Helicity Accumulation Research Articles

Global and local dynamics of X-flare-producing active regions during solar cycle 25 peak phase

Context. The configuration of the longitudinally elongated region that active regions (ARs) cluster around, known as a toroid belt, has been shown to be an indicator of intense activity. In particular, complex ARs at locations in the north and/or south toroids tend to appear “tipped-away” with respect to each other. On the other hand, magnetic helicity has been used as an indicator of flare activity in ARs. Aims. As solar cycle (SC) 25 approaches its peak, a number of significant (X-class) flares have been produced. Here, we investigate the circumstances surrounding two of the most flare-prolific ARs of solar cycle 25, namely, ARs 13590 and 13514. Two aspects of the evolution of these ARs are investigated in this work: the global-scale magnetic toroid configuration and small-scale magnetic field morphology and topology – before, during, and after the onset of major flares. Methods. We studied the global morphology of the solar magnetic fields near intense flares in terms of the spatial distribution of ARs on magnetic fields synoptic maps. On AR scales, we analyzed the magnetic helicity accumulation, as well as its current-carrying and potential components. Results. Our results are consistent with major flare-prolific ARs from solar cycles 23 and 24. In particular, we observe a consistent dominance of current-carrying magnetic helicity at the time of major flares. The evolution of global magnetic toroids, indicating the occurrence of flare-prolific ARs in the tipped-away portion of the toroid, together with the local dynamics of complex ARs, could offer a few weeks of lead time to prepare for upcoming space weather hazards.

Magnetic helicity evolution during active region emergence and subsequent flare productivity

Aims. Solar active regions (ARs), which are formed by flux emergence, serve as the primary sources of solar eruptions. However, the specific physical mechanism that governs the emergence process and its relationship with flare productivity remains to be thoroughly understood. Methods. We examined 136 emerging ARs, focusing on the evolution of their magnetic helicity and magnetic energy during the emergence phase. Based on the relation between helicity accumulation and magnetic flux evolution, we categorized the samples and investigated their flare productivity. Results. The emerging ARs we studied can be categorized into three types, Type-I, Type-II, and Type-III, and they account for 52.2%, 25%, and 22.8% of the total number in our sample, respectively. Type-I ARs exhibit a synchronous increase in both the magnetic flux and magnetic helicity, while the magnetic helicity in Type-II ARs displays a lag in increasing behind the magnetic flux. Type-III ARs show obvious helicity injections of opposite signs. Significantly, 90% of the flare-productive ARs (flare index ≥ 6) were identified as Type-I ARs, suggesting that this type of AR has a higher potential to become flare productive. In contrast, Type-II and Type-III ARs exhibited a low and moderate likelihood of becoming active, respectively. Our statistical analysis also revealed that Type-I ARs accumulate more magnetic helicity and energy, far beyond what is found in Type-II and Type-III ARs. Moreover, we observed that flare-productive ARs consistently accumulate a significant amount of helicity and energy during their emergence phase. Conclusions. These findings provide valuable insight into the flux emergence phenomena, offering promising possibilities for early-stage predictions of solar eruptions.

Filament Eruption from Active Region 13283 Leading to a Fast Halo-CME and an Intense Geomagnetic Storm on 2023 April 23

Using multi-instrument and multiwavelength observations, we studied a coronal mass ejection (CME) that led to an intense geomagnetic storm on 2023 April 23. The eruption occurred on April 21 in solar active region (AR) 13283 near the disk center. The AR was in its decay stage, with fragmented polarities and a preexisting long filament channel a few days before the eruption. The study of the magnetic field evolution suggests that the flux rope (filament) was built up by monotonous helicity accumulation over several days, and furthermore, converging and canceling fluxes led to a change in helicity injection, resulting in an unstable nature of the magnetic flux rope (MFR) and its further eruption. Importantly, the CME morphology revealed that the MFR apex underwent a rotation of up to 56° in clockwise direction owing to its positive helicity. The CME decelerates in the field of view (FOV) of the Large Angle Spectrometric Coronagraph and has a plane-of-sky velocity of 1226 km s−1 at 20 R ⊙. In the FOV of the Heliospheric Imager, the lateral expansion of the CME is tracked better than the earthward motion. This implies that the arrival time is difficult to assess. The in situ arrival of the interplanetary CME shock was at 07:30 UT on April 23, and a geomagnetic storm commenced at 08:30 UT. The flux rope fitting to the in situ magnetic field observations reveals that the magnetic cloud flux rope orientation is consistent with its near-Sun orientation, which has a strong negative Bz-component. The analysis of this study indicates that the near-Sun rotation of the filament during its eruption to the CME is the key to the negative Bz-component and consequently the intense geomagnetic storm.

