Evolution of Photospheric \edit1vector
Magnetic Field Associated With \edit1moving
Flare Ribbons as Seen by Gst
The photospheric response to solar flares, also known as coronal back reaction, is often observed as sudden flare-induced changes in vector magnetic field and sunspot motions. However, \edit1it remains obscure whether evolving flare ribbons, the flare signature closest to the photosphere, are accompanied by changes in vector magnetic field therein. Here we explore the relationship between the dynamics of flare ribbons in the chromosphere and variations of magnetic fields in the underlying photosphere, \edit1using high-resolution off-band H images and near-infrared vector magnetograms of the M6.5 flare on 2015 June 22 observed with the 1.6 m Goode Solar Telescope. We find that changes of photospheric fields occur at the arrival of the flare ribbon front, thus \edit1propagating analogously to flare ribbons. In general, the horizontal field increases and the field lines become more inclined to the surface. When ribbons sweep through regions that undergo a rotational motion, the fields transiently turn more vertical with decreased horizontal field and inclination \edit1angle, and then restore and/or become more horizontal than before the ribbon arrival. The ribbon \edit1propagation decelerates near the sunspot rotation center, where the vertical field becomes permanently enhanced. Similar magnetic field \edit1changes are discernible in magnetograms from the Helioseismic and Magnetic Imager (HMI), and \edit1an inward collapse of coronal magnetic fields is inferred from the time sequence of non-linear force-free field models extrapolated from HMI magnetograms. \edit1We conclude that photospheric fields respond nearly instantaneously to \edit1magnetic reconnection in the corona.
It is widely believed that the structural evolution and dynamics of the solar photosphere (e.g., magnetic flux emergence and shearing motion) can build up free magnetic energy in the corona that powers flares and coronal mass ejections (CMEs; Priest & Forbes 2002). The reconfiguration of coronal magnetic field due to energy release is the focus of almost all models of flares/CMEs, which generally do not consider the restructuring of magnetic and flow fields in the dense photosphere partially due to the often assumed line-tying effect (Raadu, 1972). Nonetheless, observational evidences of rapid (in minutes), significant, and permanent photospheric structural changes apparently as a response to flare/CME occurrences have been accumulated over the past 25 years from both ground- and space-based instruments (see e.g., Wang & Liu 2015 for a recent review). These include stepwise changes of line-of-sight (LOS) and vector magnetic fields (e.g., Wang, 1992; Sudol & Harvey, 2005; Wang & Liu, 2010; Liu et al., 2012a; Sun et al., 2012; Petrie, 2012; Liu et al., 2014; Song & Zhang, 2016; Sun et al., 2017; Castellanos Durán et al., 2018), morphological changes of sunspot penumbrae (e.g., Wang et al., 2004; Liu et al., 2005; Deng et al., 2005; Xu et al., 2016, 2017), changes of photospheric flow field (e.g., Tan et al., 2009; Deng et al., 2011; Wang et al., 2014, 2018a), and sunspot displacement and rotations (Anwar et al., 1993; Liu et al., 2010; Wang et al., 2014; Liu et al., 2016a; Bi et al., 2016, 2017; Xu et al., 2017). Although it is sometimes challenging to disentangle the cause-and-effect relationship between flare/CME processes and photospheric structural changes, studying this topic can shed new insights into the photosphere-corona coupling under the context of energy and momentum transportation in the flare-related phenomena, and help advance and constrain flare/CME models.
