicity balance during a filament eruption that occurred in active region NOAA 9682

Romano

et al. 2011

A&A

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Context. Photospheric shear plasma flows in active regions may be responsible for the magnetic helicity injection in the solar corona not only during the energy storage process before a solar eruption, but also during and after the release of the free magnetic energy caused by the eruption. Indeed, after a filament eruption or expansion the magnetic torque imbalance can induce shear flows that can be responsible for yet another injection of magnetic helicity into the corona. Aims. We investigated the magnetic helicity balance in an active region where a confined solar eruption occurred. This was done to verify a possible relationship between the filament expansion and the helicity transport at its footpoints. We aimed to verify if this variation in the helicity transport rate could be interpreted as a consequence of the magnetic torque imbalance caused by the tube expansion, as proposed by Chae et al. (2003, J. Kor. Astron. Soc., 36, 33). Methods. We used 171 Å TRACE data to measure some geometrical parameters of the new magnetic system produced by a filament eruption that occurred on 2001 November 1 in active region NOAA 9682. We used MDI full disk line-of-sight magnetogram data to measure the accumulation of magnetic helicity in the corona before and after the event.Results. From the measured expansion factor in the magnetic arcade, visible at 171 Å during the eruption, we estimated that the resulting torque imbalance at the photosphere ought to lead to the injection of negative helicity following the eruption. We compared this with measurements of the helicity injection using photospheric velocity and magnetogram data. Conclusions. In contradiction to the expectations from the Chae et al. model, the helicity injection after the eruption was positive. We offer the alternative interpretation that the helicity injection resulted from torque of the opposite sign, generated as the filament lost its negative helicity through magnetic reconnection with its surroundings.