Fine Structure and Formation Mechanism of a Sunspot Bipolar Light Bridge in NOAA AR 13663

Abstract

Bipolar Light Bridges (BLBs) are bright regions located between sunspot umbrae of opposite magnetic polarity. They are typically characterized by strong magnetic fields and intense flows, which are believed to be closely associated with major solar flares. Despite their importance, their fine structure, formation and evolution remain poorly understood. In this work, we analyze the observations of a well-defined BLB obtained by the Goode Solar Telescope at the Big Bear Solar Observatory and the Helioseismic and Magnetic Imager onboard the Solar Dynamics Observatory. The high-resolution GST observations reveal that the BLB is composed of fine, penumbral filament-like structures with widths of approximately 100-150 km. The corresponding Doppler velocity maps present a stable pattern of spatially adjacent red- and blueshifted patches within the BLB throughout the 5.5-hour GST observation. HMI observations show that the BLB arises from the converging and shearing motions of sunspots with opposite polarities. Penumbral regions originating from different polarities gradually evolve and interact, eventually forming the BLB. The observed Doppler velocity pattern, characterized by red- and blueshifted patches, can be interpreted as a projection effect of the Evershed flow within the penumbrae. Therefore, we argue that the BLB is formed through the compression and stretching of penumbral structures from oppositely polarized sunspots.

Figures (10)

• Figure 1: Overview of the BLB at 16:53 UT on 2024 May 3. (a) Full-disk solar image with NOAA AR 13663 marked by a blue rectangle, corresponding to the FOV shown in panel (b). (b) Zoomed-in view of NOAA AR 13663; the blue rectangle indicates the FOV shown in panels (c)–(f). (c) High-resolution GST/TiO image of the light bridge. The lime rectangle shows the FOV of Figure \ref{['figure:fig2']}. Panel (c) is available as an animation showing the BLB over the entire 5.5-hour interval starting at 16:38 UT on 3 May 2024. The real-time duration is 16 seconds. (d)–(f) SDO/HMI observations of the same region, corresponding to the continuum intensity, LOS magnetogram, and LOS Doppler velocity, respectively. Lime contours denote the regions with continuum intensity $< 0.35$$I_\mathrm{c}$. Blue and black dashed lines indicate locations where the line-of-sight magnetic field is zero.
• Figure 2: High-resolution observations of the BLB using BBSO/GST, focusing on the region of interest (ROI) shown in Figure \ref{['figure:fig1']}(c). (a) GST/BFI TiO image. (b)-(c): Line-of-sight (LOS) magnetic field and total magnetic field derived from GST/NIRIS observations. (d) GST/VIS H$\alpha$ line center. (e)-(f): Doppler velocity, and horizontal magnetic field derived from GST/NIRIS observations. Red dashed box Region1 and blue dashed box Region2 in (e) mark the FoV in Figure \ref{['figure:fig3']} and Figure \ref{['figure:fig4']}, respectively. White contours and black contours in (a)-(f) denote regions where the continuum intensity is below $0.2\space I_\mathrm{c}$ of intensity of TiO. The green curves in (a), (b), (e), and (f) mark the PIL. See Section \ref{['sub:fine structure']} for details. An animation showing the evolution of BLB based on NIRIS observations is available. The animation shows the sequence from May 3rd, 2024 from 16:40 to 21:40. The real-time duration is 1.33 seconds.
• Figure 3: Properties of Region 1(redshifted Doppler region) shown in Figure \ref{['figure:fig2']}(e). (a) Image in the TiO band. The short red lines labeled 1 and 2 indicate the positions where the filament widths are measured in panels (b1) and (b2), respectively. Panel (a) is available as an animation showing the evolution of the redshifted region of the BLB. The animation runs from 16:37:09 to 17:39:09 with a real-time duration of 8 seconds. The lime dashed curved lines mark a relatively representative penumbral filament corresponding to the NIRIS data shown in panels (c1)–(c5); the label A–B indicates the orientation. (b1)–(b2) Gaussian fitting of the intensity variation at locations 1 and 2 in panel (a), respectively. (c1)–(c5) Maps within the lime dashed curved lines in panel (a), including the TiO intensity, NIRIS intensity, inferred LOS velocity, magnetic inclination, and LOS magnetic field strength. The spatially averaged value is overlaid on each map as a solid curve.
• Figure 4: Same as Figure \ref{['figure:fig3']}, but for Region 2 (the blueshifted Doppler region). Panel (a) is available as an animation showing the evolution of the blueshifted region of the BLB. The animation runs from 16:37:09 to 17:39:09 with a real-time duration of 8 seconds. Label C--D in panel (a) indicates the orientation of a representative penumbral filament, and point C marks the location of the penumbral grain.
• Figure 5: Evolution of NOAA AR 13663. (a1)–(f1) SDO/HMI LOS magnetograms from 01:30 UT on 1 May to 16:54 UT on 3 May, 2024. (a2)–(f2) SDO/HMI continuum intensity images for the times as same as panels (a1)–(f1). Solid and dashed lime contours denote the LOS magnetic field strengths of $\pm$ 1000 G, respectively. Blue rectangles in panel (b1) and (b2) mark the ROI shown in Figure \ref{['figure:fig6']}. Cyan arrows labeled N1 (negative polarity) and P1 (positive polarity) mark the pre-exsiting sunspot pair, while red arrows labeled N2 (negative polarity) and P2 (positive polarity) indicate the emerging sunspot pair. An animation showing the evolution of NOAA AR 13663 is available. The animation runs from April 29th, 2024 20:29:49 to May 5th, 2024 23:29:05. The animation's real-time duration of 9 seconds.