Magnetic field evolution and reconnection in low resistivity plasmas

The mathematics and physics of each of the three aspects of magnetic field evolution—topology, energy, and helicity—are remarkably simple and clear. When the resistivity η is small compared to an imposed evolution, a/v, timescale, which means Rm≡μ0va/η≫1, magnetic field-line chaos dominates the evolution of field-line topology in three-dimensional systems. Chaos has no direct role in the dissipation of energy. A large current density, jη≡vB/η, is required for energy dissipation to be on a comparable timescale to the topological evolution. Nevertheless, chaos plus Alfvén wave damping explain why both timescales tend to be approximately an order of magnitude longer than the evolution timescale a/v. Magnetic helicity is injected onto tubes of field lines when boundary flows have vorticity. Chaos can spread but not destroy magnetic helicity. Resistivity has a negligible effect on helicity accumulation when Rm≫1. Helicity accumulates within a tube of field lines until the tube erupts and moves far from its original location.

Sunspot Rotation in High- and Low-Flaring Active Regions

Sunspot rotations are closely linked with flaring activity. They are thought to contribute to the accumulation of helicity in magnetic flux tubes and to triggering magnetic reconnection in large solar flares. This link to solar flares has led to sunspot rotations being used as a parameter in solar flare prediction methods, but analysis for long-period observations of rotations in the literature is scarce. In this study, the rotation profiles of sunspots in a selection of six active regions are studied over time periods of 5 – 10 days to measure how sunspot rotation varies as active regions develop. The active regions are divided into two categories: high-flaring groups, which produced at least one X-class flare, and low-flaring regions that had little flaring activity. Comparison of the rotation profiles in these regions showed that young complex sunspot groups exhibit faster angular velocities and more frequent changes in rotation than older single-spot groups and, although the most rotating groups were also the most flare-productive, sudden changes in rotation were found to not definitively indicate an imminent eruption.

Magnetic helicity and energy of emerging solar active regions and their erruptivity

Aims.We investigate the role of the accumulation of magnetic helicity and magnetic energy in the generation of coronal mass ejections (CMEs) from emerging solar active regions (ARs).Methods.Using vector magnetic field data obtained by the Helioseismic and Magnetic Imager on board the Solar Dynamics Observatory, we calculated the magnetic helicity and magnetic energy injection rates as well as the resulting accumulated budgets in 52 emerging ARs from the start time of magnetic flux emergence until they reached a heliographic longitude of 45° West (W45).Results.Seven of the ARs produced CMEs, but 45 did not. In a statistical sense, the eruptive ARs accumulate larger budgets of magnetic helicity and energy than the noneruptive ARs over intervals that start from the flux emergence start time and end (i) at the end of the flux emergence phase and (ii) when the AR produces its first CME or crosses W45, whichever occurs first. We found magnetic helicity and energy thresholds of 9 × 1041Mx2and 2 × 1032erg. When these thresholds were crossed, ARs are likely to erupt. In terms of accumulated magnetic helicity and energy budgets, the segregation of the eruptive from the noneruptive ARs is violated in one case when an AR erupts early in its emergence phase and in six cases in which noneruptive ARs exhibit large magnetic helicity and energy budgets. Decay index calculations may indicate that these ARs did not erupt because the overlying magnetic field provided a stronger or more extended confinement than in eruptive ARs.Conclusions.Our results indicate that emerging ARs tend to produce CMEs when they accumulate significant budgets of both magnetic helicity and energy. Any study of their eruptive potential should consider magnetic helicity together with magnetic energy.