The aforementioned various aspects of photospheric evolution closely associated with flares/CMEs were largely studied separately. It might be possible that they can be accommodated by the back reaction of coronal restructuring on the photosphere and interior (Hudson et al., 2008). In this scenario, the coronal magnetic field would contract inward due to magnetic energy release (Hudson, 2000), and the central photospheric field vectors may be loosely expected to tilt toward the surface (i.e., becoming more horizontal) as a result of this contraction. Such a magnetic field change would correspond to a Lorentz-force change that is exerted at and below the photosphere (Hudson et al., 2008; Fisher et al., 2012; Petrie, 2014). Furthermore, the inward collapse of coronal field might also be accompanied by an upward turning of fields in the peripheral regions (Liu et al., 2005). These are well in line with observations of flare-induced contraction of coronal loops (e.g., Liu et al., 2009; Liu & Wang, 2009, 2010; Liu et al., 2012b; Gosain, 2012; Simões et al., 2013; Wang et al., 2018b), and with photospheric observations that flaring sites usually exhibit an enhancement of horizontal magnetic field and penumbral structure at the center, surrounded by regions of weakened and penumbrae; also, \edit1the resulting Lorentz-force change seems to be able to drive the observed surface flows and sunspot motions (see references above). It should be noted that although the overall magnetic field in three dimension (3D) must become more potential after the release of magnetic energy, the near-surface field could become more stressed after flares/CMEs (e.g., Jing et al., 2008; Liu et al., 2012a).
It is worth noting that due to resolution limitation imposed by data, a majority of previous studies rely on the comparative analysis of pre- and postflare structures. Meanwhile, this approach avoids the concern that heating from flare emissions change spectral line profiles, leading to transient anomaly in the magnetic field measurement (e.g., Patterson & Zirin, 1981; Zirin & Tanaka, 1981; Qiu & Gary, 2003; Maurya et al., 2012; Sun et al., 2017). For flare-related permanent magnetic field changes, the most prominent one could be the irreversible strengthening of in regions around central flaring PILs and between double flare ribbons. This has been corroborated by results from not only observations but also MHD modeling (e.g., Li et al., 2011; Inoue et al., 2015, 2018). However, there are only rare reports about permanent changes of photospheric magnetic and flow field in association with the spatial and temporal evolution of flare emissions, specifically, flare ribbons. Using LOS magnetograms from the \edit1Global Oscillation Network Group, Sudol & Harvey (2005) pointed out in several events that the step-like LOS field change appears to propagate at a speed similar to those of ribbons. A propagating motion of enhancement across the flaring region in a major flare event was also noticed by Sun et al. (2017) using vector magnetic field data from the Helioseismic and Magnetic Imager (HMI; Schou et al., 2012) on board the Solar Dynamics Observatory (SDO). Importantly, higher resolution data at both the chromospheric and photospheric levels are needed to fully exploit the association between flare ribbon motions and magnetic/flow field changes, which could provide major clues to the origin of flare-related restructuring on the surface.
Recently, based on chromospheric H and photospheric TiO images at unprecedented resolution obtained with the 1.6 m Goode Solar Telescope (GST; Goode et al., 2010; Cao et al., 2010; Goode & Cao, 2012; Varsik et al., 2014) at Big Bear Solar Observatory (BBSO), Liu et al. (2016a) discovered that a sunspot experiences a differential rotation, where the moving front corresponds to a flare ribbon that moves across the sunspot during the 2015 June 22 M6.5 flare event (SOL2015-06-22T18:23) in NOAA active region (AR) 12371. This finding implies that the surface rotation is directly linked to the magnetic reconnection process in the corona (Aulanier, 2016). Naturally, this revives the question of whether the photospheric magnetic field would change permanently as ribbons sweep by. Motivated by our observation, Wheatland et al. (2018) presented a theoretical model in which this kind of flare-ribbon-related photospheric \edit1change results from a downward propagating shear Alfvén wave from the coronal reconnection region. Another natural question is whether the velocity of ribbon propagation would be affected concurrently by the possible field change, since under a simplified two-dimensional magnetic reconnection model, is correlated with the vertical field on the surface as , where is the electric field strength in the reconnecting current sheet (Forbes & Priest, 1984).
