Filament Footpoint Research Articles

Formation of a Long Filament Through the Connection of Two Filament Segments Observed by CHASE

We present imaging and spectroscopic diagnostics of a long filament during its formation with the observations from the Chinese Hα Solar Explorer and Solar Dynamics Observatory. The seed filament first appeared at about 05:00 UT on 2022 September 13. Afterward, it grew gradually and connected to another filament segment nearby, building up a long filament at about 20:00 UT on the same day. The CHASE Hα spectra show an obvious centroid absorption with mild broadening at the main spine of the long filament, which is interpreted as evidence of filament material accumulation. More interestingly, near the footpoints of the filament, persistent redshifts have been detected in the Hα spectra during the filament formation, indicating continuous drainage of filament materials. Furthermore, through inspecting the extreme-ultraviolet (EUV) images and magnetograms, it was found that EUV jets and brightenings appeared repeatedly at the junction of the two filament segments, where opposite magnetic polarities converged and canceled each other continuously. These results suggest the occurrence of intermittent magnetic reconnection that not only connects magnetic structures of the two filament segments but also supplies cold materials for the filament channel likely by the condensation of injected hot plasma, even though a part of the cold materials falls down to the filament footpoints at the same time.

Studying Magnetic Field Variations Accompanying the 2011 June 7 Eruptive Event, by Using Nonlinear Force-Free Field Modeling

We study the features of the magnetic field variations within the 2011 June 7 eruptive event that includes a large filament eruption, a flare, and a CME formation. The magnetic field characteristics were obtained by using vector measurements of the magnetic field with the SDO/HMI and 3D magnetic field calculations based on nonlinear force-free field (NLFFF) modeling. Strong and relatively fast variations in the photospheric field characteristics after the flare onset are shown to be observed only within a small site ($20''\times 20''$) of the eruption region in the neighborhood of the polarity inversion line (PIL). We found that the magnetic field strength, the electric current density, current helicity density and free magnetic energy density above this region are growing with height reaching their maximums at the level of ∼15 Mm. After 2011 July 7 00:00 UT, this height started gradually reducing. The NLFFF extrapolation revealed the presence of a magnetic flux rope elongated approximately along the main PIL and an arcade of magnetic field lines over it. The flux-rope axis is located at height of ∼15 Mm. The flux-rope footpoints approximately coincide with the eruptive filament footpoints. Thus, we concluded that the detected flux rope is associated with the magnetic structure of the observed filament. The detected strong variation of the magnetic field within the eruption region are most probably associated with the magnetic field reconfiguration after the filament eruption. The $T_{n}$ parameter, which is the average magnetic field twist within the flux rope, was found to increase up to 2.5 rotations before the flare onset, and to dramatically decrease afterward. This may reflect the developing of kink instability that presumably triggered this eruption.

Radio, EUV, and X-Ray Observations during a Filament Rise in the 2011 June 7 Solar Flare

Abstract The most energetic flares start with a filament rise followed by magnetic reconnection below this filament. The start of the reconnection corresponds to the beginning of the flare impulsive phase. In this paper we study processes before this phase. During the filament rise we recognize an unusual radio continuum with a starting boundary drifting toward lower frequencies. The estimated velocity of the agent generating this continuum boundary is about 400 km s−1, similar to that of the rising filament. In association with this filament rise, transient X-ray sources and extreme ultraviolet (EUV) brightenings are found near the filament footpoint and outside the locations where later two parallel flare ribbons appear. Moreover, oscillations with a ∼30 s period are found simultaneously in radio, EUV, and X-ray observations. Around the end of these oscillations the flare impulsive phase starts as seen in observations of the drifting pulsation structure and X-ray source located at the upper part of the rising filament. We interpret the unusual radio continuum and transient X-ray sources, which are located outside the two parallel flare ribbons, as those generated during an interaction of the rising filament with the above-lying magnetic loops. The EUV brightening at the filament footpoint could be a signature of the magnetic reconnection inside the magnetic rope carrying the filament. Possible scenarios of the ∼30 s period oscillations in radio, X-ray, and EUV are discussed.