Comparative case study of two methods to assess the eruptive potential of selected active regions

Solar eruptive events, like flares and coronal mass ejections, are characterized by the rapid release of energy that can give rise to emission of radiation across the entire electromagnetic spectrum and to an abrupt significant increase in the kinetic energy of particles. These energetic phenomena can have important effects on the space weather conditions and therefore it is necessary to understand their origin, in particular, what is the eruptive potential of an active region (AR). In these case studies, we compare two distinct methods that were used in previous works to investigate the variations of some characteristic physical parameters during the pre-flare states of flaring ARs. These methods consider: i) the magnetic flux evolution and magnetic helicity accumulation, and ii) the fractal and multi-fractal properties of flux concentrations in ARs. Our comparative analysisis based on time series of photospheric data obtained bythe Solar Dynamics Observatory between March 2011 and June 2013. We selected two distinct samples of ARs: one is distinguished by the occurrence of more energetic M- and X-class flare events, that may have a rapid effect on not just the near-Earth space, but also on the terrestrial environment; the second is characterized by no-flares or having just a few C- and B-class flares. We foundthat the two tested methods complement each other in their ability to assess the eruptive potentials of ARs and could be employed to identify ARs prone to flaring activity. Based on the presented case study, we suggest that using a combination of different methods may aid to identify more reliably the eruptive potentials of ARs and help to better understand the pre-flare states.

Two Classes of Eruptive Events During Solar Minimum

During solar minimum, the Sun is relatively inactive with few sunspots observed on the solar surface. Consequently, we observe a smaller number of highly energetic events such as solar flares or coronal mass ejections (CMEs), which are often associated with active regions on the photosphere. Nonetheless, our magnetofrictional simulations during the minimum period suggest that the solar corona is still dynamically evolving in response to the large-scale shearing velocities on the solar surface. The non-potential evolution of the corona leads to the accumulation of magnetic free energy and helicity, which is periodically shed in eruptive events. We find that these events fall into two distinct classes: One set of events are caused by eruption and ejection of low-lying coronal flux ropes, and they could explain the origin of occasional CMEs during solar minimum. The other set of events are not driven by destabilisation of low-lying structures but rather by eruption of overlying sheared arcades. These could be associated with streamer blowouts or stealth CMEs. The two classes differ significantly in the amount of magnetic flux and helicity shed through the outer coronal boundary. We additionally explore how other measurables such as current, open magnetic flux, free energy, coronal holes, and the horizontal component of the magnetic field on the outer model boundary vary during the two classes of event. This study emphasises the importance and necessity of understanding the dynamics of the coronal magnetic field during solar minimum.

A MULTI-INSTRUMENT ANALYSIS OF A C4.1 FLARE OCCURRING IN A δ SUNSPOT

ABSTRACT We present an analysis of multi-instrument space- and ground-based observations relevant to a C4.1 solar flare that occurred in the active region (AR) NOAA 11267 on 2011 August 6. Solar Dynamics Observatory observations indicate that at the flare’s beginning, it was localized in the preceding sunspot of the AR, which exhibits a δ configuration. Along the polarity inversion line between its opposite polarities we find a large shear angle of about 80°. The helicity accumulation shows that the AR does not obey the general hemispheric helicity rule. At the flare peak, unique observations taken with the X-Ray Telescope aboard Hinode reveal that the bulk of the X-ray emission takes place in the δ-spot region, where the plasma heats up to ≈ 1.9 · 10 7 ?> K. During the gradual phase, we observe the development of a Y-shaped structure in the corona and in the high chromosphere. An extruding structure forms, being directed from the emitting region above the δ spot toward the following sunspot. This structure cools down in a few tens of minutes while moving eastward along a direction opposite to the flare ribbon expansion. Finally, remote brightenings are found at the easternmost footpoint of this structure, appearing as a third flare ribbon in the chromosphere. After some minutes, RHESSI measurements show that the X-ray emission is localized in the region close to the crossing point of the coronal Y-shaped structure. Simultaneously, high-resolution (0.″15) observations performed at the Swedish 1 m Solar Telescope indicate a decreasing trend of the Ca ii H intensity in the flare ribbons with some transient enhancements. All these findings suggest that this event is a manifestation of magnetic reconnection, likely induced by an asymmetric magnetic configuration in a highly sheared region.