Several other works have also studied this 2015 June 22 M6.5 flare from various perspectives. Mainly using data from BBSO/GST’s Visible Imaging Spectrometer (VIS) and Near InfraRed Imaging Spectropolarimeter (NIRIS; Cao et al. 2012), Wang et al. (2017) reported small preflare brightenings near magnetic channels that may be precursors to the event onset. With nonlinear force-free field (NLFFF) modeling, Awasthi et al. (2018) revealed that the initial magnetic reconnection may occur within a multiple flux rope system. Jing et al. (2017) observed a propagating brightening in the flare decay phase, which may be linked to a slipping-type reconnection. More relevant to the present study, Wang et al. (2018a) analyzed GST TiO and HMI observations and found flare-related enhanced penumbral and shear flows as well as around the PIL, which could be attributed to the coronal back reaction. Using HMI observations and NLFFF models, Bi et al. (2017) presented that the main sunspots on either side of the PIL rotate clockwise during the flaring period, when coronal fields are found to contract significantly. In addition, with NIRIS data Deng et al. (2017) studied magnetic field property and flare-related evolution of umbral fine structures, and Xu et al. (2018) showed a transient rotation of surface field vectors seemingly associated with one flare ribbon. Related discussions will be given below.
In this paper, we further investigate the 2015 June 22 M6.5 flare event by comparatively studying high spatiotemporal resolution VIS chromospheric H off-band images and NIRIS photospheric near-infrared vector magnetograms from BBSO/GST. These state-of-the-art observations are essential for achieving our goal of scrutinizing the intimate relationship between the motion of flare ribbons and possible permanent changes of the local vector field, \edit1which was not studied before. Special attention is paid to , which is the component exhibiting the most clear flare-related changes (e.g., Wang & Liu, 2010, 2015; Fisher et al., 2012). Concerning the aforementioned flare-produced transient magnetic anomaly, we note that the contamination of NIRIS polarimetry from flare emissions was claimed not to be present in this event, as no significant changes are detected in NIRIS intensity profiles (Xu et al., 2018, \edit1also see the Appendix and Figure 7). Moreover, we mainly concern ourselves with permanent magnetic field changes associated with the flare. For the purposes of data validation and results corroboration, HMI vector magnetograms are analyzed as well. In order to examine the evolution of 3D magnetic field above the flaring AR, we also build a time sequence of NLFFF extrapolation models based on HMI data. The plan of this paper is as follows. In Section 2, we first introduce observations and data processing procedures. In Section 3, we describe results derived from analyses of observations and magnetic field models, and remark on their implications. More details of structural evolution can be seen in the accompanying animations. In Section 4, we summarize major findings and discuss the results.
2 Observations and Data Processing
BBSO/GST employs a combination of a high-order adaptive optics system with 308 subapertures (Shumko et al., 2014) and the post-facto speckle-masking image reconstruction technique (Wöger et al., 2008). During 16:50–23:00 UT on 2015 June 22, GST makes observations of the then near-disk-center (8W, 12N) NOAA AR 12371 and achieves diffraction-limited resolution under an excellent seeing condition, fully covering the M6.5 flare. The data taken include images in TiO (705.7 nm; 10 Å bandpass) by the Broad-band Filter Imager with a field of view (FOV) of 70″ at 0.1″ resolution and 15 s cadence, Fabry-Pérot spectroscopic observations around the H line center at 1.0, 0.6, and 0.0 Å (0.07 Å bandpass) by VIS with a 70″ circular FOV at 0.1″ resolution and 28 s cadence, and spectropolarimetric observations of the Fe i 1564.8 nm line (0.1 Å bandpass) by NIRIS with a 85″ round FOV at 0.24″ resolution and 87 s cadence (for a full set of Stokes measurement). Bursts of 100 and 25 frames are processed for speckle reconstruction at TiO and each H line position, respectively. In this study, we aligned H 1.0 Å images with sub-pixel precision and used these H far red-wing images to best trace the evolution of flare ribbon fronts (e.g., Deng et al., 2013).
