Fine-scale structures and material flows of quiescent filaments observed by the New Vacuum Solar Telescope

Study of the small-scale structures and material flows associated with solar quiescent filaments is very important for understanding the formation and equilibrium of solar filaments. Using high resolution Ha data observed by the New Vacuum Solar Telescope, we present the structures of barbs and material flows along the threads across the spine in two quiescent filaments on 2013 September 29 and on 2012 November 2, respectively. During the evolution of the filament barb, several paral- lel tube-shaped structures formed and the width of the structures ranged from about 2.3 Mm to 3.3 Mm. The parallel tube-shaped structures merged together accompanied by material flows from the spine to the barb. Moreover, the boundary between the barb and surrounding atmosphere was very neat. The counter-streaming flows were not found to appear alternately in the adjacent threads of the filament. However, the large-scale patchy counter-streaming flows were detected in the filament. The flows in one patch of the filament have the same direction but flows in the adjacent patch have opposite direction. The patches of two opposite flows with a size of about 10＂ were alternately exhibited along the spine of the filament. The velocity of these material flows ranged from 5.6 km s^-1 to 15.0 km s^-1. The material flows along the threads of the filament did not change their direction for about two hours and fourteen minutes during the evolution of the filament. Our results confirm that the large-scale counter- streaming flows with a certain width along the threads of solar filaments exist and are coaligned well with the threads.

The north-south asymmetry of solar filaments separately at low and high latitudes in solar cycle 23

We present the results of a study on the north-south asymmetry of solar filaments at low（〈50°） and high（〉60°） latitudes using daily filament numbers from January 1998 to November 2008（solar cycle 23）. It is found that the northern hemisphere is dominant at low latitudes for cycle 23. However, a similar asymmetry does not occur for solar filaments at high latitudes. The present study indicates that the hemispheric asymmetry of solar filaments at high latitudes in a cycle appears to have little connection with that at low latitudes. Our results support that the observed magnetic fields at high latitudes include two components： one comes from the emergence of the magnetic fields from the solar interior and the other comes from the drift of the magnetic activity at low latitudes.

Properties of filaments in solar cycle 20-23 from McIntosh Archive

A filament is a cool, dense structure suspended in the solar corona. The eruption of a filament is often associated with a coronal mass ejection(CME), which has an adverse effect on space weather. Hence,research on filaments has attracted much attention in the recent past. The tilt angle of active region(AR)magnetic bipoles is a crucial parameter in the context of the solar dynamo, which governs the conversion efficiency of the toroidal magnetic field to poloidal magnetic field. Filaments always form over polarity inversion lines(PILs), so the study of tilt angles for these filaments can provide valuable information about generation of a magnetic field in the Sun. We investigate the tilt angles of filaments and other properties using McIntosh Archive data. We fit a straight line to each filament to estimate its tilt angle. We examine the variation of mean tilt angle with time. The latitude distribution of positive tilt angle filaments and negative tilt angle filaments reveals that there is a dominance of positive tilt angle filaments in the southern hemisphere and negative tilt angle filaments dominate in the northern hemisphere. We study the variation of the mean tilt angle for low and high latitudes separately. Investigations of temporal variation with filament number indicate that total filament number and low latitude filament number vary cyclically, in phase with the solar cycle. There are fewer filaments at high latitudes and they also show a cyclic pattern in temporal variation. We also study the north-south asymmetry of filaments with different latitude criteria.

Can an injection model replenish filaments in a weak magnetic environment?

We observed an Hα surge that occurred in NOAA Active Region 12401 on 2015 August 17, and we discuss its trigger mechanism, and kinematic and thermal properties. It is suggested that this surge was caused by a chromospheric reconnection which ejected cool and dense material with transverse velocity of about 21–28 km s-1 and initial Doppler velocity of 12 km s^-1. This surge is similar to the injection of newly formed filament materials from their footpoints, except that the surge here occurred in a relatively weak magnetic environment of 100 G. Thus, we discuss the possibility of filament material replenishment via the erupting mass in such a weak magnetic field, which is often associated with quiescent filaments. It is found that the local plasma can be heated up to about 1.3 times the original temperature, which results in an acceleration of about –0.017 km s^-2. It can lift the dense material up to 10 Mm and higher with an inclination angle smaller than 50°, namely the typical height of active region filaments, but it can hardly inject the material up to those filaments higher than 25 Mm, like some quiescent filaments. Thus, we think that the injection model does not work well in describing the formation of quiescent filaments.

