Signatures of Magnetic Reconnection at the Footpoints of Fan-shaped Jets on a Light Bridge Driven by Photospheric Convective Motions

The fan-shaped jets on the LB can be found in the region R1 in Figures 1(b) and (c), corresponding to the velocity derived from the Hα line using 11 points and the IRIS SJI in the 1400 Å passband, respectively. The blue and red colors in the LOS velocity images indicate the plasma is showing upward and downward motions on the fan-shaped jets at a relatively cool temperature (less than 10,000 K). The associated online animation clearly reveals this evolution of upflow and downflows. Signatures of the fan-shaped jets can also be identified in the IRIS 1400 Å passband and the six EUV channels of AIA (94, 131, 171, 193, 211, and 335 Å), indicating that they have hot materials, covering from 10⁵ to 10⁶ K. The cool and hot components seen from the simultaneous multiwavelength observation reveal the multithermal nature of the fan-shaped jets. Moreover, the jets' fronts show brightenings (the region marked by the arrow in Figures 1(d)–(i)) in the above-mentioned passbands, while the footpoint of the jets on the LB has some transient brightenings, especially in the IRIS 1400 Å image.

Along the trajectory of the jets, a slice, labeled as L1, is selected to study the dynamical response of the different solar atmosphere layers to the jets. The corresponding spacetime diagram is presented in Figure 2. The first thing we notice is that the jets follow a parabolic trajectory. This is evident when following the shape of the brightenings (or bright fronts) observed in the six EUV channels of AIA (Figure 2, panels a to f), for example, from 17:27 to 17:36 UT. Evolution of the LOS velocity in the Hα line in Figure 2(g) reveals that the jets have blueshifted (upward motion) at the beginning, and then they gradually become redshifted (downward motion) later. Combining with the parabolic trajectory in the plane of the sky (POS), one can imagine that the jets leave the solar surface with a high initial speed, decrease their velocity due to gravity, reach their maximum height, and finally fall back to the solar surface again. The maximum speeds in the LOS and POS directions at the beginning of the jet's eruption are about 25 and 15 km s⁻¹ near 17:27 UT. The corresponding values near 17:43 UT are about 30 km s⁻¹ each. The two times are selected because line wing enhancements in the Hα line are found in the meantime. The total speeds (square root of the summation of LOS velocity squared and POS velocity squared) of the launched jets at the two times are about 29 and 42 km s⁻¹, which are much larger than the typical local sound speed in the chromosphere (less than 10 km s⁻¹). The large initial velocity values are likely to promote shock waves, so the bright fronts visible in IRIS and SDO can be interpreted as the effect of shock compression of TRs and coronal material as the cool jet slams into it (Robustini et al. 2016). The fast nature of shocks can also produce departures from statistical equilibrium in the different populations of the emitting ions. Therefore, enhanced brightenings in the bright front of the jets could also be related to nonequilibrium effects as they occur in surges due to the quick action of entropy sources (Nóbrega-Siverio et al. 2018).

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Figure 2(h) shows temporal sequences of the Hα spectrum of VIS at the location marked by the dashed line in Figure 2(g). Both wings of the Hα spectrum are extremely enhanced near 17:24 and 17:47 UT, while the line core remains unperturbed, similar to the profiles of Ellerman bombs (EBs; Ellerman 1917; Robustini et al. 2016; Chen et al. 2017; Hong et al. 2017a, 2017b; Song et al. 2017). Generally, the appearance of EB spectra indicates that magnetic reconnection is happening in the upper photosphere or the lower chromosphere, namely near the temperature minimum region. Comparing Figure 2(h) with Figure 2(g), one can find that the line wing's enhancements in the Hα spectrum are associated with the blueshift as shown in the LOS velocity, especially near 17:47 UT. At the same time, all of the EUV channels also show brightenings. The velocities found in the jets, together with the enhancements of the wings in the Hα spectrum, seem to indicate that the fan-shaped jets studied in this paper are caused by magnetic reconnection.