It is notable that this M6.5 flare is one of the first major flare events observed by NIRIS, which is dedicated to the 1564.8 nm doublet band observation. This spectral line is the most Zeeman sensitive probe (with the maximum splitting factor Landé ) of the magnetic field within a small height range at the atmospheric minimum opacity, the deepest photosphere (Solanki et al., 1992), and is the best spectral line for umbral magnetic field observations in the entire electromagnetic spectrum (Harvey & Hall, 1975; Livingston & Watson, 2015). Although it has a lower diffraction limit than some visible lines and the issue of thermal noise has to be mitigated, the 1564.8 nm line has lower scattered light, produces more stable images under the circumstances of atmospheric turbulence, and only exhibits emissions in some extremely energetic flares. Equipped with two Fabry-Pérot etalons in a dual-beam optical design, NIRIS captures two simultaneous polarization states and images them side-by-side onto half of a closed-cycle, helium-cooled 2048 2048 HgCdTe infrared array. Significant efforts have been devoted to develop the NIRIS data processing pipeline at BBSO (Ahn et al., 2016; Ahn & Cao, 2017), which essentially includes dark and flat field correction, image alignment and destretching for dual beams (with 60 wavelength sampling), calibration of instrumental crosstalk (by measuring the detector response to pure states of polarization passing through the telescope optics), and Stokes inversion using the Milne-Eddington (M-E) atmospheric approximation (with initial parameters pre-calculated to resemble the observed Stokes profiles). For a proper exploration of NIRIS vector field measurement, we further resolved the 180 azimuthal ambiguity using the ME0 code originally developed for Hinode vector data (Leka et al., 2009a, b) that is based on the “minimum energy” algorithm (Metcalf, 1994; Metcalf et al., 2006), removed the projection effect by transforming the observed vector fields to heliographic coordinates (Gary & Hagyard, 1990), and conducted a validation of data processing by comparing to HMI data products (see the Appendix and Figure 6). The NIRIS vector magnetograms deduced from the above procedures were used in our previous study of this event (Wang et al., 2017). Note that following the convention of Hinode, the disambiguated azimuth angle in this paper ranges counterclockwise from 180 to 180, with the direction of zero azimuthal angle pointed to the solar west. In order to minimize the seeing effect (spatially varying image motion) in the ground-based observations, in this work we also performed image destretching to intensity images from the inversion, and then applied the determined destretch to the time sequence of NIRIS vector magnetograms. NIRIS intensity images were also used to accurately co-align NIRIS vector field observations with H far red-wing images through matching sunspot and plage areas.
The SDO/HMI observations used to accompany the NIRIS data analysis are full-disk vector magnetograms at 1″ resolution and 135 s cadence (Sun et al., 2017). The HMI instrument takes filtergrams of Stokes parameters at six wavelength positions around the Fe i 617.3 nm spectral line. The Stokes inversion technique implemented to routinely analyze HMI pipeline data is also based on the M-E approximation (Borrero et al., 2011), and a variant of the ME0 code is used for azimuthal disambiguation (Hoeksema et al., 2014). The retrieved HMI data were processed (mainly for combining disambiguation results with azimuth, and deprojection) using standard procedures in the Solar SoftWare (SSW) provided by the HMI team, and were expanded in size to match and align with NIRIS. For NLFFF extrapolations, we remapped HMI magnetograms of the entire AR at original resolution using Lambert (cylindrical equal area) projection centered on the middle point of the AR. After adjusting the photospheric boundary with a preprocessing procedure to better suit the force-free condition (Wiegelmann et al., 2006), we constructed a time sequence of NLFFF models using the “weighted optimization” method (Wheatland et al., 2000; Wiegelmann, 2004) optimized for HMI data (Wiegelmann & Inhester, 2010; Wiegelmann et al., 2012). The calculation was made using 2 2 rebinned magnetograms within a box of 472 224 224 uniform grid points (corresponding to about 348 165 165 Mm). In addition, soft- and hard X-ray (HXR) emissions of the 2015 June 22 M6.5 flare were recorded by the Geostationary Operational Environmental Satellite (GOES)-15 and Fermi Gamma-Ray Burst Monitor (Meegan et al., 2009), respectively. In GOES 1.6–12.4 keV energy flux, the flare of interest starts at 17:39 UT, peaks at 18:23 UT, and ended at 18:51 UT, with the first main peak in Fermi 25–50 keV HXR flux at 17:52:31 UT (Liu et al., 2016a).
