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Strong Transverse Photosphere Magnetic Fields and Twist in Light Bridge Dividing Delta Sunspot of Active Region 12673  

Haimin Wang   Submitted: 2018-01-11 15:22

Solar Active Region (AR) 12673 is the most flare productive AR in the solar cycle 24. It produced four X-class flares including the X9.3 flare on 06 September 2017 and the X8.2 limb event on 10 September. Sun and Norton (2017) reported that this region had an unusual high rate of flux emergence, while Huang et al. (2018) reported that the X9.3 flare had extremely strong white-light flare emissions. Yang at al. (2017) described the detailed morphological evolution of this AR. In this report, we focus on usual behaviors of the light bridge (LB) dividing the delta configuration of this AR, namely the strong magnetic fields (above 5500 G) in the LB and apparent photospheric twist as shown in observations with a 0.1 arcsec spatial resolution obtained by the 1.6m telescope at Big Bear Solar Observatory.

Authors: Wang, Haimin; Yurchyshyn, Vasyl; Liu, Chang; Ahn, Kwangsu; Toriumi, Shin and Cao, Wenda
Projects: BBSO/NST,Hinode/SOT

Publication Status: Published in RNAAS
Last Modified: 2018-01-14 00:01
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Extending Counter-Streaming Motion from an Active Region Filament to Sunspot Light Bridge  

Haimin Wang   Submitted: 2017-12-19 21:15

We analyze the high-resolution observations from the 1.6m telescope at Big Bear Solar Observatory that cover an active region filament. Counter-streaming motions are clearly observed in the filament. The northern end of the counter-streaming motions extends to a light bridge, forming a spectacular circulation pattern around a sunspot, with clockwise motion in the blue wing and counterclockwise motion in the red wing as observed in Hα off-bands. The apparent speed of the flow is around 10 to 60 km s-1 in the filament, decreasing to 5 to 20 km s-1 in the light bridge. The most intriguing results are the magnetic structure and the counter-streaming motions in the light bridge. Similar to those in the filament, magnetic fields show a dominant transverse component in the light bridge. However, the filament is located between opposite magnetic polarities, while the light bridge is between strong fields of the same polarity. We analyze the power of oscillations with the image sequences of constructed Dopplergrams, and find that the filament's counter-streaming motion is due to physical mass motion along fibrils, while the light bridge's counter-streaming motion is due to oscillation in the direction along the line-of-sight. The oscillation power peaks around 4 minutes. However, the section of the light bridge next to the filament also contains a component of the extension of the filament in combination with the oscillation, indicating that some strands of the filament are extended to and rooted in that part of the light bridge.

Authors: Haimin Wang, Rui Liu, Qin Li, Chang Liu, Na Deng, Yan Xu, Ju Jing, Yuming Wang, Wenda Cao
Projects: BBSO/NST,SDO-HMI

Publication Status: ApJ Letters, Accepted
Last Modified: 2017-12-20 10:32
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High-resolution observations of flare precursors in the low solar atmosphere  

Haimin Wang   Submitted: 2017-03-28 16:41

Solar flares are generally believed to be powered by free magnetic energy stored in the corona, but the build up of coronal energy alone may be insufficient to trigger the flare to occur. The flare onset mechanism is a critical but poorly understood problem, insights into which could be gained from small-scale energy releases known as precursors. These precursors are observed as small pre-flare brightenings in various wavelengths and also from certain small-scale magnetic configurations such as opposite-polarity fluxes, where the magnetic orientation of small bipoles is opposite to that of the ambient main polarities. However, high-resolution observations of flare precursors together with the associated photospheric magnetic field dynamics are lacking. Here we study precursors of a flare using the unprecedented spatiotemporal resolution of the 1.6-m New Solar Telescope, complemented by new microwave data. Two episodes of precursor brightenings are initiated at a small-scale magnetic channel (a form of opposite-polarity flux) with multiple polarity inversions and enhanced magnetic fluxes and currents, lying near the footpoints of sheared magnetic loops. Microwave spectra corroborate that these precursor emissions originate in the atmosphere. These results provide evidence of low-atmospheric small-scale energy release, possibly linked to the onset of the main flare.