Manifestations of Three-dimensional Magnetic Reconnection in an Eruption of a Quiescent Filament: Filament Strands Turning to Flare Loops

Abstract We report on observations of the conversion of bright filament strands into flare loops during the 2012 August 31 filament eruption. Prior to the eruption, individual bright strands composing one of the legs of the filament were observed in the 171 Å filter channel data of the Atmospheric Imaging Assembly. After the onset of the eruption, one of the hooked ribbons started to propagate and contract, sweeping footpoints of the bright filament strands as well as coronal loops located close by. Later on, hot flare loops appeared in regions swept by the hook, where the filament strands were rooted. Timing and localization of these phenomena suggest that they are caused by reconnection of field lines composing the filament at the hook, which, to our knowledge, has not been observed before. This process is not included in the standard flare model (CSHKP), as it does not address footpoints of erupting flux ropes and ribbon hooks. It has, however, been predicted using the recent three-dimensional extensions to the standard flare model. There, the erupting flux rope can reconnect with surrounding coronal arcades as the hooked extensions of current ribbons sweep its footpoints. This process results in formation of flare loops rooted in previous footpoints of the flux rope. Our observations of sweeping of filament footpoints are well described by this scenario. In all observed cases, all of the footpoints of the erupting filament became footpoints of flare loops. This process was observed to last for about 150 minutes, throughout the whole eruption.

Can we determine the filament chirality by the filament footpoint location or the barb-bearing?

We attempt to propose a method for automatically detecting the solar filament chirality and barb bearing. We first introduce the concept of an unweighted undirected graph and adopt the Dijkstra shortest path algorithm to recognize the filament spine. Then, we use the polarity inversion line (PIL) shift method for measuring the polarities on both sides of the filament, and employ the connected components labeling method to identify the barbs and calculate the angle between each barb and the spine to determine the bearing of the barbs, i.e., left or right. We test the automatic detection method with Hα filtergrams from the Big Bear Solar Observatory (BBSO) Hα archive and magnetograms observed with the Helioseismic and Magnetic Imager (HMI) on board the Solar Dynamics Observatory (SDO). Four filaments are automatically detected and illustrated to show the results. The barbs in different parts of a filament may have opposite bearings. The filaments in the southern hemisphere (northern hemisphere) mainly have left-bearing (right-bearing) barbs and positive (negative) magnetic helicity, respectively. The tested results demonstrate that our method is efficient and effective in detecting the bearing of filament barbs. It is demonstrated that the conventionally believed one-to-one correspondence between filament chirality and barb bearing is not valid. The correct detection of the filament axis chirality should be done by combining both imaging morphology and magnetic field observations.

Can we determine the filament chirality by the filament footpoint location or the barb-bearing?or the Barb-bearing?

We attempt to propose a method for automatically detecting the solar filament chirality and barb bearing. We first introduce the concept of an unweighted undirected graph and adopt the Dijkstra shortest path algorithm to recognize the filament spine. Then, we use the polarity inversion line (PIL) shift method for measuring the polarities on both sides of the filament, and employ the connected components labeling method to identify the barbs and calculate the angle between each barb and the spine to determine the bearing of the barbs, i.e., left or right. We test the automatic detection method with Hα filtergrams from the Big Bear Solar Observatory (BBSO) Hα archive and magnetograms observed with the Helioseismic and Magnetic Imager (HMI) on board the Solar Dynamics Observatory (SDO) . Four filaments are automatically detected and illustrated to show the results. The barbs in different parts of a filament may have opposite bearings. The filaments in the southern hemisphere (northern hemisphere) mainly have left-bearing (right-bearing) barbs and positive (negative) magnetic helicity, respectively. The tested results demonstrate that our method is efficient and effective in detecting the bearing of filament barbs. It is demonstrated that the conventionally believed one-to-one correspondence between filament chirality and barb bearing is not valid. The correct detection of the filament axis chirality should be done by combining both imaging morphology and magnetic field observations.

