ENHANCEMENT OF A SUNSPOT LIGHT WALL WITH EXTERNAL DISTURBANCES

Above sunspot light bridges, some dynamic activities, such as brightenings and surges, have been observed in the lower solar atmosphere (Asai et al. 2001; Shimizu et al. 2009; Louis et al. 2014; Tian et al. 2014; Toriumi et al. 2015b; Robustini et al. 2016). By examining the Doppler characters or the intensity variations in light bridges, a number of authors found evidence for the existence of oscillations in light bridges (Sobotka et al. 2013; Yuan et al. 2014; Yuan & Walsh 2016). With the high tempo-spatial resolution observations from the Interface Region Imaging Spectrograph (IRIS; De Pontieu et al. 2014), Yang et al. (2015) have found an ensemble of bright structures in the lower atmosphere rooted in a light bridge of NOAA Active Region (AR) 12192, and named this ensemble light wall. The most distinct characteristic of the light wall is the existence of a much brighter wall top observed in IRIS 1330 Å images. The light wall showed evidence of oscillations in height, appearing as successive upward and downward motions of the wall top. This kind of oscillation, i.e., joint rising and falling motion of neighboring bright structures, was also noted by Bharti (2015). We have interpreted that these oscillations of the light wall are due to the leakage of p-modes from below the photosphere (Yang et al. 2015). As the global resonant acoustic oscillations, the p-modes are deemed to leak sufficient energy to power shocks and thus drive upward flows (De Pontieu et al. 2004; Klimchuk 2006). Hou et al. (2016a) examined IRIS observations between 2014 December and 2015 June and found that most light walls originate above light bridges.

In this Letter, we report our findings that the light wall can be significantly disturbed by external events, such as falling material. Due to the external disturbances, the wall base brightens and the wall height increases.

We use two series of slit-jaw images (SJIs) observed by IRIS on 2016 February 12 in 1330 Å. The 1330 Å passband includes a strong emission from the C ii 1334/1335 Å lines, which are formed in the chromosphere and lower transition region and the continuum emission from the upper photosphere and lower chromosphere. The first series of SJIs were obtained from 17:23 UT to 18:34 UT and the second one from 22:15 UT to 23:25 UT. Both of the two series have a cadence of 10 s and a plate scale of 0166. Their field of view (FOV) is 119'' × 119'', covering the main sunspot of NOAA AR 12497. The two data sets have the same region (i.e., AR 12497) as target with solar-rotation tracking. The spectral data were obtained on the large coarse four-step raster mode, and the slit crosses the path of the falling material.

During the two IRIS observational periods, we also obtained simultaneous intensity maps and magnetograms from the Helioseismic and Magnetic Imager (HMI; Scherrer et al. 2012; Schou et al. 2012) on board the Solar Dynamics Observatory (SDO; Pesnell et al. 2012). The full-disk HMI data have a plate scale of 05 and a cadence of 45 s. In addition, we use the 1600 Å images from the SDO Atmospheric Imaging Assembly (AIA; Lemen et al. 2012). The pixel size and cadence of the 1600 Å line are 06 and 24 s, respectively. Each sequence of the HMI and AIA data are calibrated to Level 1.5 with the standard procedure aia_prep.pro and are differentially rotated to a reference time. The reference times for the two sequences are 17:46 UT and 22:38 UT, respectively.

An overview of AR 12497 in IRIS 1330 Å is shown in Figure 1(a). In the HMI intensity map (panel (b)) at 17:43 UT, there is a light bridge (outlined by the red parentheses) across the main sunspot. In the corresponding HMI magnetogram, the magnetic field at the light bridge is much weaker than the surrounding negative fields. The light bridge can be identified both in the AIA 1600 Å image (panel (d)) and IRIS 1330 Å image (panel (e)). Moreover, a light wall, i.e., an ensemble of bright structures rooted in the light bridge, is observed in the IRIS 1330 Å image, as denoted by the square brackets. The bright top of the light wall is somewhat conspicuous. The light wall cannot be identified in the AIA 1600 Å image.

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3.1. Disturbance by the Falling Material

In the south–west area near the light wall, a C6.8 flare occurred, as outlined by the blue circle in Figure 2(a). After that, a great deal of material was ejected into the corona (outlined by the blue dotted circle in the animation of Figure 2), and some of it descended downward to the solar surface (denoted by the arrows in the animation of Figure 2). At 17:42:10 UT (see panel (b1)), the wall base and wall top are marked by the green dashed and dotted lines, respectively. Then, the falling material (indicated by the blue arrows in panels (b2) and (b3)) fell toward the base of the light wall. When the falling material reached the wall base at 17:44:28 UT, the landing point (outlined by the blue rectangle in panel (b3)) brightened. Meanwhile, more material as denoted by the green arrows in panel (b3) was falling down. This material hit the wall base at 17:45:17 UT, and there was also a brightening at the landing point (outlined by the green circle in panel (b4)). Then, 39 s later, the brightening became more obvious (panel (b5)). Subsequently, the wall top began to rise (see panels (b5)–(b7)). At 17:46:45 UT, the wall top reached a higher position (panel (b7)) compared to that of at 17:45:56 UT and 17:46:26 UT, respectively. Note that a large number of bright threads connecting the top and base of the light wall can be identified (marked by the blue dotted lines in panel (b7)), revealing the fanning out structure of the light wall. This can be used to explain why the wall with large increase in height is much larger than the brightened region at the wall base. Several minutes later, the top of the light wall began to fall back (see panel (b8)). We found that the bottom part of the light wall started to rise up prior to the main impact of the downflows. The reason may be that material was already falling prior to the main impact, which remains invisible at the IRIS resolution and temperature response.

