Junwei Zhao1, Wei Liu2,3, and Jean-Claude Vial4
1. W. W. Hansen Experimental Physics Laboratory, Stanford University, Stanford, CA 94305-4085, USA
2. Lockheed Martin Solar and Astrophysics Laboratory, Palo Alto, CA94304, USA
3. Bay Area Environmental Research Institute, NASA Research Park, Moffett Field, CA 94035, USA
4. Institut d’Astrophysique Spatiale, Université Paris-Saclay, C.N.R.S., Batiment 121, ORSAY 91405, France
White-light flares (WLFs) are rare flaring events that cause continuum emission in excess of the photospheric background. Off-limb WLFs, with flare-loop brightening observed in white light (WL) beyond the solar limb, are even rarer with only a few cases reported, and among them the SOL2017-09-10 X8.2 is well observed by a suite of ground- and space-based instruments. As shown in Figure 1 and its associated video, this off-limb post-flare loop was captured by SDO/HMI FeI 6173 Å continuum and all of the SDO/AIA’s channels, as well as by the RHESSI X-ray imager.
Figure 1| Selected images of the flare loops at the wavelength channels of (a) 304 Å near its peak intensity at 16:07:05 UT; (b) 1700 Å near its peak intensity at 16:12:29UT; and (c) 335 Å at 16:27:36UT, overplotted with a magenta WL contour near the WL peak time. (d)-(e) Selected composite images of HMI WL (blue-red background image) and AIA 335 Å (orangewhite semi-transparent foreground image), with RHESSI 10-16 keV intensity over-plotted as white contours.
Time-Height Relation. To analyze the time–height relations of the loop-top observed in HMI WL, AIA’s UV channels (1700 Å and 1600 Å), and EUV channels (335 Å and 304 Å) through the period of interest from 15:56 UT to 17:12 UT, for each time step and for each altitude we integrate the intensity for the total flux in a 10″-wide band that includes the loop-top (Figure 2). It can be seen that through all the five wavelength channels, the loop-top shows four distinct brightness-enhancements patches, each of which is marked by one contour. Based on the time and location of the brightness enhancement patches, the five wavelength channels form two groups, with one group being the WL and UV channels, and another group the EUV channels. Within each group, the height and the duration of the four brightness enhancements observed in different channels are quite similar but do not exactly overlap. The brightness enhancements in the EUV channels occur earlier in time and higher in altitude than those in the WL and UV channels, and the time lags and height differences between the two groups of enhancement patches increase as the flare loop-top evolves to higher altitude. The X-ray centroid locations of the loop-top, obtained from the RHESSI’s 6-10 keV and 25-60 keV channels during 15:53UT and 16:17UT, can be found at least 10″ higher and at least 10 minutes earlier than the AIA’s EUV channels.
Figure 2| Time–height relations obtained for the flare’s loop-top after an integration over a 10″-wide band for each time step, using observations from (a) HMI WL, (b) 1700 Å, (c) 1600 Å, (d) 335 Å, and (e) 304 Å. Contours in each of these images mark the 75% of intensity level at the peak of their corresponding brightness enhancement patches. The WL contours are also plotted in panel (d) to better compare the WL emission relative to EUV emissions. (f) Contours in panels (a) – (e) are plotted together and numbers “1” – “4” are marked corresponding to the four brightening patches. The centroid locations obtained from RHESSI’s 6-10 keV and 25-60 keV soft X-ray data are also overplotted as ‘+’ signs for a comparison.
Continuing Growth in WL. To better compare the temporal variations of the intensity for all the WL/UV/EUV channels, we further integrate the time–height diagrams over heights, and study the total flux change over time. Figure 3a shows all the flux curves, which are normalized to 1.0 at their respective major brightness peak between 16:05 – 16:15 UT. It can be seen that all the UV and EUV channels show similar trend of growth and decay, but the WL light flux continues to grow to a brighter peak around 16:28 UT after a 5-min dip in brightness following the first peak. That is, the WL and UV channels show sharply different behaviors after about 16:17 UT despite their many similarities in other aspects, highlighting the different emission mechanisms in the WL and UV wavelengths at the flare’s loop-top.
Figure 3| (A) Temporal evolution of the flux for the HMI and AIA wavelength channels, normalized relative to their respective peak fluxes during 16:05 – 16:15UT. (B) Comparison of detrended flux curves between the WL and the UV channels, as well as EUV channels in (C). Dark arrows in both lower panels point to the four peak times in the detrended WL curve, and the magenta arrows indicate the peak times of the detrended 335 Å curve.
Quasi-Periodic Pulsations. To study the shorter-period pulsations in the brightness observed in all the wavelengths, we detrend the brightness curves (Figure 3A) by subtracting their respective running averages with a window width of 300 sec. Figure 3B (3C) shows comparisons of the detrended WL and UV (EUV) flux curves. The WL brightness peaks show quasi-periodic pulsations with a period of approximately 8.0 min. The pulsations in the two UV channels occurred slightly earlier than those in the WL, lasting a total of about 40 minutes with a similar period. The pulsations in the two EUV channels lead the WL pulsations by ~2.4 – 6.5 min. These EUV pulsations are less regular in periodicity with an averaged period of ~6.6 min, significantly shorter than that of the WL pulsations.
Discussion. As Figure 2f shows, the emission generally progresses with time toward lower altitudes from X-ray to EUV and then UV/WL, corresponding to the emitting plasma at progressively lower temperatures. This is a well-known trend that can result from the interplay of (a) upward development of magnetic reconnection toward higher altitude with time in the standard flare model, (b) downward contraction of newly reconnected hot flaring loops that undergo cooling at the same time. The observation of that WL intensity continues to grow while UV/EUV intensities decay may indicate that, after 16:10UT and above the height of approximately 22″, the temperature has dropped substantially and resulted in reduced UV/EUV emissions; however, the increasing electron density may still keep the WL intensity growing for a longer time, primarily because the Paschen and Brackett recombination and free-free emission depend quadratically on the electron density.
For details of this work, please refer to our full publication Ref. .
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