Authors: Haimin Wang, Chang Liu, Kwangsu Ahn, Yan Xu, Ju Jing, Na Deng, Nengyi Huang, Rui Liu, Kanya Kusano, Gregory D. Fleishman, Dale E. Gary and Wenda Cao
Projects: BBSO/NST

Publication Status: Nature Astronomy, Published
Last Modified: 2017-03-29 13:50
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Irreversible Rapid Changes of Magnetic Field Associated wi th the 2012 October 23 Circular Near-limb X1.8 Flare  

Haimin Wang   Submitted: 2016-02-08 07:51

It has been found that photospheric magnetic fields can change in accordance with the three-dimensional magnetic field restructuring following solar eruptions. Previous studies mainly use vector magnetic field data taken for events near the disk center. In this paper, we analyze the magnetic field evolution associated with the 2012 October 23 X1.8 flare in NOAA AR 11598 that is close to the solar limb, using both the 45 s cadence line-of-sight and 12 minute cadence vector magnetograms from the Helioseismic and Magnetic Imager on board Solar Dynamic Observatory. This flare is classified as a circular-ribbon flare with spine-fan type magnetic topology containing a null point. In the line-of-sight magnetograms, there are two apparent polarity inversion lines (PIL). The PIL closer to the limb is affected more by the projection effect. Between these two PILs there lie positive polarity magnetic fields, which are surrounded by negative polarity fields outside the PILs. We find that after the flare, both the apparent limb-ward and disk-ward negative fluxes decrease, while the positive flux in-between increases. We also find that the horizontal magnetic fields have a significant increase along the disk-ward PIL, while in surrounding area, they decrease. Synthesizing the observed field changes, we conclude that the magnetic fields collapse toward the surface above the disk-ward PIL as depicted in the coronal implosion scenario, while the peripheral field turns to a more vertical configuration after the flare. We also suggest that this event is an asymmetric circular-ribbon flare: a flux rope is likely present above the disk-ward PIL. Its eruption causes the instability of the entire fan-spine structure and the implosion near that PIL.

Authors: Dandan Ye, Chang Liu and Haimin Wang
Projects: SDO-HMI

Publication Status: Accepted by Research in Astronomy and Astrophysics
Last Modified: 2016-02-09 14:59
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Witnessing magnetic twist with high-resolution observation from the 1.6-m New Solar Telescope  

Haimin Wang   Submitted: 2015-05-27 07:32

Magnetic flux ropes are highly twisted, current-carrying magnetic fields. They are crucial for the instability of plasma involved in solar eruptions, which may lead to adverse space weather effects. Here we present observations of a flaring using the highest resolution chromospheric images from the 1.6-m New Solar Telescope at Big Bear Solar Observatory, supplemented by a magnetic field extrapolation model. A set of loops initially appear to peel off from an overall inverse S-shaped flux bundle, and then develop into a multi-stranded twisted flux rope, producing a two-ribbon flare. We show evidence that the flux rope is embedded in sheared arcades and becomes unstable following the enhancement of its twists. The subsequent motion of the flux rope is confined due to the strong strapping effect of the overlying field. These results provide a first opportunity to witness the detailed structure and evolution of flux ropes in the low solar atmosphere.

Authors: Haimin Wang, Wenda Cao, Chang Liu, Yan Xu, Rui Liu, Zhicheng Zeng, Jongchul Chae and Haisheng Ji
Projects: SDO-HMI

Publication Status: Nature Communications, 6, 7008
Last Modified: 2015-05-27 14:13
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Structure and Evolution of Magnetic Fields Associated with Solar Eruptions  

Haimin Wang   Submitted: 2015-01-03 17:42

This paper reviews the studies of solar photospheric magnetic field evolution in active regions and its relationship to solar flares. It is divided into two topics, the magnetic structure and evolution leading to solar eruptions and the rapid changes of photospheric magnetic field associated with eruptions. For the first topic, we describe the magnetic complexity, new flux emergence, flux cancellation, shear motions, sunspot rotation, and magnetic helicity injection, which may all contribute to the storage and buildup of energy and triggering of solar eruptions. For the second topic, we concentrate on the observations of rapid and irreversible changes of photospheric magnetic field associated with flares, and the implication on the restructuring of three-dimensional magnetic field. In particular, we emphasize the recent advances in observations of photospheric magnetic field, as state-of-the-art observing facilities (such as Hinode and Solar Dynamic Observatory) become available. The linkage between observations and theories and future prospectives in this research area are also discussed.