THE FORMATION AND ERUPTION OF A SMALL CIRCULAR FILAMENT DRIVEN BY ROTATING MAGNETIC STRUCTURES IN THE QUIET SUN

We present the first observation of the formation and eruption of a small circular filament driven by a rotating network magnetic field (RNF) in the quiet Sun. In the negative footpoint region of an inverse J-shaped dextral filament, the RNF was formed by the convergence to supergranular junctions of several magnetic flux patches of the same polarity, and it then rotated counterclockwise (CCW) for approximately 11 hr and showed up as a CCW rotating EUV cyclone, during which time the filament gradually evolved into a circular filament that surrounded the cyclone. When the calculated convergence and vortex flows appeared around the RNF during its formation and rotation phases, the injected magnetic helicity calculation also showed negative helicity accumulation during the RNF rotation that was consistent with the dextral chirality of the filament. Finally, the RNF rotation stopped and the cyclone disappeared, and, probably due to an emerging bipole and its forced cancellation with the RNF, the closure filament underwent an eruption along its axis in the (clockwise) direction opposite to the rotation directions of the RNF and cyclone. These observations suggest that the RNFs might play an important role in the formation of nearby small-scale circular filaments as they transport and inject magnetic energy and helicity, and the formation of the EUV cyclones may be a further manifestation of the helicity injected into the corona by the rotation of the RNFs in the photosphere. In addition, the new emerging bipole observed before the filament eruption might be responsible for destabilizing the system and triggering the magnetic reconnection which proves useful for the filament eruption.

UNWINDING MOTION OF A TWISTED ACTIVE REGION FILAMENT

To better understand the structures of active-region filaments and the eruption process, we study an active-region filament eruption in active region NOAA 11082 in detail on June 22, 2010. Before the filament eruption, the opposite unidirectional material flows appeared in succession along the spine of the filament. The rising of the filament triggered two B-class flares at the upper part of the filament. As the bright material was injected into the filament from the sites of the flares, the filament exhibited a rapid uplift accompanying the counterclockwise rotation of the filament body. From the expansion of the filament, we can see that the filament is consisted of twisted magnetic field lines. The total twist of the filament is at least 5$\pi$ obtained by using time slice method. According to the morphology change during the filament eruption, it is found that the active-region filament was a twisted flux rope and its unwinding motion was like a solar tornado. We also find that there was a continuous magnetic helicity injection before and during the filament eruption. It is confirmed that magnetic helicity can be transferred from the photosphere to the filament. Using the extrapolated potential fields, the average decay index of the background magnetic fields over the filament is 0.91. Consequently, these findings imply that the mechanism of solar filament eruption could be due to the kink instability and magnetic helicity accumulation.

Twist and Writhe of the Magnetic Flux in the Super Active Region NOAA 11429

We used full-disk line-of-sight magnetograms taken by the Helioseismic and Magnetic Imager (HMI) onboard the Solar Dynamics Observatory (SDO) to study the variation of coronal magnetic helicity in the Active Region (AR) NOAA 11429, where several GOES M- and X-class flares and coronal mass ejections (CMEs) occurred. The magnetic flux, total magnetic-helicity flux, and helicity accumulation over the period of interest, i.e. 6 to 11 March 2012, were measured and are discussed. We also evaluated the tilt-angle evolution within the standard polarity flux-weighted centroids approach. The AR displays a shearing motion of the magnetic structures along the polarity inversion line, reaching values of about 1.0 km s−1. The variations of magnetic helicity flux and the tilt-angle seem to be time-correlated, and both display three-phase evolutionary patterns. We also found that the flare/CME activity is higher during the first observation phase when the tilt-angle decreases and the negative magnetic helicity is accumulated. The main changes in the accumulated helicity curve are observed only after the onset of the two strongest flare/CME events. After the major event (GOES X5.4 class/CME of 7 March) there was a decrease in the occurrence of flares and CMEs. This phase is marked by a decrease of the flux of magnetic helicity from the convection zone to the corona and a change in the orientation of the tilt of the AR. This behavior suggests that the combination of these two quantities might be important in the description of the magnetic complexity accumulated by an AR during its lifetime.