It is worth mentioning that the LOS velocity in the Hα line (Figure 2, panel g) shows a continuous blueshift from 17:42 to 17:54 UT, lasting for 12 minutes. Meanwhile, wave- or oscillation-like patterns occur four times at the base of the slice L1 covering the LB and adjacent umbra, evidenced by the alternating appearance of the blueshifts and redshifts. The long time interval of the blueshift in the jets possibly indicates that they are not driven by the oscillations. The brightenings in the bright fronts of jets in the AIA 171, 193, and 211 Å channels have some fine structures, manifested as two mini jet-like (marked by the two arrows in Figure 2(f)) brightenings on the background of the jets' fronts near 17:47 UT, together with the brightening wings in the Hα line. The interval of the two jets is about 1 minute. Moreover, two similar mini jet-like structures with an interval of about 2 minutes can also be found near 17:23 UT (marked by the two arrows in Figure 2(b)) and are most clearly seen in the 131 Å channel, but they are present in the others. The mini jets in such a short time interval are not possibly caused by the leakage of waves, supporting its correlation with magnetic reconnection. These mini and short-lived jets possibly reveal some fine structures during the magnetic reconnection process.

The line wing enhancement in the Hα line occurs not only at the point shown in Figure 2(h), but also at other points on the footpoints of the jets located on the LB. Figure 3 presents the brightenings on the LB observed at −0.8 Å from the Hα line center for different stages of the evolution. In Figures 3(a) to (c), they show that the jets move to higher layers, evidenced by the increasing length of the jets along their propagation direction. Note that the footpoints of the jets get bright and look like a ribbon at 17:20:38 UT. From Figures 3(d) to (f), one can find that the ribbon-like brightening propagates along the LB at a speed of about 9.46 km s⁻¹, indicated by the green solid line. Meanwhile, the jets on the LB move in the same direction as the brightenings (marked by the arrows), which is almost perpendicular to the jets' propagation direction. According to Figures 3(g)–(i), the jets occur first at both ends of the LB with remarkable brightenings at their footpoints. Then, the brightenings at both ends move in opposite directions with a speed of about 30 km s⁻¹. At 17:48:53 UT, one can also see a long ribbon appearing along the entire LB (see the online animation associated with Figure 3). The jets are launched along the entire LB soon afterward. The ribbon motions seem to be the signatures of slipping reconnection, which is generally found in the flare ribbons (Aulanier et al. 2006; Li & Zhang 2014; Li et al. 2016; Jing et al. 2017). The result reveals the three-dimensional nature of the fan-shaped jets. The magnetic reconnection occurs successively at different locations on the LB so that we can see the brightening motions. The width of the ribbon is about 03, much smaller than the spatial resolution of AIA.

In this coordinated campaign, IRIS merely has one scan during the time interval of observations with 64 steps. The upper panels of Figure 4 show the slit scanning image near the line centers of the Si iv 1402.77 Å and Mg ii K2v lines at 2795.94 Å. Similar to the bright features of SDO/AIA mentioned above, both the footpoints and the front of the jets are also bright in Si iv 1402.77 Å and Mg II K2v. Note that although Mg ii K2v and Hα are both chromospheric lines, the latter does not show any bright fronts, possibly due to the increase in source function at the selected wavelength position (Leenaarts et al. 2013). Those brightenings suggest that there is heating of the chromospheric and TR region plasma on the jets.

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In order to analyze the IRIS spectral information, we choose three points on the bright fronts of the fan-shaped jets (marked by P1, P2, and P3) and two points on their footpoints (P4 and P5). Their corresponding spectrum in C ii 1335, Si iv 1394, and 1403 Å is shown in the panels in the lower two rows of Figure 4. The black line is the reference spectrum from the average of the bottom rows of the slit scanning image with a region of 12'' ×  2''. The three points on the jets' bright fronts show three different types of spectra, and all of them have line broadening relative to the reference spectral profiles, as seen from the Si iv 1403 and 1394 Å lines. The green line, located at the brightest region (P2), has the highest emission. The blue line for P3, whose emission is at the intermediate level, shows a strong blue asymmetry in all of the lines of Si iv 1397, 1403 Å and C ii 1335 Å characterized by multicomponent profiles. The red line located at P1 has the weakest emission among the three points and manifests as two Gaussian emission structures, which is an indication of plasma motions with two components. This idea could be supported by the results of Nóbrega-Siverio et al. (2017, 2018). In those papers, the authors show that for surges, the alignment of the LOS with the orientation of the ejections is key to understanding the remarkable TR brightenings as well as the multicomponent profiles. This result for surges could also be applied for our jets, so the multicomponent profiles found may be due to the integration of the multiple-crossing emitting layers of the folded TR on the fan-shaped jets with different LOS velocities.