3 Analyses and Results
Figure 1 presents an overview of the evolution of chromospheric ribbons and photospheric field in the 2015 June 22 M6.5 flare. Here the FOV of BBSO/GST covers the central core region of the flare. From animations of VIS H 1.0 Å and NIRIS (available in the online journal), it can be clearly seen that (1) two main flare ribbons move away from the PIL and sweep through two sunspot regions of opposite polarities (also see Figures 1(a) and (b)), and (2) both sunspots undergo a clockwise rotation during the flare period, which is unambiguously demonstrated with flow tracking using the differential affine velocity estimator for vector magnetograms (DAVE4VM; Schuck, 2008) method (see Figure 1(c)). This is consistent with previous studies using TiO and HMI observations (Liu et al., 2016a; Bi et al., 2017). Interestingly, the southern part of the eastern ribbon apparently slows down when approaching the center of the eastern rotating sunspot (cf. Figures 1(b) and (c)). A similar but less obvious slowdown is discernible for the central part of the western ribbon. A comparison between pre- and postflare images (see Figures 1(d) and (e), and also the animation) shows that there is a pronounced enhancement of in an extended region mainly along the PIL (red-colored region in Figure 1(f)). To better disclose the evolution, we make fixed difference images relative to a preflare time. From the time-lapse movie, it is remarkable to notice that the enhancement of not only shows up around the PIL (Wang et al., 2018a), but also moves away from the PIL and spreads across the flaring region, mimicking the flare ribbon motion. More intriguingly, a negative front, meaning a transient weakening of , appears to precede the moving enhancement, especially at the southern portion of the eastern ribbon and the entire western ribbon.
To accurately characterize the evolution associated with the flare ribbon motion, time-distance maps along slits S1–S4 (drawn in Figure 1(b)) based on the fixed difference images are presented as the backgrounds of Figure 2. They are overplotted with contours of the same time-distance maps but based on the running difference H 1.0 Å images that highlight the ribbon fronts. We constructed these slits by orientating elongated windows (with various length but a common short side of 0.78″) approximately perpendicular to the observed ribbon motion at 26 counterclockwise from the solar west, and averaged the pixels across the short sides. The distance shown is measured from the ends of slits closest to the PIL. In Figure 3, the temporal evolution of H 1.0 Å emission (blue) is compared with that of vector magnetic field (red; in terms of , , inclination angle relative to the vertical direction, magnetic shear, and azimuth angle) at several representative positions P1, P2a, P2b, and P3 along the slits (as marked in Figure 1(b); values averaged over 7 7 pixels centered on them). Here the magnetic shear for evaluating the nonpotentiality is computed as (Wang et al., 1994, 2006), where and , with the subscript representing the potential field, which we derived using the fast-Fourier transform method (Alissandrakis, 1981). When appropriate, we also fit these time profiles of magnetic properties with a step function (green lines; Sudol & Harvey, 2005). Based on these results, we observe the \edit1following.
Along the slit S1, there exists a close spatial and temporal correlation between the motion of the eastern flare ribbon and the enhancement of (Figure 2(a)), especially after the time of the first main HXR peak (vertical dashed line). At P1 (see Figure 3, first column), with the arrival of ribbon front the photospheric field turns more inclined relative to the surface, with and inclination angle increased stepwise by 24424 G and 6.40.6 in 0.5 and 1.5 minutes, respectively; also, magnetic shear sharply increases by 250% but then returns to the preflare level in about 20 minutes. In contrast, evolved more gradually without an abrupt change. In the meantime, a transient increase of azimuth angle meaning a temporary counterclockwise rotation of field vectors can be noticed (Xu et al., 2018).
