White-light Continuum Observation of the Off-limb Loops of the SOL2017-09-10 X8.2 Flare: Temporal and Spatial Variations

White-light flares (WLFs; Hudson et al., 2006; Hudson, 2011) 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 (e.g., Hiei et al., 1992; Martínez Oliveros et al., 2014). Three off-limb WLFs were so far observed in the continuum near Fe I 6173 Å by the Helioseismic and Magnetic Imager (HMI; Scherrer et al., 2012; Schou et al., 2012) onboard the spacecraft Solar Dynamics Observatory (SDO; Pesnell et al., 2012), i.e., SOL2013-05-13T02:16 (X1.7), SOL2013-05-13T16:01 (X2.8), and SOL2017-09-10T15:35 (X8.2). These observations offered unprecedented opportunities for solar physicists to study properties of WLFs that are otherwise impossible: e.g., Martínez Oliveros et al. (2014) compared the WL brightening sources with those observed in extreme ultraviolet (EUV) and soft X-ray; Saint-Hilaire et al. (2014) analyzed the linear polarization observed in the WL flare loops; and Heinzel et al. (2017) studied emission mechanisms of the WL off the solar limb.

One popular phenomenon that is associated with solar flaring events and has been widely studied by various authors is quasi-periodic pulsations (QPPs) in the time-dependent intensity curves observed in different wavelengths during flares (Nakariakov & Melnikov, 2009; Van Doorsselaere et al., 2016). This phenomenon, with a repeating period of a few seconds to a few minutes, has been observed almost across all the electromagnetic spectrum, such as in $\gamma$-rays (Nakariakov et al., 2010), EUV and soft X-rays (e.g., Dennis et al., 2017; Dominique et al., 2018), chromospheric lines (e.g., Brosius et al., 2016; Tian et al., 2016), and also in radio bursts (e.g., Li et al., 2015). Due to the rare observation of WLFs, no known QPP has yet been reported in white light. The physical mechanism for the generation of QPP is not very clear, but it is believed that this phenomenon is either due to the characteristics of the flaring loops that exhibit magnetohydrodynamic (MHD) oscillations at the flaring sites or MHD waves passing through the flaring loops (e.g., Nakariakov & Melnikov, 2009), or due to the quasi-periodic magnetic reconnection and energy release that power the intensity enhancements of the spectrum lines (e.g., Murray et al., 2009).

The X8.2 limb flare of 2017 September 10 (SOL2017-09-10T15:35), whose loops rose right above the Sun’s west limb and which was well observed by a suite of ground- and space-based instruments, has been studied from many different perspectives to tackle various physics problems related to flare dynamics and energy releases. For example, using EUV observations Warren et al. (2018) studied the formation and evolution of the current sheet following the eruption; Coupling hard X-ray and ground-based radio observations, Gary et al. (2018) studied the evolving spatial and energy distribution of high-energy electrons in the flaring region; Coupling with GOES X-ray and EUV observations, Hayes et al. (2019) found QPPs with periods of about 65 s and 150 s; Using the SDO/HMI’s WL observations, Jejčič et al. (2018) modeled the physical conditions inside the flare loops and explored the WL emission mechanisms; Heinzel et al. (2020) identified signatures of He continuum in the cool flare loops; Using Ca II Stokes components observed by the Swedish 1m Solar Telescope, Kuridze et al. (2019) reported a strong magnetic field of 350 G at the flare’s loop-top; Using microwave observations, Fleishman et al. (2020) estimated that the coronal magnetic field in the flaring loops decayed with a rate of $\sim 5$ G s^-1 during the flare, and Yu et al. (2020) analyzed the bidirectional outflows from the magnetic reconnection sites.