Authors: Haimin Wang and Chang Liu
Projects: None

Publication Status: Accepted for Publication, Research in Astronomy and Astrophysics
Last Modified: 2015-01-06 10:57
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Study of Two Successive Three-Ribbon Solar Flares on 2012 July 6  

Haimin Wang   Submitted: 2013-12-23 19:02

This Letter reports two rarely observed three-ribbon flares (M1.9 and C9.2) on 2012 July 6 in NOAA AR 11515, which we found with Hα observations of 0.1'' resolution from the New Solar Telescope and CaII H images from Hinode. The flaring site is characterized with an intriguing ''fish-bone-like'' morphology evidenced by both Hα images and a nonlinear force-free field (NLFFF) extrapolation, where two semi-parallel rows of low-lying, sheared loops connect an elongated, parasitic negative field with the sandwiching positive fields. The NLFFF model also shows that the two rows of loops are asymmetric in height and have opposite twists, and are enveloped by large-scale field lines including open fields. The two flares occurred in succession in half an hour and are located at the two ends of the flaring region. The three ribbons of each flare run parallel to the PIL, with the outer two lying in the positive field and the central one in the negative field. Both flares show surge-like flows in Hα apparently toward the remote region, while the C9.2 flare is also accompanied by EUV jets possibly along the open field lines. Interestingly, the 12-25 keV hard X-ray sources of the C9.2 flare first line up with the central ribbon then shift to concentrate on the top of the higher branch of loops. These results are discussed in favor of reconnection along the coronal null-line producing the three flare ribbons and the associated ejections.

Authors: Haimin Wang, Chang Liu, Na Deng, Zhicheng Zeng, Yan Xu, Ju Jing and Wenda Cao
Projects: None

Publication Status: ApJL accepted
Last Modified: 2013-12-24 06:12
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Study of Rapid Formation of a Delta Sunspot Associated with the 2012 July 2 C7.4 Flare Using High-resolution Observations of New Solar Telescope  

Haimin Wang   Submitted: 2013-08-12 18:48

Rapid, irreversible changes of magnetic topology and sunspot structure associated with flares have been systematically observed in recent years. The most striking features include the increase of horizontal field at the polarity inversion line (PIL) and the co-spatial penumbral darkening. A likely explanation of the above phenomenon is the back reaction to the coronal restructuring after eruptions: a coronal mass ejection carries the upward momentum while the downward momentum compresses the field lines near the PIL. Previous studies could only use low resolution (above 1'') magnetograms and white-light images. Therefore, the changes are mostly observed for X-class flares. Taking advantage of the 0.1'' spatial resolution and 15s temporal cadence of the New Solar Telescope at Big Bear Solar Observatory, we report in detail the rapid formation of sunspot penumbra at the PIL associated with the C7.4 flare on 2012 July 2. It is unambiguously shown that the solar granulation pattern evolves to alternating dark and bright fibril structure, the typical pattern of penumbra. Interestingly, the appearance of such a penumbra creates a new delta sunspot. The penumbral formation is also accompanied by the enhancement of horizontal field observed using vector magnetograms from the Helioseismic and Magnetic Imager. We explain our observations as due to the eruption of a flux rope following magnetic cancellation at the PIL. Subsequently the re-closed arcade fields are pushed down towards the surface to form the new penumbra. NLFFF extrapolation clearly shows both the flux rope close to the surface and the overlying fields.

Authors: Haimin Wang, Chang Liu, Shuo Wang, Na Deng, Yan Xu, Ju Jing and Wenda Cao
Projects: SDO-HMI

Publication Status: Ap.J. Letters, accepted
Last Modified: 2013-08-13 11:03
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CIRCULAR RIBBON FLARES AND HOMOLOGOUS JETS  

Haimin Wang   Submitted: 2012-10-10 18:59

Solar flare emissions in the chromosphere often appear as elongated ribbons on both sides of the magnetic polarity inversion line (PIL), which has been regarded as evidence of a typical configuration of magnetic reconnection. However, flares having a circular ribbon have rarely been reported, although it is expected in the fan?spine magnetic topology involving reconnection at a three-dimensional (3D) coronal null point. We present five circular ribbon flares with associated surges, using high-resolution and high-cadence Hα blue wing observations obtained from the recently digitized films of Big Bear Solar Observatory. In all the events, a central parasitic magnetic field is encompassed by the opposite polarity, forming a circular PIL traced by filament material. Consequently, a flare kernel at the center is surrounded by a circular flare ribbon. The four homologous jet-related flares on 1991 March 17 and 18 are of particular interest, as (1) the circular ribbons brighten sequentially, with co-spatial surges, rather than simultaneously, (2) the central flare kernels show an intriguing ?round-trip? motion and become elongated, and (3) remote brightenings occur at a region with the same magnetic polarity as the central parasitic field and are co-temporal with a separate phase of flare emissions. In another flare on 1991 February 25, the circular flare emission and surge activity occur successively, and the event could be associated with magnetic flux cancellation across the circular PIL. We discuss the implications of these observations combining circular flare ribbons, homologous jets, and remote brightenings for understanding the dynamics of 3D magnetic restructuring.

