Homologous White Light Solar Flares

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Number: 341
1st Author: Paolo Romano
2nd Author: Abouazza Elmhamdi
Published: 31 December 2018
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At the end of the last solar cycle, a very huge and very active region appeared on the Sun. On the first two weeks of September 2017 this active region, NOAA 12673, was the theater of many flares: about 40 C-class GOES events, 20 M-class and 4 X-class. Several earlier Nuggets have dealt with these phenomena, from different points of view: "last best" and radio waves, in particular. The presence of very prolific magnetic sites at the conclusion of a solar cycle is quite common. In this case, as Cycle 24 ended, this activity captured the interest of many researchers due to some peculiarities which characterized the region. In fact, it was the site of strong magnetic flux emergence and persistent horizontal displacements of the magnetic features forming the main sunspot. These two aspects have been, probably, the main drivers of many of the flares that occurred in the region. Among these events, we draw your attention to two homologous solar flares. Homologous white-light flares (WLF) are really interesting especially for their magnetic field configuration and for their timing of the emissions at different wavelengths, and this has provided some constraints for the modeling of such events (Refs. [1] and [2]).

The peculiarities of the flares of September 6, 2017

On September 6, 2017, an X2.2 flare began at 08:57 UT and reached its peak at 9:10 UT, while an X9.3 (the most powerful flare in more than a decade) started during the main phase of the previous one, at 11:53 UT, and reached its peak at 12:02 UT. Standardized names for these events would be SOL2017-09-06T09 and SOL2017-09-06T12, respectively. Both events were characterized also by emissions at the photospheric level ("white light"), clearly visible in the continuum filtergrams taken by HMI/SDO in the Fe I photospheric absorption line at 617.3 nm. The events were observed by many instruments at different wavelengths, corresponding to different layers of the solar atmosphere: AIA/SDO, HINODE, INAF-Catania Astrophysical Observatory, the GONG network and RHESSI. This wealth of data allows us to make a detailed study of the magnetic field configuration at the base of the two homologous flares. The active region consisted of by a main delta sunspot and a scattering of smaller spots. During some hours preceding the two flares and also during the main phase of the first flare we observed a significant horizontal displacement of the negative umbra of the delta spot, migrating northward with an average velocity of 0.4 km/s, reaching values as high as 0.6 km/s. These peculiar motions, together with a strong magnetic flux emergence occurred in the previous days, were the main sources of the magnetic configuration at the base of the two flares. The tracking of the main sunspot features, by the application of the "DAVE" method to the HMI vector magnetograms, shows also southward motions of the remaining parts of the delta spot (Figure 1). These produced strong shear in the overlying magnetic system as confirmed by the values of the shear and inclination angles measured around the polarity inversion line of the sunspot.

Figure 1: Horizontal velocity fields measured over the delta spot of AR NOAA 12673 by means of the DAVE tracking processing method (see text). The background images correspond to the average of the vertical component of the magnetic field measured by HMI during the time interval considered for the velocity computations. The red arrow in the top left corner indicates a scale velocity of 0.5 km/s. From Ref. [2].

The photospheric area of the shear motions was located in the central portion of the S-shape chromospheric ribbons and under a bundle of loops forming a sigmoid at 171 Â. Moreover, during the flash phases of the two flares the hard X-ray sources registered by RHESSI at 12-25 KeV and at 25-50 keV were located in the center of the delta spot. From extrapolation of the magnetic field we also identified the presence of two related 3D null points in the low corona, i.e. at about 5000 km and 3000 km above the boundary level for the X2.2 and X9.3 flares, respectively. In principle, the magnetic reconnection responsible for the two flares occurred near those null points, accelerating the particles along the traced field lines and, hence, explaining the observed ribbons at the different layers of the solar atmosphere. We found also that the lower 3D null point corresponded to the stronger flare. We suggest three possible explanations of the variation of the null point height between the two events: (i) the changes of the magnetic field topology due to the emergence of new magnetic flux; (ii) the stretching and lowering of the field lines due to the peculiar photospheric horizontal motions; (iii) a combination of the previously cited mechanisms. In any case, the homology of the two white light flares is recognizable not only by the correspondence between the ribbon locations of the two flare, but more interestingly by the location of the emission registered by RHESSI and by the similar magnetic field configuration deduced by the extrapolations.

A last but not least result concerns the timing of the emission at different wavelengths. In fact, exploiting these two white light flares, we were able to highlight that for both events the emission at photospheric level was observed several minutes before the emissions in X-rays, EUV, and Hα lines reached their maxima (Figure 2). This is a further constraint for the modeling of such events.

Figure 2: GOES X-ray light curves (blue curve: 1-8 Â; red curve: 0.5-4 Â). Overplotted is the HXR light curve deduced from RHESSI at 25-50 keV (yellow curve), the SDO/AIA light curve taken at 171 Â (green), and the Hα light curve (red boxes). From Ref. [1].


If the white light flares are rare events, homologous white light flares much rarer still. For this reason, we analyzed the X2.2 and X9.3 flares that occurred on Spetember 6, 2017, providing some clues for a better understanding of how homologous flares may occur in short time intervals (few hours) and how their energy may involve the lower layers of the solar atmosphere. In these events, we found that the height of the reconnection site may play an important role in the amount of released energy. We also think that this kind of event may offer clues for the interpretation of stellar superflares, such as those recently reported by the Kepler mission. In fact, the white-light emission observed during superflares in young Sun-like stars may be explained by huge numbers of particles accelerated during the occurrence of magnetic reconnection at low altitudes of their atmospheres.


[1] "Homologous White Light Solar Flares Driven by Photospheric Shear Motions"

[2] "Two strong white-light solar flares in AR NOAA 12673 as potential clues for stellar superflares"

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