The footpoints of the fan-shaped jets (P4 and P5 in Figure 4) show line broadening and intensity enhancement. We also find absorption structures of Ne ii 1393.33 and 1335.2 Å superimposed on the lines of Si iv 1393.76 Å and C ii 1335 Å, especially for P5 (green line) with a higher emission. The appearance of the absorption features in enhanced TR spectral profiles is an indication of a hot reconnection region below the dense and cold plasma (Peter et al. 2014; Tian et al. 2018). We also checked the Hα images at −0.8 Å from the line center and found that there are intermittent intensity enhancements at the same time. These observational characteristics also support the idea that the fan-shaped jets are caused by magnetic reconnection. Note, however, that the absorption features observed here are not akin to the one discussed by Tian et al. (2018). In their paper, significant line broadening of the Si iv, C ii, and Mg ii h&k lines is found, and the Mg h&k lines show strong absorption due to large opacity. Almost all of the lines have absorption line structures. In our data, no significant absorption structures are found in the Si iv 1403 Å line, possibly because the emission at P4 and P5 is relatively weaker.

Concerning Hinode/SP data, two scans were performed from 17:15 to 17:55 UT. Figure 5(a) shows the longitudinal magnetic field in level 2 for region R1 as shown in Figure 1(a). It can be found that its value is much lower than that in the pore or umbra. The arrows superimposed on the panel indicate the strength and direction of the transverse magnetic field. The transverse field changes its direction on LB, indicating the existence of magnetic shear. The current sheet is formed at the magnetic shear region, storing enough nonpotential energy for the fan-shaped jets (Toriumi et al. 2015a). We also obtained the inclination angles on the LB and show them in Figure 5(b). The inclination angles, with values close to 90°, are much larger than those in the nearby pores and umbra, revealing an almost horizontal magnetic structure.

We derived the horizontal velocity on the LB with the high-resolution TiO images from GST following the method explained by Liu et al. (2018). The result is arranged in Figures 5(c) and (d), where the corresponding background images are the SP LOS velocity and the TiO image, respectively. The presented horizontal velocity corresponds to the times 17:27 and 17:44 UT. When plotting the horizontal velocity, a running average of the ones calculated from the method of Liu et al. (2018) with a time interval of 100 s is adopted so as to reduce the noise. Two large-scale flow structures occur during the 40 minutes of observation (see the online animation associated with Figure 5). They move in the opposite direction from almost the center of the LB, with the upper left to one end of the LB and that in the lower right to the other end of the LB. The maximum speed of the horizontal flow is about 1.8 km s⁻¹. From the background image in Figure 5(c), one can also find that the upper left part of the LB has an upflow being blueshifted, while the lower right part has a downflow. In the upper left part (region R2), the plasma on the LB shows an upward motion and changes to downward motions at the boundary between the LBs and the nearby pore, indicating the convective motions of the LBs.

The small-scale photospheric convection on the LB can be found from the evolution of the TiO images in region R2, covering the upper left parts of the LB. The result is shown in Figure 6. From 17:20 to 17:25 UT, one can find some fragmental structures near the location of P6. At time 17:27, they develop into a new convection cell that expands after comparing with the subsequent images. In addition, the direction of horizontal velocity in the region as shown in Figure 5(c) shows that the flow moves from the cell center to its boundary. Combining the upflows in the LOS velocity, it indicates the emergence of a new convection cell. Intensity enhancements at 17:27 UT near P6 on the TiO image are obvious. The red contour marks the brightenings seen from the simultaneous Hα −0.8 Å passband. It indicates that there is a response in the photosphere during the EBs, probably related to the magnetic reconnection heating. From 17:40 to 17:51 UT, the motion of a dark lane (marked by the arrow) is notable. It first occurs near the boundary of the penumbra and the LB, then moves to the upper left direction, and finally reaches the upper boundary of the LB. At the times 17:49 and 17:51 UT, one can also find intensity enhancements near the dark lane on the TiO images, which are accompanied by the brightenings in the Hα −0.8 Å passband. The scenario shows that the small-scale photospheric convection plays an important role in triggering the EBs and subsequent fan-shaped jets. Recently, Pasechnik (2018) also reported an increase in the core intensity of the studied photospheric lines, correlated spatially with the enhanced emission in the Hα line wing.