Hαspectroscopy and multiwavelength imaging of a solar flare caused by filament eruption

We study a sequence of eruptive events including filament eruption, a GOES C4.3 flare and a coronal mass ejection. We aim to identify the possible trigger(s) and precursor(s) of the filament destabilisation; investigate flare kernel characteristics; flare ribbons/kernels formation and evolution; study the interrelation of the filament-eruption/flare/coronal-mass-ejection phenomena as part of the integral active-region magnetic field configuration; determine H\alpha\ line profile evolution during the eruptive phenomena. Multi-instrument observations are analysed including H\alpha\ line profiles, speckle images at H\alpha-0.8 \AA\ and H\alpha+0.8 \AA\ from IBIS at DST/NSO, EUV images and magnetograms from the SDO, coronagraph images from STEREO and the X-ray flux observations from FERMI and GOES. We establish that the filament destabilisation and eruption are the main trigger for the flaring activity. A surge-like event with a circular ribbon in one of the filament footpoints is determined as the possible trigger of the filament destabilisation. Plasma draining in this footpoint is identified as the precursor for the filament eruption. A magnetic flux emergence prior to the filament destabilisation followed by a high rate of flux cancelation of 1.34$\times10^{16}$ Mx s$^{-1}$ is found during the flare activity. The flare X-ray lightcurves reveal three phases that are found to be associated with three different ribbons occurring consecutively. A kernel from each ribbon is selected and analysed. The kernel lightcurves and H alpha line profiles reveal that the emission increase in the line centre is stronger than that in the line wings. A delay of around 5-6 mins is found between the increase in the line centre and the occurrence of red asymmetry. Only red asymmetry is observed in the ribbons during the impulsive phases. Blue asymmetry is only associated with the dynamic filament.

Filament eruption in association with rotational motion near the filament footpoints

The active region magnetic field surrounding the filament plays an important role in filament formation, their evolution and disruption. We investigated a filament eruption that occurred in southern hemisphere of the Sun on July 08, 2011 using AIA and HMI data. The filament was located in a region close to the active region NOAA 11247 with its West-most footpoint anchored in the negative polarity plage region and the East-most in the positive polarity plage region. During observations, the magnetic flux was emerging in the active region and also in the plage regions. The flux emergence was stopped in West-most footpoint of the plage region about an hour before the filament eruption. A converging motion was also observed for many hours in the Western footpoint of the filament. The filament had left-handed twist and the net injected magnetic helicity was positive in both footpoints. Both sign of magnetic helicity were observed in the Western footpoint of the filament where the eruption has initiated. Further, an anti-clockwise rotational motion was observed in both the footpoints just after the onset of filament eruption which lasted for 6min during the eruption process. The emerging flux, converging motion and injection of opposite magnetic helicity could be responsible for destabilizing of the Western footpoint of the filament leading to eruption. The torque imbalance between the expanded portion of the flux tube and the photosphere may have caused the rotation in the footpoint region which changed the trend in the injected magnetic helicity after the filament eruption.

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

Context. In recent years it has been stressed that the accumulation of magnetic helicity via emergence of new magnetic flux and/or shearing photospheric motions, can play an important role in the destabilization processes leading to eruptive phenomena occurring in the solar atmosphere. Aims. In this paper we want to highlight a specific aspect of the process of magnetic helicity accumulation, providing new observational evidences of the role played by the interaction of magnetic fields characterized by opposite magnetic helicity signs in triggering solar eruption. Methods. We used 171 u TRACE data to describe a filament eruption occurred on Nov 1st, 2001 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 the methods proposed by Chae (2001) and Pariat et al. (2005) for the estimation of 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 found that only the DAVE method allowed us to obtain the crucial information on the horizontal velocity field near the magnetic inversion line.