Interaction and eruption of two filaments observed by Hinode, SOHO, and STEREO

We investigate the interaction between two filaments and the subsequent filament eruption event observed from different viewing angles by Hinode, the Solar and Heliospheric Observatory, and the Solar Terrestrial Relations Observatory. In the event, the two filaments rose high, interacted with each other, and finally were ejected along two different paths. We measure the bulk-flow velocity using spectroscopic data. We find significant outflows at the speed of a few hundreds of km s 1 during the filament eruption, and also some downflows at a few tens of km s-1 at the edge of the eruption region in the late stage of the eruption. The erupting material was composed of plasmas with a wide temperature range of 10-4–106 K. These results shed light on the filament nature and the coronal dynamics.

The Chiralities of Three Filaments in AR7500

An analysis of structure and evolution of three filaments in AR7500 ON May, 1993 is presented. The three filaments show the following remarkable peculiarities: 1) a sinistral filament lies between two dextral filaments and meets them at two cusps. One of the cusps is rooted in positive polarity plage region while the other in a small decaying spot with negative polarity. 2) Both chromospheric plagette associated fibrils near the filaments and the transverse magnetic fields of the photosphere beneath the filaments almost parallel to the long axis of these filaments, suggesting the dominance of axial field of filaments; the spots near filament’s ends, however, do not show clearly counterclockwise or clockwise superpenumbral fibril pattern. 3) The three filaments do not merge into a single filament during 8 day observation. This can be explained by considering that the magnetic fields along their long axes are in different directions at two cusps. When one of dextral filaments and the sinistral filarment eventually dissipate into fibrils connecting the opposite polarity regions, their jointing cusps can be still clearly observed due to their opposite chiralities.

A new comprehensive dataset of solar filaments of one-hundred-year interval (Ⅱ) the poleward migration of polar crown filaments

Solar filaments,hypothermia and dense structures suspended in the solar corona are formed above the magnetic polarity inversion line.Polar crown filaments(PCFs)at high-latitude regions of the Sun are of profound significance to the periodic variation of solar activities.In this paper,we statistically analyze PCFs by using full disk Ha data from 1912 to 2018,which were obtained by Kodaikanal Solar Observatory(KODA,India),National Solar Observatory(NSO,USA),Kanzelhohe Solar Observatory(KSO,Austria),Big Bear Solar Observatory(BBSO,USA),and Huairou Solar Observing Station(HSOS,China).We first manually identify PCFs from every solar image based on the centennial data,and record the latitude and other features corresponding to the PCFs.Then we plot the PCF latitude distribution as a function of time,which clearly shows that PCFs rush to the poles at the ascending phase of each solar cycle.Our results show that the filaments drift toward mid-latitude covering solar cycle 15 to 24 after the PCFs reach the highest latitudes.The poleward migration rates of PCFs are calculated in ten solar cycles,and the range is about 0.12 degree to 0.50 degree per Carrington Rotation(CR).We also investigate the north-south(N-S)asymmetry of migration rates and the normalized N-S asymmetry index.

A comparison study of a solar active-region eruptive filament and a neighboring non-eruptive filament