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From the SP longitudinal magnetograms in level 2, we did not find small-scale magnetic structures with opposite polarity on the LB. However, abnormal Stokes V profiles do exist in the level 1 data. The locations of the abnormal Stokes V profiles are marked by P6 and P7 in Figure 6. The corresponding Stokes I, Q, U, and V profiles can be found in Figure 7, acquired at 17:27 and 17:44 UT, respectively. Note that there is an enhanced emission at the Hα −0.8 Å passband near the two times, possibly indicating the line wing brightening has a close connection with these abnormal Stokes V profiles. The asymmetry in the red wings of Stokes I profiles is obvious in Figure 7. The abnormal Stokes V profiles have three lobes, which is an indicator of the complexity of the LB magnetic field configuration. Taking a look at the TiO images, we can find that there are fine structures in both the P6 and P7 regions, pointing again to the highly complex LB environment. Similar abnormal Stokes V profiles have also been found in the penumbra (Bellot Rubio et al. 2004; Ruiz Cobo & Asensio Ramos 2013; Lagg et al. 2014; Franz et al. 2016), which is thought to be the indication of returning magnetic flux. Louis et al. (2014) and Felipe et al. (2017) also reported the Stokes V profiles with multiple lobes on the LBs. The three lobes in Stokes V profiles are generally interpreted as the signature of opposite polarity in the LOS direction.

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In order to fit the observed Stokes I, Q, U, and V profiles, we employ the NICOLE code (Socas-Navarro et al. 2015), which is very powerful in dealing with these abnormal profiles thanks to its flexibility in accommodating complex atmospheric models. The level 2 data of Hinode/SP are generated with the analytical solution of the polarized radiative transfer equation under the assumption of a MilneCEddington atmosphere model, which is unable to generate the asymmetric profiles. The solid lines in Figure 7 are the best-fitting results. In total, eight nodes in the temperature, four nodes in the LOS bz, and four nodes in the LOS velocity are used in the inversion. The LOS bx and by have one node. NICOLE can reproduce most of the observing profiles, especially the three lobes in Stokes V profiles. We present the atmospheric parameters output from NICOLE in Figure 8.

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The error bars in the figure have been computed with a Monte Carlo approach, by performing many inversions with different initializations. The errors reflect the statistical spread of the solutions that converged to a fit of quality similar to the best inversion. This approach is optimal for studying the uncertainties due to line sensitivity or possible ambiguities in the solution (possible uniqueness issues); it is constrained to the specific model scenario chosen in this work. For example, it does not rule out other possible solutions obtained with a larger number of nodes or a more complex scenario.

The fitted temperature at P6 covering the enhanced emission in the TiO images also shows a temperature enhancement near the height of 250 km, reconfirming the above conclusion that the photosphere is disturbed during the EBs. The temperature at P7 also has a temperature enhancement, but the value is about 500 K lower than that at P6. From the lower panel in Figure 6, P7 contains the dark lane and its nearby convective cells with higher emission. The fitted temperature at P7 possibly indicates the hot (the temperature enhancements) and cool (dark lane) components. The bx and by are 0 and 600 G at P6, while their values are 320 and −800 G at P7, respectively. The key point in generating the three lobes in the Stokes V profiles is to add stratification with height for the bz and LOS velocity. The bz in the low height has a negative value, while it changes to positive values in the upper height. The scenario reveals that there are mixed polarities at P6 and P7. Regarding the LOS velocity, it manifests as redshift in low height while being near zero in the upper height. The redshift (downflow) corresponds to negative polarity, contrary to the adjacent magnetic fields. The scenario is possibly consistent with the result from Lagg et al. (2014), who show that the downflowing material at the boundary of the LB is able to drag down magnetic field lines, creating a region where additional heating might occur. Felipe et al. (2016) also claimed that the LBs are in a plasma-dominated region with a plasma β larger than 1, and the convective flow observed on the LBs is able to bend the magnetic field lines to produce field reversals.