Photospheric flux cancellation and associated flux rope formation and eruption

Abstract

Aims. We study an evolving bipolar active region that exhibits flux cancellation at the internal polarity inversion line, the formation of a soft X-ray sigmoid along the inversion line and a coronal mass ejection. The aim is to investigate the quantity of flux cancellation that is involved in flux rope formation in the time period leading up to the eruption. Methods. The active region is studied using its extreme ultraviolet and soft X-ray emissions as it evolves from a sheared arcade to flux rope configuration. The evolution of the photospheric magnetic field is described and used to estimate how much flux is reconnected into the flux rope. Results. About one third of the active region flux cancels at the internal polarity inversion line in the 2.5 days leading up to the eruption. In this period, the coronal structure evolves from a weakly to a highly sheared arcade and then to a sigmoid that crosses the inversion line in the inverse direction. These properties suggest that a flux rope has formed prior to the eruption. The amount of cancellation implies that up to 60% of the active region flux could be in the body of the flux rope. We point out that only part of the cancellation contributes to the flux in the rope if the arcade is only weakly sheared, as in the first part of the evolution. This reduces the estimated flux in the rope to ∼30% or less of the active region flux. We suggest that the remaining discrepancy between our estimate and the limiting value of ∼10% of the active region flux, obtained previously by the flux rope insertion method, results from the incomplete coherence of the flux rope, due to nonuniform cancellation along the polarity inversion line. A hot linear feature is observed in the active region which rises as part of the eruption and then likely traces out the field lines close to the axis of the flux rope. The flux cancellation and changing magnetic connections at one end of this feature suggest that the flux rope reaches coherence by reconnection immediately before and early in the impulsive phase of the associated flare. The sigmoid is destroyed in the eruption but reforms quickly, with the amount of cancellation involved being much smaller than in the course of its original formation.

Evolution of Active Regions

“…The limiting axial flux given in Savcheva and van Ballegooijen (2009) for a fourth, strongly decayed and sigmoidal active region, compares with the amount of unsigned flux in the region to a limiting ratio of roughly 14%. This contrasts sharply with Green et al (2011), who showed, using an observational study, that a flux rope formed by flux cancellation may contain as much as 60% of the unsigned flux of the active region. Savcheva et al (2012) carried out a study to try and reconcile the above discrepancy between the observational value of flux rope flux content and that found for a modelled flux rope.…”

“…The results for four flux ropes for which it was possible to compare the rope's axial flux to the unsigned flux of the active region just before the eruption showed the ratios 18%, 16%, 58% and 50%. The region studied in Green et al (2011) was also modelled and there was an excellent agreement in the value found between these two approaches.…”

“…Flux cancellation within the AR can be as large is 10%/day of the total AR flux (Sterling et al, 2010;Baker et al, 2012;Green et al, 2011). The size of the ratio of in-AR cancellation to dispersing flux may be inversely related to the AR size (flux content) being the largest in ephemeral ARs.…”

“…Baker et al (2012) and Sterling et al (2010) studied different regions that both showed that 20% of the flux was lost over just two days and Green et al (2011) tracked an active region from emergence to decay and found that 34% of the active region flux was cancelled along the active region's internal polarity inversion within 3.5 days of the flux emergence ending.…”

Proper horizontal photospheric flows in a filament channel

“…The local correlation tracking (LCT) method is mainly applied to magnetic field polarities to explain eruptions or flares with MDI data (1.96 arcsec pixel size) and only recently with HMI data (0.5 arcsec pixel size). The aim of these analyses mainly concerns the onsets of the eruptions or flares (Ahn et al 2010;Zhou et al 2006;Green et al 2011;Liu et al 2012). The LCT method has also been applied to tracking granules.…”