Solar active region （AR） 11283 is a very magnetically complex region and it has produced many eruptions. However, there exists a non-eruptive filament in the plage region just next to an eruptive one in the AR, which gives us an opportunity to perform a comparison analysis of these two filaments. The coronal magnetic field extrapolated using our CESE-MHD-NLFFF code reveals that two magnetic flux ropes （MFRs） exist in the same extrapolation box supporting these two filaments, respectively. Analysis of the magnetic field shows that the eruptive MFR contains a bald-patch separatrix surface （BPSS） co- spatial very well with a pre-eruptive EUV sigmoid, which is consistent with the BPSS model for coronal sigmoids. The magnetic dips of the non-eruptive MFRs match Hα observation of the non-eruptive filament strikingly well, which strongly supports the MFR-dip model for filaments. Compared with the non-eruptive MFR/filament （with a length of about 200 Mm）, the eruptive MFR/filament is much smaller （with a length of about 20 Mm）, but it contains most of the magnetic free energy in the extrapolation box and holds a much higher free energy density than the non-eruptive one. Both the MFRs are weakly twisted and cannot trigger kink instability. The AR eruptive MFR is unstable because its axis reaches above a critical height for torus instability, at which the overlying closed arcades can no longer confine the MFR stably. On the contrary, the quiescent MFR is very firmly held by its overlying field, as its axis apex is far below the torus-instability threshold height. Overall, this comparison investigation supports that an MFR can exist prior to eruption and the ideal MHD instability can trigger an MFR eruption.

Disappearance of a coronal hole induced by a filament activation

We present a rare observation of direct magnetic interaction between an activating filament and a coronal hole （CH）. The filament was a quiescent one located at the northwest of the CH. It underwent a nonradial activation, during which filament material constantly fell and intruded into the CH. As a result, the CH was clearly destroyed by the intrusion. Brightenings appeared at the boundaries and in the interior of the CH, meanwhile, its west boundaries began to retreat and the area gradually shrank. It is noted that the CH went on shrinking after the end of the intrusion and finally disappeared entirely. Following the filament activation, three coronal dimmings （D1-D3） were formed, among which D1 and D2 persisted throughout the complete disappearance of the CH. The derived coronal magnetic configuration shows that the filament was located below an extended loop system, which obviously linked D1 to D2. By extrapolating this result, our observations imply that the interaction between the filament and the CH involved direct intrusion of the filament material to the CH and the disappearance of the CH might be due to interchange reconnection between the expanding loop system and the CH＇s open field.

Formation of Transient Coronal Holes during the Eruption of a Quiescent Filament and its Overlying Sigmoid

By using Hα, He I 10830, EUV and soft X-ray （SXR） data, we examined a filament eruption that occurred on a quiet-sun region near the center of the solar disk on 2006 January 12, which disturbed a sigmoid overlying the filament channel observed by the GOES-12 SXR Imager （SXI）, and led to the eruption of the sigmoid. The event was associated with a partial halo coronal mass ejection （CME） observed by the Large Angle and Spectrometric Coronagraphs （LASCO） on board the Solar and Heliospheric Observatory （SOHO）, and resulted in the formation of two flare-like ribbons, post-eruption coronal loops, and two transient coronal holes （TCHs）, but there were no significantly recorded GOES or Hα flares corresponding to the eruption. The two TCHs were dominated by opposite magnetic polarities and were located on the two ends of the eruptive sigmoid. They showed similar locations and shapes in He I 10830, EUV and SXR observations. During the early eruption phase, brightenings first appeared on the locations of the two subsequent TCHs, which could be clearly identified on He I 10830, EUV and SXR images. This eruption could be explained by the magnetic flux rope model, and the two TCHs were likely to be the feet of the flux rope.

Coupling of small-and large-scale filament eruptions

We present observations of the eruption of a large-scale quiescent filament （LF） that is associated with the formation and eruption of a miniature filament （MF）. As a result of convergence and subsequent cancelation of opposite-polarity magnetic flux, MF was formed just below the spine of the LF＇s right seg- ment. Probably triggered by a nearby newly emerging flux, MF underwent a failed eruption immediately after its full development, which first ejected away from the spine of LF and then drained back to the Sun. This eruption no sooner started than the overlying LF＇s right segment began to rise slowly and the LF＇s other parts were also disturbed, and eventually the whole LF erupted bodily and quickly. These observa- tions suggest that the MF can serve as an intermediary that links the photospheric small-scale magnetic-field activities to the eruption of the overlying large filament. It appears that, rather than directly interacting with the supporting magnetic field of LF, small-scale flux cancelation and emergence in the LF＇s channel can manifest themselves as the formation and eruption of MF and so indirectly affect the stability of LE