HELICITY CONDENSATION AS THE ORIGIN OF CORONAL AND SOLAR WIND STRUCTURE

Three of the most important and most puzzling features of the Sun's atmosphere are the smoothness of the closed field corona, the accumulation of magnetic shear at photospheric polarity inversion lines (PIL), and the complexity of the slow wind. We propose that a single process, helicity condensation, is the physical mechanism giving rise to all three features. A simplified model is presented for how helicity is injected and transported in the closed corona by magnetic reconnection. With this model we demonstrate that helicity must condense onto PILs and coronal hole boundaries, and estimate the rate of helicity accumulation at PILs and the loss to the wind. Our results can account for many of the observed properties of the closed corona and wind.

The helicity, induced electric field and Poynting flux of AR 11158 and their relationship with the X-class flare

AbstractWith the photospheric vector magnetic fields provided by SDO/HMI team, the helicity accumulation, induced electric field and Poynting flux is calculated for AR 11158 by using the local correlation tracking technique (LCT). It is found that the helicity accumulation reaches 6000×1040Mx2, the average densities of the induced electric field about 0.15-0.35 V cm−1, and that of the Poynting flux about 50-240 W m−2, within 50 hours. One main flare of X2.2 occurs in the increasing phase of the helicity accumulation, which also corresponds to the decreasing phase of the induced electric field and the gradual change phase of the Poynting flux. Before the flare, all these quantities increase rapidly for about 20 hours firstly, then increase gradually or even decrease for 8-9 hours.

Helicity transport from solar convection zone to interplanetary space

AbstractMagnetic helicity is a physical quantity that describes field topology. It is also a conserved quantity as Berger in 1984 demonstrated that the total magnetic helicity is still conserved in the corona even when there is a fast magnetic reconnection. It is generally believed that solar magnetic fields, together with their helicity, are created in the convection zone by various dynamo processes. These fields and helicity are transported into the corona through solar photosphere and finally released into the interplanetary space via various processes such as coronal mass ejections (CMEs) and solar winds. Here I will give a brief review on our recent works, first on helicity observations on the photosphere and how to understand these observations via dynamo models. Mostly, I will talk about what are the possible consequences of magnetic helicity accumulation in the corona, namely, the formation of magnetic flux ropes, CMEs taking place as an unavoidable product of coronal evolution, and flux emergences as a trigger of CMEs. Finally, I will address on in what a form magnetic field in the interplanetary space would accommodate a large amount of magnetic helicity that solar dynamo processes have been continuously producing.

MAGNETIC HELICITY OF SELF-SIMILAR AXISYMMETRIC FORCE-FREE FIELDS

In this paper we continue our theoretical studies on addressing what are the possible consequences of magnetic helicity accumulation in the solar corona. Our previous studies suggest that coronal mass ejections (CMEs) are natural products of coronal evolution as a consequence of magnetic helicity accumulation and the triggering of CMEs by surface processes such as flux emergence also have their origin in magnetic helicity accumulation. Here we use the same mathematical approach to study the magnetic helicity of axisymmetric power-law force-free fields, but focus on a family whose surface flux distributions are defined by self-similar force-free fields. The semi-analytical solutions of the axisymmetric self-similar force-free fields enable us to discuss the properties of force-free fields possessing a huge amount of accumulated magnetic helicity. Our study suggests that there may be an absolute upper bound on the total magnetic helicity of all bipolar axisymmetric force-free fields. And with the increase of accumulated magnetic helicity, the force-free field approaches being fully opened up, with Parker-spiral-like structures present around a current-sheet layer as evidence of magnetic helicity in the interplanetary space. It is also found that among the axisymmetric force-free fields having the same boundary flux distribution, the one that is self-similar is the one possessing the maximum amount of total magnetic helicity. This possibly gives a physical reason why self-similar fields are often found in astrophysical bodies, where magnetic helicity accumulation is presumably also taking place.