Authors: Haimin Wang and Chang Liu
Projects: None

Publication Status: Ap,J., accepted
Last Modified: 2012-10-11 08:04
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The Relationship between the Sudden Change of the Lorentz Force and the Magnitude of Associated Flares  

Haimin Wang   Submitted: 2012-08-15 18:54

The rapid and irreversible change of photospheric magnetic fields associated with flares has been confirmed by many recent studies. These studies showed that the photospheric magnetic fields respond to coronal field restructuring and turn to a more horizontal state near the magnetic polarity inversion line (PIL) after eruptions. Recent theoretical work has shown that the change in the Lorentz force associated with a magnetic eruption will lead to such a field configuration at the photosphere. The Helioseismic Magnetic Imager has been providing unprecedented full-disk vector magnetograms covering the rising phase of the solar cycle 24. In this study, we analyze 18 flares in four active regions, with GOES X-ray class ranging from C4.7 to X5.4. We find that there are permanent and rapid changes of magnetic field around the flaring PIL, the most notable of which is the increase of the transverse magnetic field. The changes of fields integrated over the area and the derived change of Lorentz force both show a strong correlation with flare magnitude. It is the first time that such magnetic field changes have been observed even for C-class flares. Furthermore, for seven events with associated CMEs, we use an estimate of the impulse provided by the Lorentz force, plus the observed CME velocity, to estimate the CME mass. We find that if the time scale of the back reaction is short, i.e., in the order of 10 s, the derived values of CME mass (1015 g) generally agree with those reported in literature.

Authors: Shuo Wang, Chang Liu, Haimin Wang
Projects: SDO-HMI

Publication Status: ApJ Letters, accepted
Last Modified: 2012-08-21 19:06
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Rapid Transition of Uncombed Penumbrae to Faculae during Large Flares  

Haimin Wang   Submitted: 2012-03-13 09:20

In the past two decades, the complex nature of sunspots has been disclosed with high-resolution observations. One of the most important findings is the ''uncombed'' penumbral structure, where a more horizontal magnetic component carrying most of Evershed Flows is embedded in a more vertical magnetic background (Solanki & Montavon 1993). The penumbral bright grains are locations of hot upflows and dark fibrils are locations of horizontal flows that are guided by nearly horizontal magnetic field. On the other hand, it was found that flares may change the topology of sunspots in delta configuration: the structure at the flaring polarity inversion line becomes darkened while sections of peripheral penumbrae may disappear quickly and permanently associated with flares (Liu et al. 2005). The high spatial and temporal resolution observations obtained with Hinode/ SOT provide an excellent opportunity to study the evolution of penumbral fine structure associated with major flares. Taking advantage of two near-limb events, we found that in sections of peripheral penumbrae swept by flare ribbons, the dark fibrils completely disappear, while the bright grains evolve into faculae that are signatures of vertical magnetic flux tubes. The corresponding magnetic fluxes measured in the decaying penumbrae show stepwise changes temporally correlated with the flares. These observations suggest that the horizontal magnetic field component of the penumbra could be straightened upward (i.e., turning from horizontal to vertical) due to magnetic field restructuring associated with flares, which results in the transition of penumbrae to faculae.

Authors: Haimin Wang, Na Deng, Chang Liu
Projects: GONG,Hinode/SOT,SoHO-MDI

Publication Status: Published in ApJ 2012, 748, 76
Last Modified: 2012-03-13 13:19
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Study of White-Light Flares Observed by Hinode  

Haimin Wang   Submitted: 2008-12-03 07:39

White-light flares are considered as the most energetic flaring events that are observable in optical broad-band continuum of solar spectrum. They have not been commonly observed. Observations of white-light flares with sub-arcsecond resolution have been very rare. The continuous high resolution observations of Hinode provide an unique opportunity to systematically study the white-light flares with a spatial resolution around 0.2 arcsec. We surveyed all the flares above GOES magnitude C5.0 since the launch of Hinode in 2006 October, 13 of them were covered by Hinode G-band observations. We analyzed the peak contrast and equivalent area (calculated via integrated excess emission contrast) of these flares as a function of GOES X-ray flux, and found that the cut-off visibility is likely around M1 flares under the observing limit of Hinode. Many other observational and physical factors should affect the visibility of white-light flares; as the observing conditions are improved, smaller flares are likely to have detectable white-light emissions. We are cautious that this limit visibility is an overestimate, as G-band observations contains emissions from upper atmosphere. Among 13 events analyzed, only the M8.7 flare of 2007 June 4 had near-simultaneous observations in both G-band and blue continuum. The blue continuum had a peak contrast of 94% vs. 175% in G-band for this event. The equivalent area in blue continuum is an order of magnitude lower than that in G-band. Very recently, Jess et al. studied a C2.0 flare with a peak contrast of 300% in blue continuum. Comparing to the events presented in this Letter, that event is probably a unusual white-light flare: a very small kernel with a large contrast that can be detected in high resolution observations.