Magnetic non-potentiality on the quiet Sun and the filigree

From the observed vector magnetic fields by the Solar Optical Telescope/ Spectro-Polarimeter aboard the satellite Hinode, we have examined whether or not the quiet Sun magnetic fields are non-potential, and how the G-band filigrees and Ca II network bright points （NBPs） are associated with the magnetic non-potentiality. A sizable quiet region in the disk center is selected for this study. The new findings by the study are as follows. （1） The magnetic fields of the quiet region are obviously non-potential. The region-average shear angle is 40°, the average vertical current is 0.016A m^-2, and the average free magnetic energy density, 2.7× 10^2erg cm^-3. The magnitude of these non-potential quantities is comparable to that in solar active regions. （2） There are overall correlations among current helicity, free magnetic energy and longitudinal fields. The magnetic non-potentiality is mostly concentrated in the close vicinity of network elements which have stronger longitudinal fields. （3） The filigrees and NBPs are magnetically characterized by strong longitudinal fields, large electric helicity, and high free energy density. Because the selected region is away from any enhanced network, these new results can generally be applied to the quiet Sun. The findings imply that stronger network elements play a role in high magnetic non-potentiality in heating the solar atmosphere and in conducting the solar wind.

Dependence of the length of solar filament threads on the magnetic configuration

High-resolution Ha observations indicate that filaments consist of an as- sembly of thin threads. In quiescent filaments, the threads are generally short, whereas in active region filaments, the threads are generally long. In order to explain these observational features, we performed one-dimensional radiative hydrodynamic sim- ulations of filament formation along a dipped magnetic flux tube in the framework of the chromospheric evaporation-coronal condensation model. The geometry of a dipped magnetic flux tube is characterized by three parameters, i.e., the depth （D）, the half-width （w） and the altitude （h） of the magnetic dip. A survey of the parame- ters in numerical simulations shows that when allowing the filament thread to grow in 5 days, the maximum length （Lth） of the filament thread increases linearly with w, and decreases linearly with D and h. The dependence is fitted into a linear function Lth = 0.84w --0.88D - 2.78h ＋ 17.31 （Mm）. Such a relation can qualitatively explain why quiescent filaments have shorter threads and active region filaments have longer threads.

A study of the relation between intensity oscillations and magnetic field parameters in a sunspot: Hinode observations

We present properties of intensity oscillations of a sunspot in the photo- sphere and chromosphere using G band and Ca u H filtergrams from Hinode. Intensity power maps as function of magnetic field strength and frequency reveal reduction of power in the G band with an increase in photospheric magnetic field strength at all frequencies. In Ca II H, however, stronger fields exhibit more power at high frequen- cies, particularly in the 4.5-8.0 mHz band. Power distributions in different locations of the active region show that the oscillations in Ca II H exhibit more power compared to that of the G band. We also relate the power in intensity oscillations with differ- ent components of the photospheric vector magnetic field using near simultaneous spectro-polarimetric observations of the sunspot from the Hinode spectropolarime- ter. The photospheric umbral power is strongly anti-correlated with the magnetic field strength and its line-of-sight component but there is a good correlation with the trans- verse component. A reversal of this trend is observed in the chromosphere except at low frequencies （V≤ 1.5 mHz）. The power in sunspot penumbrae is anti-correlated with the magnetic field parameters at all frequencies （1.0 ≤ v ≤ 8.0 mHz） in both the photosphere and chromosphere, except that the chromospheric power shows a strong correlation in the frequency range 3-3.5 mHz.

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 beating. 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 Ha filtergrams from the Big Bear Solar Observatory （BBSO） Ha 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 （fight- 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.

MHD Seismology of a loop-like filament tube by observed kink waves

We report and analyze observational evidence of global kink oscillations in a solar filament as observed in Ha by instruments administered by National Solar Observatory （NSO）/Global Oscillation Network Group （GONG）. An M1.1-class flare in active region （AR） 11692 occurred on 2013 March 15 and induced a global kink mode in the filament lying towards the southwest of AR 11692. We find periods of about 61-67 minutes and damping times of 92-117 minutes at positions of three ver- tical slices chosen in and around the filament apex. We find that the waves are damped. From the observed period of the global kink mode and damping timescale using the theory of resonant absorption, we perform prominence seismology. We estimate a lower cut-off value for the inhomogeneity length scale to be around 0.34-0.44 times the radius of the filament cross-section.