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To investigate the evolution of falling material and light wall, we make two time–distance diagrams along slices "A–B" and "C–D" (marked in Figure 2(b5)), respectively. The time–distance diagrams are presented in Figure 3. The base of the light wall is marked by the dashed line in each panel. The falling material appears as bright structures, and the leading and trailing edges are outlined by the two dotted lines in panel (a). When the falling material reaches the wall base with the projected velocity of 71 km s⁻¹, a range of brightenings appeared in the area outlined by the blue ellipse. The "+" symbol in panel (a) shows the first landing point of the falling material and the first brightening point around 17:45:30 UT. In panel (b), the green dotted curve outlines the variation of the bright wall top. When the first landing point became brightened (marked by the "+" symbol), the wall top rose quickly with an average velocity of 60 km s⁻¹. Due to the successive falling of material, several brightenings at the wall base appeared and the wall top moved upward (between the two blue dotted lines). The short streaks between the blue dotted lines are the characteristic of material due to the side motion of bright threads. The projected height of the light wall reached at the maximum of about 6.5 Mm at 17:50 UT, which is much higher than its height of 1.3 Mm just before the interaction with the falling material. Due to the projection effect, the real height of the light wall may be actually larger. Then, the height of the light wall began to decrease. At 17:54 UT, the projected height was 1.2 Mm, which is comparable to the one before the base brightenings. We analyzed the spectral profiles of Si iv 1393.78 Å in the path of the falling material (marked by the "+" symbol in Figure 2(a)). At 17:42:59 UT, the falling material was flowing across the slit, and by applying the Gaussian fitting method, we found that the redshift relative to the background is about 29 km s⁻¹. Since the projected speed is 71 km s⁻¹, the total speed of the downflow is estimated to be 77 km s⁻¹.

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3.2. Disturbance by the Flare

In the larger FOV, outlined by the white window in Figure 1(a), next we study another flare and its effect on the light wall. At 22:17:57 UT, the base and top of the light wall can be well identified, and there was no observable explosive event in the surrounding area (see Figure 4(a)). At 22:34:50 UT, the region outlined by the blue ellipse began to brighten (panel (b)), and then a C3.0 flare occurred (panel (c)). We note that, during the flare process, a bright channel that looks like a tail of the flare appeared, as indicated by the white arrow in panel (c). The bright channel extended from the flare toward the light wall. Then, the base of the light wall brightened and the height of the light wall increased. The bright channel was more evident at 22:38:16 UT (panel (d)). Although the flare decayed, the bright channel linking the flare and wall base remained clear for a while (panels (e)–(f)).

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In order to study the response of the light wall during the flare, we display in Figures 5(a1)–(a6) a sequence of IRIS 1330 Å images in a small FOV (outlined by the green square in Figure 4(d)). In Figures 5(a1)–(a6), the dashed lines and dotted lines outline the wall base and wall top, respectively. Before the disturbance by the flare, the projected height of the light wall is about 36 at 22:35:59 UT (panel (a1)). At 22:37:47 UT, the light wall was affected by the flare. Therefore, the base of the light wall was brightened as denoted by the arrow in panel (a2), and the brightness of the wall top also increased, compared with that of 2 minutes earlier. Moreover, the wall top moved upward quickly (panels (a2)–(a4)). A set of bright threads (outlined by the blue dotted lines in panel (b4)) connecting the wall base and wall top formed a fan-shaped structure. After that, the emission of the wall base decayed and the wall top moved downward again (panels (a4)–(a6)), reaching a lower height. In order to study the evolution of the light wall in more detail, we make a time–distance plot along slice "A–B" marked in panel (a3) and present it in panel (b). In the time–distance plot, the green dashed line and dotted curve outline the wall base and wall top, respectively. We may see that, before 22:37 UT (marked by the left vertical line), the top of the light wall moved upward and downward with an average projected height of about 2.5 Mm. At 22:37 UT, the height of the light wall itself was about 2.5 Mm, as denoted by arrow "M." Then, the wall base began to brighten at 22:37 UT, as marked by the blue "+" symbol. Subsequently, the height of the light wall increased quickly in the following 50 s (marked by the right vertical line) and became approximately 4.5 Mm (denoted by arrow "N") in height. In the next 3 minutes, the wall base brightened continually and the wall top maintained its relatively high altitude. When the brightness of the wall base decreased, the top of the light wall fluctuated to lower heights. At last, the height of the light wall was comparable to that before the disturbance.