Along the slit S2 across the center of the eastern rotating sunspot, the propagation of the eastern flare ribbon exhibits a prominent deceleration, and the arrival of the ribbon front is coincident with a transient decrease of followed by an increase (see Figure 2(b)). At P2a (see Figure 3, second column), and inclination angle temporarily decrease by 300 G and 8 and then increases by 600 G and 13 in 30 minutes, respectively; meanwhile, magnetic shear shows a step-like increase by 86% in 15 minutes. After a transient increase like at P1, azimuth angle begins to decrease, connoting the observed clockwise sunspot rotation (Liu et al., 2016a) that drags the magnetic field with it. Compared to P2a, the magnetic field evolution at P2b (around the rotation center) bears a resemblance but displays a more prolonged decrease of and inclination angle; remarkably, at P2b undergoes a permanent increase of 26620 G in 13 minutes around 18 UT (see Figure 3, third column), when the speed of the flare ribbon has evidently reduced (Figure 2(b)). Since darker umbrae evince stronger vertical fields (Martínez Pillet & Vázquez, 1993), the irreversible increase of of this rotating sunspot is also evidenced by a 7% decrease of its overall intensity in TiO and 1564.8 nm after the flare (Liu et al., 2016a; Deng et al., 2017). \edit1A line profile analysis further corroborates that the transient decrease of (and also increase of ) at P2b is irrelevant to magnetic anomaly due to flare heating (see the Appendix and Figure 7). Assuming a uniform reconnecting electric field along the entire eastern ribbon, the observed slowdown of flare ribbon motion with concurrent increase of at a portion of the ribbon could be expected (Forbes & Priest, 1984). From about 18:08 UT, the northern section of the ribbon curves southward and overtakes the motion of the ribbon along S2.
In Figure 4, we further compare the locations of the eastern flare ribbon front, , and the horizontal Lorentz-force change (Fisher et al., 2012), at a time close to the first main HXR peak. Note that the newly brightened ribbon region (e.g., the ribbon front) is cospatial with the region of decreased . This, together with the increased at the region just swept by the ribbon, yields a vortex pattern in the map. Obviously, the torque provided by this vortex has the same direction (i.e., clockwise) as the observed sunspot rotation. This implies that the decrease preceding its increase may create a moving horizontal Lorentz-force change to drive the differential sunspot rotation as observed (Liu et al., 2016a).
Along the slit S3, the motion of the western ribbon is correlated with magnetic field changes in a way similar to those found along the slit S2, e.g., showing a transient decrease followed by an increase of and inclination angle (see Figure 2(c) and light curves of P3 in the fourth column of Figure 3). A ribbon deceleration together with a prolonged decrease of also seems to be present along the slit S4 across the center of the western rotating sunspot (see Figure 2(d)). It is worthwhile to mention that despite of a lower resolution, vector magnetograms from HMI show very similar magnetic field changes related to flare ribbon motions as described above (see the Appendix and Figure 8), which substantiates the NIRIS results.
Finally, we investigate the flare-related coronal field evolution in terms of the distribution of horizontal component of the total electric current density in several vertical slices to the extrapolated 3D coronal magnetic field. These slices intersect with the surface at locations of the same slits S1 and S3 as above through the regions of flare ribbons and another slit SC perpendicularly across the central PIL (as denoted in Figure 1(b)). Plotted in the top and bottom rows of Figure 5 are the distributions of for the pre- and postflare states, respectively, in these vertical slices, which are superimposed with arrows representing the transverse magnetic field vectors. From the results and also the supplementary animation spanning the flaring period, it transpires that a downward collapse of coronal field occurs intimately associated with the flare \edit1(e.g., Sun et al., 2012; Liu et al., 2012a, 2014). This is visualized by the dramatic change of the coronal currents above the PIL, from a vertically elongated source reaching 12″ to a substantially enhanced, horizontally elongated source concentrated close to the surface below 9″ (cf. Figures 5(c) and (d)). \edit1We further show that the collapse is also manifested by the clockwise (counterclockwise) turning of magnetic field vectors in the east (west) side of the PIL (except that the near-surface region in S3 has a clockwise turning), which leads to a more horizontal (i.e., inclined) configuration of magnetic fields at and above regions of the PIL and flare ribbons, conforming to the observed surface enhancement therein. We note that (1) field vectors in the far east portion of S1 (and also the upper portion of S3) become more vertical after the flare. This reflects the fact that in the outer flaring region, is observed to decrease (see Figures 1(f) and 2(a)) together with weakened penumbral features (not shown), which may be coherent with the collapse of the central fields (e.g., Liu et al., 2005). (2) Although not well demonstrated by the present extrapolations, it is plausible to expect that a time sequence of coronal field models with higher spatial and temporal resolution might show the successive turning of field vectors with the motion of flare ribbons.