In this Letter, combining simultaneous observations of the SOL2017-09-10 flare obtained in the SDO/HMI’s continuum intensity and SDO/AIA’s (Atmospheric Imaging Assembly; Lemen et al., 2012) ultraviolet (UV) and EUV observations, along with data from the Geostationary Operational Environmental Satellite (GOES) and Reuven Ramaty High Energy Solar Spectroscopic Imager (RHESSI; Lin et al., 2002), we analyze the temporal and spatial variations of the brightening observed in different wavelength channels, as well as the QPPs in all these channels. It is believed that these unprecedented simultaneous observations in different wavelength channels offer us a rare opportunity to understand the emission mechanism of white light, heat dissipation process, and energy releases during the flare. This Letter is organized as follows: we introduce observations in Section 2, present our analysis and results in Section 3, and discuss our results in Section 4.

The SDO/HMI uses Fe I 6173 Å line to observe the Sun with 1″ spatial resolution and 45 sec temporal cadence, and provides full-disk continuum intensity data, along with Doppler velocity and magnetic field. The field of view of SDO/HMI is nominally $45\arcsec$ wider than the photospheric solar disk, allowing detection of features bright in visible light immediately beyond the solar limb. The SOL2017-09-10 X8.2 flare started at approximately 15:35 UT in NOAA Active Region 12673, when most of the active region rotated around the west limb into far side of the Sun. The sudden brightening at the footpoints of the flare in the chromosphere and the rise of the bright flare loops, up to about $40\arcsec$ above the limb, were observed by the SDO/HMI in its continuum intensity, i.e., white light. As shown in Figure 1 and its associated online animation, the flare loops become visible above the limb at around 16:03:00 UT. Its loop-top brightness gradually enhances with the increase of height, reaching an intensity maximum at about 16:12:00 UT when the SDO/AIA’s UV channels also reach their respective maxima (Cf. Figure 4b). While continuing to rise in height, the WL intensity drops for about 5 min, and then starts to enhance again, reaching a even brighter maximum at about 16:27:45 UT. The WL brightening of the flare loops then gradually fade away, lasting a total of about 70 min.

The flare was also observed in all the SDO/AIA wavelength channels. However, due to the powerful intensity of this flare, a number of AIA channels either saturated near and during the peak of the flare, mostly at the location of the loop-top, or had irregular exposure times in order not to saturate the observations. The data from those channels with irregular exposure times may still be useful for some types of analyses, but cannot be used in this analysis, which focuses on temporal variations of the loop-top intensity and is sensitive to the alteration of exposure times. This leaves only observations from the wavelengths of two UV channels (1700 Å and 1600 Å) and two EUV channels (335 Å and 304 Å) useful for the analysis in this study. According to Lemen et al. (2012), the UV channel 1700 Å (1600 Å) is sensitive to the temperature of $10^{3.7}$ K ($10^{5.0}$ K) corresponding to the upper photosphere (the upper photosphere and transition region), the EUV channel 304 Å (335 Å) is sensitive to the temperature of $10^{4.7}$ K ($10^{6.4}$ K) corresponding to the chromosphere and transition region (active region corona). However, it must be stressed that these properties of different channels are assessed mostly for on-disk observations, and may not strictly apply to the off-limb observations. Figure 2 shows key snapshots of the flare loops for some wavelengths, along with selected composite images showing the relative spatial and temporal relations of WL and EUV channels. Particularly, as seen in Figure 2c, the WL loop-top, at its peak brightness, is located below the 335Å loop-top in its absorption area.

GOES captured the total soft X-ray flux of the Sun in its both channels: 1–8 Å and 0.5–4 Å (Figure 4a). RHESSI’s X-ray images did not cover the entire flare event, but its coverage during the early impulsive phase of the flare provides useful information that can be compared with the WL/UV/EUV locations (Figure 3f). Selected RHESSI X-ray images are also plotted as contours with the WL and EUV images for comparison of loop-top locations (Figure 2d-f).