Authors: Haimin Wang
Projects: Hinode/SOT

Publication Status: Research in Astronomy and Astrophysics, 2009, in press
Last Modified: 2008-12-03 13:24
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Evidence of Rapid Flux Emergence Associated with the M8.7 Flare on 2003 July 26  

Haimin Wang   Submitted: 2003-11-24 20:48

In this paper, we present a detailed study of the M8.7 flare that occurred on July 26, 2002, using data from Big Bear Solar Observatory (BBSO), Ramaty High Energy Solar Spectroscopic Imager (RHESSI), Transition Region and Coronal Explorer (TRACE) and Solar and Heliospheric Observatory (SOHO). This flare has similar interesting properties to a number of flares that we studied previously, such as a rapid increase of magnetic flux in one polarity, and an increase in transverse fields and magnetic shear associated with the flare. However, this event had most comprehensive observations, in particular, the high resolution high cadence BBSO vector magnetograph observations. At the time of the flare, across the flare neutral line, there was a sudden emergence of magnetic flux at the rate of 1020 Mx/hr in both the longitudinal and transverse components. The emerging flux mostly occurred at the sites of the flare. It was very inclined and led to impulsively enhanced shear in the magnetic fields. We discuss these observations in the context of flux injection model.

Authors: H. Wang, J. Qiu, J. Jing, T. Spirock, V. Yurchyshyn, V. Abramenko, H. Ji and P.R. Goode
Projects: RHESSI,Soho-MDI,TRACE

Publication Status: ApJ, (submitted)
Last Modified: 2003-11-24 20:48
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Rapid Penumbral Decay Following Three X-Class Flares  

Haimin Wang   Submitted: 2003-11-10 12:35

We show strong evidence that penumbral segments decayed rapidly and permanently right after three X-class flares. Two of the three events occurred very recently in solar active region NOAA~10486, an X17 flare on 2003 October~28 and an X10 flare on 2003 October~29, 2003. The third X2.3 flare was observed in solar active region NOAA~9026 on 2000 June~6. The locations of penumbral decay are closely related to hard X-ray flare kernels or coincide with the central region of flare ribbons seen in extra-ultraviolet (EUV) images obtained by the Transition Region and Coronal Explorer (TRACE). We present difference images high-lighting the rapid changes between pre- and post-flare states of flaring active region, which show distinct decaying penumbral segments and neighboring umbral cores becoming darker. This indicates that magnetic fields change from a highly inclined to an almost vertical configuration within approximately one hour after the flares, i.e., part of the penumbral magnetic field is converted into umbral fields.

Authors: Haimin Wang, Cheng Liu, Jiong Qiu, Na Deng, Phil Goode and Carsten Denker
Projects: RHESSI,TRACE

Publication Status: Ap.J. Letters (submitted)
Last Modified: 2003-11-18 14:18
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Abstracts by Author
Strong Transverse Photosphere Magnetic Fields and Twist in Light Bridge Dividing Delta Sunspot of Active Region 12673
Extending Counter-Streaming Motion from an Active Region Filament to Sunspot Light Bridge
High-resolution observations of flare precursors in the low solar atmosphere
Irreversible Rapid Changes of Magnetic Field Associated wi th the 2012 October 23 Circular Near-limb X1.8 Flare
Witnessing magnetic twist with high-resolution observation from the 1.6-m New Solar Telescope
Structure and Evolution of Magnetic Fields Associated with Solar Eruptions
Study of Two Successive Three-Ribbon Solar Flares on 2012 July 6
Study of Rapid Formation of a Delta Sunspot Associated with the 2012 July 2 C7.4 Flare Using High-resolution Observations of New Solar Telescope
CIRCULAR RIBBON FLARES AND HOMOLOGOUS JETS
The Relationship between the Sudden Change of the Lorentz Force and the Magnitude of Associated Flares
Rapid Transition of Uncombed Penumbrae to Faculae during Large Flares
Study of White-Light Flares Observed by Hinode
Evidence of Rapid Flux Emergence Associated with the M8.7 Flare on 2003 July 26
Rapid Penumbral Decay Following Three X-Class Flares

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