MAGNETIC HELICITY TRANSPORTED BY FLUX EMERGENCE AND SHUFFLING MOTIONS IN SOLAR ACTIVE REGION NOAA 10930

We present a new methodology which can determine magnetic helicity transport by the passage of helical magnetic field lines from sub-photosphere and the shuffling motions of foot-points of preexisting coronal field lines separately. It is well known that only the velocity component which is perpendicular to the magnetic field ($\upsilon_{\perp B}$) has contribution to the helicity accumulation. Here, we demonstrate that $\upsilon_{\perp B}$ can be deduced from horizontal motion and vector magnetograms, under a simple relation of $\upsilon_t = \mu_t + \frac{\upsilon_n}{B_n} B_t$ as suggested by D$\acute{e}$moulin & Berger (2003). Then after dividing $\upsilon_{\perp B}$ into two components, as one is tangential and the other is normal to the solar surface, we can determine both terms of helicity transport. Active region (AR) NOAA 10930 is analyzed as an example during its solar disk center passage by using data obtained by the Spectro-Polarimeter and the Narrowband Filter Imager of Solar Optical Telescope on board Hinode. We find that in our calculation, the helicity injection by flux emergence and shuffling motions have the same sign. During the period we studied, the main contribution of helicity accumulation comes from the flux emergence effect, while the dynamic transient evolution comes from the shuffling motions effect. Our observational results further indicate that for this AR, the apparent rotational motion in the following sunspot is the real shuffling motions on solar surface.

Flare occurrence and the spatial distribution of the magnetic helicity flux

Context. The accumulation of magnetic helicity via emergence of new magnetic flux and/or shearing photospheric motions is considered an important tool for understanding the processes that lead to eruptive phenomena. Aims. We highlight a specific aspect of the magnetic helicity accumulation, providing new observational evidence of the role played by the interaction of magnetic field systems that are characterized by opposite signs of the magnetic helicity flux in triggering solar eruptions. Methods. The amount of magnetic helicity injected into the corona through the photosphere in a sample of active regions (ARs) during their passage across the solar disk was measured by inferring the apparent motion of photospheric footpoints of magnetic field lines from a time series of MDI full-disk line-of-sight magnetograms. The temporal variation of the maps of magnetic helicity flux was analysed by measuring the fragmentation of the patches that are characterized by the flux of magnetic helicity. The temporal correlation between the number of these patches and the flare and coronal mass ejection (CME) occurrence has also been studied. Results. The fragmentation of the patches singled out in the maps of the magnetic helicity flux provides a useful indication of the evolution of the AR complexity. The more fragmented the maps of the magnetic helicity flux are, the higher is the flare and CME frequency. Moreover, most of the events occur for low values (~3 ÷ 17) of the difference of the number of patches with opposite signs of magnetic helicity flux. Conclusions. These results indicate that not only the accumulation of magnetic helicity in the corona, but also its positive and negative fragmentation and distribution should be taken into account to provide a more confident indication of AR complexity and flare/CME productivity. In particular, the interaction of magnetic systems characterized by opposite sign of magnetic helicity flux may be responsible for many observed eruptions.

SIMULATIONS OF TURBULENT DYNAMOS DRIVEN BY THE MAGNETIC HELICITY FLUX

We present results of numerical simulations of driven magnetohydrodynamic turbulence in a periodic box with an imposed large-scale sinusoidal shear. We produce strong large-scale magnetic fields through dynamo action. This dynamo action is highly dependent on the spectral properties of the forcing function. In particular, we can produce either dynamo or antidynamo action by choosing particular forcing functions for the small-scale turbulence. Our results show that the large-scale electromotive force is well correlated with the local accumulation of eddy-scale magnetic helicity. Conversely, the electromotive force is very weakly correlated with the large-scale magnetic field strength or its product with kinetic helicity. We argue that in these simulations large-scale dynamo action is driven by an inverse cascade of magnetic helicity, ultimately caused by the tendency of anisotropic turbulence to create a magnetic helicity flux.

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

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 171ATRACE 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 Aduring 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.