We have analyzed the rare white-light observation of the evolution of an off-limb loop system in the SOL2017-09-10 X8.2 flare, and found that (1) there are quasi-periodic pulsations in the WL and UV channels with a period of around 8.0 min, and less regular pulsations with an average period of 6.6 min in the EUV channels; (2) intensity-enhancements appear sequentially in WL and UV/EUV channels with time delays and altitude decreases corresponding to progressively lower temperatures of the emitting plasma; and (3) the WL flux of the flare’s loop-top continues to grow for about 16 more minutes while the UV/EUV fluxes decay.

The quasi-periodic pulsations observed across the WL/UV/EUV channels, with different periods and irregular time delays between different channels, implies that these pulsations are unlikely of the MHD wave nature or oscillatory properties of the flaring plasma. Through analyzing the evolution of a strong on-disk flare, Asai et al. (2004) suggested that repeated enhancements of X-ray/EUV emissions were indicative of energy release and particle acceleration episodes resulting from magnetic reconnection. Our observation of the limb-flare loop presents a different view angle of a similar phenomenon, whose quasi-periodic brightenings are also possibly caused by episodic magnetic reconnection above. For this same flaring event, Reeves et al. (2021) found damped oscillations in Doppler velocity with a period $\sim$400 s in the region south of the the main flare arcade, and suggested that the oscillations are caused by magnetic reconnection outflows. Despite different areas of the same event analyzed, the loop-top of the main arcade in our study and the loop-top south of the main arcade in theirs, the similarities in the pulsation periods may point to a common physical cause – pulsed magnetic reconnection.

The brightness-enhancement patches across different wavelengths can serve as tracers of the flare’s energy release and subsequent thermodynamic evolution across different temperature and density ranges. As Figure 3f 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. Such contractions have various observational manifestations, such as shrinkage of flare loops (Švestka et al/, 1987) at speeds of the order of 10 km s^-1 seen in soft X-rays (Forbes & Acton, 1996; Reeves et al., 2008), and the so-called supra-arcade downflows (e.g., McKenzie & Hudson, 1999; Savage & McKenzie, 2011; Liu et al., 2013). In our analysis (see Figure 3f), the altitudes for the first two groups of brightenings, around 16:02UT and 16:10UT across all wavelengths, are similar at about 10″ and 18″ above the limb, while the time delays between the EUV and UV emissions are about 2 and 4 min, respectively. For the other two groups of later brightenings, the EUV emission originates about 7″ higher than the UV/WL emission with time delays of 3 – 5 min, which translates to a speed of $\sim 17-28$ km s^-1 if we assume this results from loop shrinkage. This trend agrees with the increase with time of the separation between the 6-10 and 25-60 keV X-ray emission centroids (especially after about 16:05UT; cf., Liu et al., 2004).

Despite many similarities in WL and UV observations, their integrated light curves show different trends: the WL flux continues to grow while the UV flux starts to decay after the peak around 16:10UT. This implies, unsurprisingly, that the UV and WL have different emission mechanisms. Off-limb flare loops are visible in WL primarily because of a combination of the following processes: hydrogen Paschen and Brackett recombination, hydrogen free-free continuum emission, and Thomson scattering of the solar radiation (Heinzel et al., 2017; Jejčič et al., 2018). It is likely that before 16:10UT plasma temperature is relatively high at the flare’s loop-top, and all the WL emission mechanisms play a role. After 16:10UT and above the height of approximately $22\arcsec$ (see Figure 3a), the temperature may have 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 density. However, one should also realize that high densities also correspond to non-negligible opacity leading to a decrease of emission. In fact, a detailed comparison of WL and EUV 335Å emissions in Figures 2c and 3d, along with the sequence of 335Å images, shows that during the EUV decay period, the WL emissions are located in an area of reduced 335 Å emission that appears to be a dark absorption. This is also evident on other AIA EUV channels, such as 193Å and 211Å (cf. Figure 1 of Heinzel et al. 2020 and Figure 3 of Jejčič et al. 2018), providing another piece of evidence that plasma has cooled substantially in the areas where WL intensity enhances.