4 Summary and Discussions
In this paper, we take advantage of BBSO/GST high-resolution observations of both chromospheric ribbons in VIS off-band H and NIRIS photospheric vector magnetic fields in near infrared during the 2015 June 22 M6.5 flare to carry out a detailed investigation of photospheric vector magnetic field changes with related to flare ribbon motions, \edit1which were not studied before. This large and complex event shows not only the separation of flare ribbons but also the flare-related rotations of sunspots. We analyzed the permanent surface magnetic field changes in the flare ribbon regions with a focus on , using time-distance maps and temporal evolution plots. We also explored the 3D coronal restructuring with aid from the NLFFF modeling based on SDO/HMI vector magnetograms. Major findings are summarized as follows.
In the photosphere, increases with the flare occurrence and this enhancement propagates away from the central PIL across the flaring region, exhibiting a close spatial and temporal correlation with the flare ribbon motion especially after the first main HXR peak (Figure 2). As seen in several representative positions (Figure 3), the strengthening of (by 300 G) at the arrival of the flare ribbon front is accompanied by an increase of inclination angle (by 6), indicating that magnetic field becomes more inclined to the surface; also, the nonpotentiality as represented by magnetic shear generally enhances.
In the corona, a downward collapse of coronal magnetic field by 3″ toward the photosphere is clearly portrayed by the evolution of the vertical profiles of around the PIL \edit1(Sun et al., 2012; Liu et al., 2012a, 2014), which changes from a vertically elongated source to an enhanced, horizontally elongated source close to the surface (Figure 5). Correspondingly, above the PIL and flare ribbon regions, magnetic field becomes more inclined, which is consistent with the observed enhancement of . We surmise that a successive turning of field vectors associated with the flare ribbon motion might be visualized given coronal field models with a sufficiently high resolution.
The increase of at flaring PILs between flare ribbons has been known from the previous observations (e.g., Sun et al., 2012; Liu et al., 2012a; Wang et al., 2012). The distinctive finding made in this investigation is that enhances not only at the PIL region, but at the locations of the flare ribbon fronts. As the flare ribbons move away from the PIL, such enhancements also propagate successively with the ribbons. This discovery of the flare-ribbon-related photospheric field changes could be made owing to the high resolution of NIRIS observations, and is also substantiated by the HMI data. Since it has been well established that flare ribbon fronts are the footpoints of the newly reconnected field lines in the corona, the vector field changes spatiotemporally correlated with the ribbon fronts must be a nearly instantaneous response of photospheric fields to the coronal restructuring, specifically, the reconnection of individual flux bundles.
