Jonathan H. Lee¹, Xudong Sun², & Maria D. Kazachenko^3,4
1 Institute for Astronomy, University of Hawai’i at Mānoa, Honolulu, HI 96822
2 Institute for Astronomy, University of Hawai’i at Mānoa, Pukalani, HI 96768
3 Department of Astrophysical and Planetary Sciences, University of Colorado Boulder, Boulder, CO 80305
4 National Solar Observatory, University of Colorado Boulder, Boulder, CO 80303

Solar active regions (ARs) are localized regions of strong magnetic field. The complex coronal magnetic field associated with these sites frequently drives explosive solar activities such as flares and coronal mass ejections. In the core of ARs, photospheric magnetic vectors typically point from positive to negative sides across polarity inversion lines (PILs), where vertical fields become zero and change signs. In this case, the low-lying field lines form Ω-shaped arcades that straddle the PILs.

In some cases, however, they may also point from the negative to the positive sides. The field lines are expected to be U-shaped and tangentially touch the photosphere at the PILs. These segments of the PILs are known as bald patches (BPs)^[1]. BPs may form in a variety of magnetic conditions and are observationally identified in photospheric vector magnetograms.

Figure 1| AR 12673 before (11:37:30 UT) and after (12:25:30 UT) the flare. The background shows the positive (negative) polarity in white (black). Vectors show horizonal field with foot points in the positive (negative) in blue (red). The PIL and BPs are shown by the green line and yellow/orange circles, respectively. The gray contour is for =300 G. Purple color shows pixels that are excluded from our analysis due to weaker fields or larger uncertainties. The ellipse and the box indicate our selected regions of BP evolution analysis: Region 1 for BP disintegration, and Region 2 for BP survival. The Zenodo animation shows BP evolution from 10:49:30 UT to 13:00:00 UT.

NOAA AR 12673 (Figure 1) produced the most intense flare of solar cycle 24 on September 6, 2017. In this work^[2], we report on the BP evolution in AR 12673 during this major eruption. The GOES X9.3 flare (SOL2017-09-06T11:53) has a start, peak, and end time at 11:53 UT, 12:02 UT, and 12:10 UT, respectively. We analyze 88 frames of SDO/HMI vector magnetograms with a 90 s cadence^[3], which span roughly two hours centered around the flare peak. For each time step, we extract a 175×175-pixel map in a cylindrical equal area projection. We adopt a local Cartesian coordinate where the unit vectors point west, north, and upward, respectively.

We use the zero vertical field contour, G, to locate the “PIL pixels” to sub-pixel accuracy and identify the “BP pixels” on the PIL using the criterion in Ref. [1]. We calculate the following variables of interest for the PIL pixels: the horizontal field strength , the parallel component of horizonal field with respect to the PIL , the corresponding, perpendicular component , and the magnetic shear angle . In our convention, is positive for BPs, and negative otherwise. The shear angle is zero if the field is perpendicular to the PIL and points from positive to negative polarity. Sheared arcades have 0° < < 90°; BPs have > 90°.

Figure 2| Time series of median , , , and at PIL pixels in Region 1 (left) and Region 2 (right). The error bars show the 1σ confidence interval of the median derived from the Monte-Carlo method. The vertical gray bar (dotted line) shows the flare duration (peak time). The horizontal dotted line is for =90°.

For each time step, we calculate the BP criterion^[1] and other variables of interest for all the PIL pixels with low statistical uncertainty. We select two subregions, Region 1 and 2 (Figure 1), for further analysis. We exclude the data taken during the flare (12:12–12:36 UT), because the flare emission creates significant transient signals at ribbon loci. As directly discernible in Figure 1, the BP appears to have “disintegrated” in Region 1 during the eruption. For Region 1, 73% of PIL pixels are identified as BPs at 11:37:30 UT (pre-flare). This is in sharp contrast with the 12:25:30 UT (post-flare) frame, where only 39% of pixels are identified as BPs. For Region 2, 63% of PIL pixels are identified as BPs pre-flare, and 71% of pixels post-flare. The BP appears to have survived the eruption.

To better characterize this distinct behavior, the median values for our variables of interest within both Regions at each time step were determined. The temporal profiles of the median values are shown in Figure 2. Region 1 exhibited a rapid and permanent increase of roughly 1 kG in during the flare, which mostly comes from . Interestingly, the median and increased between 11:20 UT and 11:35 UT, suggesting rapid formation of new BPs. Their values subsequently decreased after the flare onset, which caused the apparent BP disintegration. In comparison, the temporal variations in Region 2 were much smaller.

Figure 3| AIA 1600 Å flare ribbons in teal during the flare onset (left, 11:55:27 UT) and shortly before the flare peak (right, 12:00:15 UT). The Zenodo animation shows the flare ribbon evolution from 10:49:30 UT to 13:00:00 UT. During the onset of eruption, ribbons developed along the PIL within Region 1 and exterior to the PIL in Region 2. Prior to the flare peak, flare ribbons propagated along and separated away from the PIL covering the entirety of Region 1 and Region 2.

Flare ribbon evolution^[4] is shown in Figure 3. At the flare onset, the ribbons first appeared close to the northern portion of the PIL, including Region 1 (left panel of Figure 3). This suggests that the BPs in Region 1 may have participated in the flare reconnection early on, when the reconnection flux rate was near its maximum. The ribbons subsequently extended along the PIL to cover Region 2 (right panel), when the reconnection flux rate was much reduced. Meanwhile, the ribbons also moved away from the PIL, suggesting that the coronal reconnection proceeded to higher altitudes. In Ref. [5], 3D MHD simulations showed that BPs disappeared due to intense reconnection proceeding to very low altitudes. Our observations appear to be consistent with this scenario, particularly with the contrasting behaviors of Regions 1 and 2. The early, more intense reconnection likely occurred at lower altitudes in the current sheet above Region 1, and the magnetic configuration was converted to sheared arcade. Region 2 was involved later when the reconnection proceeded to higher altitudes and the BPs survived.

Our observations show that a segment of BP in AR 12673 rapidly disintegrated during the X9.3 flare. The disintegration is a consequence of the changing and θ_s, as the local magnetic azimuth angles rotated by about 9° over half an hour. The post-flare pointed from the positive to the negative polarity, and the low-lying field lines assumed a sheared arcade topology instead. Our observations also show that increased by about 1 kG, in line with previous reports of magnetic imprint^[3]. More details can be found in our recent paper [2]. Animations are available at https://doi.org/10.5281/zenodo.5585623.

References

[1] Titov, V. S., Priest, E. R., & Démoulin, P. 1993, A&A, 276, 564
[2] Lee, J. H., Su, X., & Kazachekno, M. D. 2021, ApJL, 921, L23
[3] Sun, X., Hoeksema, J. T., Liu, Y., Kazachenko, M. D., & Chen, R. 2017, ApJ, 839, 67
[4] Kazachenko, M. D., Lynch, B. J., Welsch, B. T., & Sun, X. 2017, ApJ, 845, 49
[5] Fan, Y., & Gibson, S. E. 2007, ApJ, 668, 1232