EVIDENCE OF FILAMENT UPFLOWS ORIGINATING FROM INTENSITY OSCILLATIONS ON THE SOLAR SURFACE

A filament footpoint rooted in an active region (NOAA 11032) was well observed for about 78 minutes with the 1.6 m New Solar Telescope at the Big Bear Solar Observatory on 2009 November 18 in Hα ±0.75 A. This data set had high cadence (~15 s) and high spatial resolution (~01) and offered a unique opportunity to study filament dynamics. As in previous findings from space observations, several dark intermittent upflows were identified, and they behave in groups at isolated locations along the filament. However, we have two new findings. First, we find that the dark upflows propagating along the filament channel are strongly associated with the intensity oscillations on the solar surface around the filament footpoints. The upflows start at the same time as the peak in the oscillations, illustrating that the upflow velocities are well correlated with the oscillations. Second, the intensity of one of the seven upflows detected in our data set exhibits a clear periodicity when the upflow propagates along the filament. The periods gradually vary from ~10 to ~5 minutes. Our results give observational clues on the driving mechanism of the upflows in the filament.

Magnetic helicity and active filament configuration

Context. The role of magnetic helicity in active filament formation and destabilization is still under debate. Aims. Although active filaments usually show a sigmoid shape and a twisted configuration before and during their eruption, it is unclear which mechanism leads to these topologies. In order to provide an observational contribution to clarify these issues, we describe a filament evolution whose characteristics seem to be directly linked to the magnetic helicity transport in corona. Methods. We applied different methods to determine the helicity sign and the chirality of the filament magnetic field. We also computed the magnetic helicity transport rate at the filament footpoints. Results. All the observational signatures provided information on the positive helicity and sinistral chirality of the flux rope containing the filament material: its forward S shape, the orientation of its barbs, the bright and dark threads at 195 A. Moreover, the magnetic helicity transport rate at the filament footpoints showed a clear accumulation of positive helicity. Conclusions. The study of this event showed a correspondence between several signatures of the sinistral chirality of the filament and several evidences of the positive magnetic helicity of the filament magnetic field. We also found that the magnetic helicity transported along the filament footpoints showed an increase just before the change of the filament shape observed in Hα images. We argued that the photospheric regions where the filament was rooted might be the preferential ways where the magnetic helicity was injected along the filament itself and where the conditions to trigger the eruption were yielded.

X-ray precursors to flares and filament eruptions

Aims. To study preflare X-ray brightenings as diagnostics of the destabilisation of flare-associated erupting filaments/prominences. Methods. We combine new observations from the Transition Region and Coronal Explorer (TRACE) and the Reuven Ramaty High Energy Solar Spectroscopic Imager (RHESSI), as well as revisit events reported in the literature to date, in order to scrutinise the preflare activity during eight flare-associated filament eruptions. Results. The preflare activity occurs in the form of discrete, localised X-ray brightenings observed between 2 and 50 min before the impulsive phase of the flare and filament acceleration. These transient preflare brightenings are situated on or near (within 10 �� of) the polarity inversion line (PIL), coincident with emerging and/or canceling magnetic flux. The filaments begin to rise from the location of the preflare brightenings. In five out of eight events, the preflare brightenings were observed beneath the filament channel, close to the filament footpoint first observed to rise. Both thermal and nonthermal hard X-ray emissions during the preflare enhancement were detected with RHESSI, suggesting that both plasma heating and electron acceleration occurred at this time. The main energy release during the impulsive phase of the flare is observed close to (within 50 �� of) the preflare brightenings. The fast-rise phase of the filament eruption starts at the same time as the onset of the main flare or up to 5 min later. Conclusions. The preflare brightenings are precursors to the flare and filament eruption. These precursors represent distinct, localised instances of energy release, rather than a gradual energy release prior to the main flare. The X-ray precursors represent clearly observable signatures in the early stages of the eruption. Together with the timing of the filament fast-rise at or after the main flare onset, the X-ray precursors provide evidence for a tether-cutting mechanism initially manifested as localised magnetic reconnection being a common trigger for both flare emission and filament eruption.