We also want to point out that the correlation between the eastern flare ribbon and its related vector field change is complicated by the fact that ahead of the eastern ribbon, there is another elongated small brightening that propagates from north to south (see the movie), along a line of high values of the squashing factor (Titov et al. 2002; see the Appendix and Figure 9(a)). The high- lines correspond to the footprints of quasi-separatrix layers (Démoulin et al., 1996, 1997), which are known to be favorable positions of flare ribbons. This brightening joins with the main eastern ribbon in the north (out of the FOV of GST) to form a continuous ribbon structure. To check whether this extra flare ribbon introduces magnetic field changes, we place a slit S5 perpendicular to the northern portion of the eastern ribbon (Figure 9(a)) and repeat the analysis as done in Figure 2. Both the results using NIRIS and HMI data evince that the enhancement of not only appears to follow the movement of the main eastern ribbon, but also occurs ahead of it, distending to the region of the extra ribbon (see Figures 9(b) and (c)). We consider this as an additional piece of evidence that the photospheric vector magnetic field may respond nearly instantaneously to the coronal reconnection.
There are a few models that may help understanding the present observations. The series of force-free field models give only snapshots of equilibrium states rather than dynamic evolution; nevertheless, the disclosed redistribution of electric current system may reflect a coronal field restructuring following magnetic energy release in the corona (e.g., Hudson, 2000). A back reaction of such coronal magnetic reconfigurations on the photosphere and interior may be expected (Hudson et al., 2008), but it only loosely points to a more horizontal photospheric field, i.e., an increase of ; further, it does not necessarily explain why the magnetic shear should also increase. The shear Alfvén wave model (Wheatland et al., 2018) can explain both the increase of and magnetic shear, in which the shear Alfvén waves launched from the coronal reconnection region travel downward to impact the flare ribbon regions. In 3D, these waves correspond to the torsional Alfvén waves so that the rotation of plasma and magnetic field at the ribbon location is also expected. In addition, we have presented an idea that the decrease preceding its increase may create a moving horizontal Lorentz-force change (Figure 4) to drive the differential sunspot rotation as observed (Liu et al., 2016a). It remains puzzling why decreases at the region of the newly brightened ribbon.
Our main intention of this study is to present the details of the new phenomenon of the flare-ribbon-related photospheric magnetic field changes. It remains to see whether these vector field changes as found in this event are a generic feature of all flares or simply a peculiarity of this event. Certainly, more simultaneous high-resolution observations of chromospheric flare ribbons and photospheric vector magnetic fields throughout the flaring period are much desirable to further elucidate the photosphere-corona coupling in the flare-related phenomena.
Appendix A Appendix information
As a validation of GST/NIRIS data processing procedures, in Figure 6 we compare vector magnetograms of NOAA AR 12371 from NIRIS and SDO/HMI obtained at about the same time right before the 2015 June 22 M6.5 flare. The HMI data used is the full-disk vector magnetic field product (
hmi.B_720s), processed using standard procedures in SSW. It is clear that for the flare core region (white box in Figures 6(a) and (b)), both and field vectors derived from NIRIS and HMI measurements have a high correlation (see Figures 6(c)–(f)). The slope of 0.8 shown by the scatter plots indicates that NIRIS tends to produce stronger fields, presumably due to the fact that NIRIS observes at a deeper atmosphere than HMI.
1In Figure 7, we present 1564.8 nm Stokes profiles at P2b before and after the arrival of the flare ribbon, at 17:34:03 UT and 18:04:34 UT, respectively. Comparing the results, we see that the Stokes component shows no clear and systematic changes (Figure 7(a)), suggesting that flare heating does not alter the spectral line profiles. In contrast, the Stokes combination that measures the overall linear polarization magnitude (e.g., Leka & Steiner, 2001; Deng et al., 2010) obviously weakens (Figure 7(b)), while the Stokes component representing the circular polarization enhances (see Figure 7(c) and note the difference profile in orange). As this AR is close to the disk center at the time of the M6.5 flare, these changes of Stokes profiles are consistent with the observed decrease (increase) of the horizontal (vertical) field at this location, as presented in Figure 3 (third column).
The presented analyses applied to NIRIS data were also carried out using HMI vector magnetograms, and generally similar results were obtained. In Figure 8, we show the time-distance maps along the slits S1–S4 based on the 135 s cadence HMI data. The results, despite of having a lower resolution, show evolutionary patterns that are almost identical to those obtained using the NIRIS data (see Figure 2).
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