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With IRIS observations, we study the dynamical evolution and response to external perturbations of a light wall. A flare occurred near the light wall followed by material ejected into the corona from the lower solar atmosphere. Some of the ejected material fell back to the solar surface. When the falling material reached the light bridge, i.e., the base of the light wall, sudden brightenings appeared at the wall base and, most importantly, the wall top rose quickly, performing an increase of the wall height. Once the brightenings of the wall base faded, the height of the light wall began to decrease. When another nearby flare took place, a bright channel was formed and extended from the flare toward the light bridge. Then, the wall base brightened and the height of the light wall began to increase again. Once again, when the brightness of the wall base decreased, and the wall top fluctuated to lower heights.

Falling material slides down along magnetic field lines (Reale et al. 2013; Kleint et al. 2014; Innes et al. 2016; Jing et al. 2016). We suggest that the falling material shown in Figure 2 causes the brightenings of the wall base and the increase of the light wall height. The proposed scenario is as follows: when the falling material hits the light bridge, the kinetic energy is converted to the thermal energy. Due to the local associated heating, the light bridge brightens and the pressure of the plasma therein increases concurrently. The magnetic configuration of light wall is thought to be a group of magnetic field lines rooted in the light bridge (Hou et al. 2016b). Thus, the material is lifted at a much higher speed along magnetic field lines by the increased pressure at the bottom, leading to the observational phenomenon that the top of the light wall is powered to reach a greater height. A given magnetic flux loop often guides a large number of falling elongated blobs (Antolin et al. 2010; Antolin & Rouppe van der Voort 2012). Indeed, we can identify several blobs from the falling material (as shown in the animation of Figure 2), which appear as multi-trajectories marked between two dotted lines in Figure 3(a). The falling material continuously crashes into the wall base, leading to successive brightenings as outlined by the ellipses in Figures 3(a)–(b). Consequently, multi-trajectories of upward motion of the wall top are observed, as marked by two dotted curves in Figure 3(b).

Applying the methods of Gilbert et al. (2013) and Innes et al. (2016), the mass of falling material in Figure 3(a) is estimated to be about 1.6 × 10¹² g. Since the total speed of the downflow is 77 km s⁻¹, then the kinetic energy is ∼4.8 × 10²⁵ erg. This energy can power the light wall material to increase by 11 Mm in height. The observed height increase of the light wall shown in Figure 3(b) is consistent with this estimate. In addition, the rebound shock generated by the impact of falling material could indeed play some role in leading to the light wall's increase in height. Figure 2 and its animation show that the falling material and the light wall are two separated structures in the corona, but they are joined at the footpoint (very close to each other). Thus, the impact of the falling material seems to affect a large region in the corona. This also implies that the falling material does not need to fall along the same path as the light wall to perturb it after the impact.

For the C3.0 flare, only a bright channel was observed. We suggest that, although no obvious material flow along the bright channel was found, some amount of ejected material may still reach the light bridge. A somewhat similar process has been reported briefly by Yang et al. (2014). In that study, on three homologous confined flares, one remote region brightened when each flare occurred, which is also deemed to be caused by ejected material. For the present study, when the ejected particles along the bright channel impacted on the light bridge, kinetic energy is likely be converted to thermal energy that heated the wall base and powered the light wall. For this case, we cannot fully exclude the possibility that the disturbance originates from below, such as small-scale reconnection in the lower atmosphere due to, e.g., convection motion (Toriumi et al. 2015a, 2015b). We note that there was a brightening at the wall base at 22:33 UT (indicated by the first arrow in the animation of Figure 4). This brightening only corresponds to a small increase of the wall height, much smaller than that resulted from the bright channel. Although it is difficult to determine the exact cause of the brightening at 22:33 UT, we think it may be caused by the small-scale reconnection due to the convection motion (Toriumi et al. 2015a, 2015b).

We conjecture that a light wall is a slab-like structure (maybe even consisting of thin magnetic threads) embedded in vertically stratified plasma. Oscillations of the light wall have earlier been interpreted to be powered by the leakage of p-modes from below the photosphere (Yang et al. 2015). Our new results reveal that the light wall can also be enhanced by external disturbances, such as falling material and an avalanche of particles caused by nearby flares.

We thank the referee for helpful suggestions and constructive comments. The data are used courtesy of IRIS and SDO science teams. This work is supported by the National Natural Science Foundations of China (11673035, 11533008, 11373004, 11303049, 11221063), the Strategic Priority Research Program (No. XDB09000000), the Youth Innovation Promotion Association of CAS (2014043), the CAS Project KJCX2-EW-T07, and the Young Researcher Grant of National Astronomical Observatories of CAS. R.E. is grateful to STFC (UK) for the awarded Consolidated Grant, The Royal Society for the support received in a number of mobility grants. He also thanks the Chinese Academy of Sciences Presidents International Fellowship Initiative, grant No. 2016VMA045, for support received.