Observational evidence of the primary role played by photospheric motions in magnetic helicity transport before a filament eruption

Many filament eruptions can be suitably described in the framework of the kink instability model, although it is not always easy to discriminate whether the helical flux rope writhes due to new emerging flux or to photospheric horizontal motions. In this paper we provide observational evidence of the important role which can be played by horizontal motions in filament instability and eruption. More precisely, we describe the analysis of the eruption of a reverse-S-shaped filament associated with a flare of class M6.3, that occurred on 15 June, 2001 in the active region NOAA 9502. Using TRACE 195 A images we studied the morphological evolution of the EUV filament channel. Using I minute cadence MDI full-disc longitudinal magnetograms we analyzed the magnetic evolution of the entire active region. The geometrical parameters of the EUV filament channel and the horizontal velocities in the areas corresponding to the filament footpoints were determined and agreed with the kink instability. Moreover, the analysis of MDI magnetograms showed that a sudden and strong increase in the magnetic helicity transport rate to the corona preceded and accompanied the filament eruption. During the same time interval, on the one hand the emergence of magnetic flux in both polarities became negligible, but on the other hand the velocity pattern at the filament ends showed horizontal, counterclockwise motions, which could make a significant contribution to the transformation, from twist to writhe, of the magnetic helicity accumulated along the filament before its eruption. This result seems to indicate that in this event the transport of magnetic helicity exceeding the limit for the kink instability is primarily due to photospheric motions, while the contribution from the emerging flux is negligible.

Plasma Flows Observed in Magnetic Flux Concentrations and Sunspot Fine Structure Using Adaptive Optics

We present diffraction-limited observations of magnetic flux concentrations and penumbral and umbral fine structure within an active region observed at disk center. We recorded G-band images, magnetograms, Dopplergrams, and narrowband filtergrams, using the Universal Birefringent Filter (UBF) at the Dunn Solar Telescope (DST). The National Solar Observatory (NSO) adaptive optics system at the DST was used to achieve diffraction-limited long-exposure imaging with a high signal-to-noise ratio. The main results can be summarized as follows: Strong and spatially narrow downflows are observed at the edge of magnetic structures, such as small flux concentrations (sometimes also referred to as flux tubes), pores, a light bridge, and the sunspot umbrae. For the particular sunspot observed, we find strong evidence for what appear to be vigorous, small-scale convection patterns in a light bridge. We observe extremely narrow (<02) channels or sheets of downflowing plasma. Flux concentrations as seen in intensity expand from a height close to where the continuum is formed to the height of formation for the G band. These observations indicate that the G band forms in the mid-photosphere. We are able to identify individual penumbral fibrils in our data and find a bright (hot) upflow and a more vertical field structure at the filament footpoint near the umbral boundary. The observations are consistent with a filament geometry in which the field and flow turn to a nearly horizontal, dark structure over a distance of about 02. In the deep photosphere we observe strong upflows of the order of 1 km s-1 in umbral dots. We compare our results with theoretical model predictions.

Magnetic helicity transport in corona and filament eruptions

Using a 28-hour time series of line-of-sight magnetograms taken by the Michelson Doppler Imager (MDI), we determined the magnetic flux variations and the rate of magnetic helicity transport at the footpoints of a filament in active region NOAA 8375. The filament was characterized by a positive helicity change due to shearing motions in both footpoints and showed several partial eruptions during the observing time. In particular, we considered 4 events registered in the Hα daily reports of Solar Geophysical Data. We found a strong temporal correlation between filament eruptions and helicity transport from the photospheric magnetic structures at the filament footpoints into the corona: in at least one footpoint, all of the events were preceded by an evident increase and followed by a small decrease of the emerging magnetic flux and of the magnetic helicity change due to shearing motions. We compared these two mechanisms of helicity transport and found that the predominant role to drive filament instability is played by emergence of new magnetic flux from the convection zone.

An analysis of the flare of 1966 March 24

This paper analyses numerically the data on the large two-ribbon flare of 1996 March 24 obtained at Purple Mountain Observatory. The results show that filament activation plays an important role in the triggering of the flare, that both the emerging flux of opposite polarity and photospheric shearing motion near the filament footpoint are responsible for the filament eruption. The emerging flux causes changes in the background field and the shearing motion which twists the axial magnetic field inside the filament leads to an increase in the electric current, so the filament ceases to be in equilibrium and moves upwards. Lastly, the physical relation between the ascending motion of the filament and the eruption of the flare is analysed and discussed.