A time series of filament eruptions observed by three eyes from space:from failed to successful eruptions

We present stereoscopic observations of six sequential eruptions of a filament in the active region NOAA 11045 on 2010 Feb 8, with the advantage of the STEREO twin viewpoints in combination with Earth＇s viewpoint from SOHO instruments and ground-based telescopes. The last one of the six eruptions is a coronal mass ejection, but the others are not. The flare in this successful one is more intense than in the others. Moreover, the velocity of filament material in the successful one is also the largest among them. Interestingly, all the filament velocities are found to be proportional to the power of their flares. We calculate magnetic field intensity at low altitude, the decay indexes of the external field above the filament, and the asymmetry properties of the overlying fields before and after the failed eruptions and find little difference between them, indicating the same coronal confinement exists for both the failed and successful eruptions. The results suggest that, besides the confinement of the coronal magnetic field, the energy released in the low corona should be another crucial element affecting a failed or successful filament eruption. That is, a coronal mass ejection can only be launched if the energy released exceeds some critical value, given the same initial coronal conditions.

Studies of grand minima in sunspot cycles by using a flux transport solar dynamo model

We propose that grand minima in solar activity are caused by simultane- ous fluctuations in the meridional circulation and the Babcock-Leighton mechanism for the poloidal field generation in the flux transport dynamo model. We present the following results： （a） fluctuations in the meridional circulation are more effective in producing grand minima; （b） both sudden and gradual initiations of grand minima are possible; （c） distributions of durations and waiting times between grand minima seem to be exponential; （d） the coherence time of the meridional circulation has an effect on the number and the average duration of grand minima, with a coherence time of about 30 yr being consistent with observational data. We also study the occurrence of grand maxima and find that the distributions of durations and waiting times between grand maxima are also exponential, like the grand minima. Finally we address the question of whether the Babcock-Leighton mechanism can be operative during grand minima when there are no sunspots. We show that an a-effect restricted to the upper portions of the convection zone can pull the dynamo out of the grand minima and can match various observational requirements if the amplitude of this a-effect is suitably fine-tuned.

Some interesting topics provoked by the solar filament research in the past decade

Solar filaments are an intriguing phenomenon,like cool clouds suspended in the hot corona.Similar structures exist in the intergalactic medium as well.Despite being a long-studied topic,solar filaments have continually attracted intensive attention because of their link to coronal heating,coronal seismology,solar flares and coronal mass ejections(CMEs).In this review paper,by combing through the solar filament-related work done in the past decade,we discuss several controversial topics,such as the fine structures,dynamics,magnetic configurations and helicity of filaments.With high-resolution and highsensitivity observations,combined with numerical simulations,it is expected that resolving these disputes will definitely lead to a huge leap in understanding the physics related to solar filaments,and even shed light on galactic filaments.

What does the Sun tell and hint now?

Some historical records, which have held since the beginning of modern solar activity cycles, are being broken by the present Sun： cycle 23 records the longest cycle length and fall time; latitudes of high-latitude sunspots belonging to a new cycle around the minimum time of the cycle are statistically the lowest at present, compared with those of other cycles; there are only one or no sunspots in a month appearing at high latitudes for 58 months, which is the first time that such a long duration has been observed. The solar dynamo is believed to be slowing down due to： （1） the minimum smoothed monthly mean sunspot number is the smallest since cycle 16 onwards, and even probably among all modern solar cycles; and （2） once the time interval between the first observations of two neighboring sunspot groups is larger than 14 d, it should be approximately regarded as an observation of no sunspots on the visible solar disk, called a spotless event. Spotless events occur with the highest frequency around the minimum time of cycle 24, and the longest spotless event also appears around the minimum time for observations of the Sun since cycle 16. Cycle 24 is expected to have the lowest level of sunspot activity from cycle 16 onwards and even probably for all of the modern solar cycles.