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Register a new userInvestigation of white-light emission in circular-ribbon flares
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Using observations by the Solar Dynamics Observatory from June 2010 to December 2017, we have performed the first statistical investigation of circular-ribbon flares (CFs) and examined the white-light emission in these CFs. We find 90 CFs occurring in 36 active regions (ARs), including 8 X-class, 34 M-class, 48 C- and B-class flares. The occurrence rate of white-light flares (WLFs) is 100% (8/8) for X-class CFs, $sim$62% (21/34) for M-class CFs, and $sim$8% (4/48) for C- and B-class CFs. Sometimes we observe several CFs in a single AR, and nearly all of them are WLFs. Compared to normal CFs, CFs with white-light enhancement tend to have a shorter duration, smaller size, stronger electric current and more complicated magnetic field. We find that for X-class WLFs, the white-light enhancement is positively correlated with the flare class, implying that the white-light enhancement is largely determined by the amount of released energy. However, there is no such correlation for M- and C-class WLFs, suggesting that other factors such as the time scale, spatial scale and magnetic field complexity may play important roles in the generation of white-light emission if the released energy is not high enough.
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In this study, we investigated the energy partition of four confined circular-ribbon flares (CRFs) near the solar disk center, which are observed simultaneously by SDO, GOES, and RHESSI. We calculated different energy components, including the radiative outputs in 1$-$8, 1$-$70, and 70$-$370 {AA}, total radiative loss, peak thermal energy derived from GOES and RHESSI, nonthermal energy in flare-accelerated electrons, and magnetic free energy before flares. It is found that the energy components increase systematically with the flare class, indicating that more energies are involved in larger flares. The magnetic free energies are larger than the nonthermal energies and radiative outputs of flares, which is consistent with the magnetic nature of flares. The ratio $frac{E_{nth}}{E_{mag}}$ of the four flares, being 0.70$-$0.76, is considerably higher than that of eruptive flares. Hence, this ratio may serve as an important factor that discriminates confined and eruptive flares. The nonthermal energies are sufficient to provide the heating requirements including the peak thermal energy and radiative loss. Our findings impose constraint on theoretical models of confined CRFs and have potential implication for the space weather forecast.
Solar flares with a fan-spine magnetic topology can form circular ribbons. The previous study based on Halpha line observations of the solar flares during March 05, 2014 by Xu et al. (2017) revealed uniform and continuous rotation of the magnetic fan-spine. Preliminary analysis of the flare time profiles revealed quasi-periodic pulsations (QPPs) with similar properties in hard X-rays, Halpha, and microwaves. In this work, we address which process the observed periodicities are related to: periodic acceleration of electrons or plasma heating? QPPs are analysed in the Halpha emission from the centre of the fan (inner ribbon R1), a circular ribbon (R2), a remote source (R3), and an elongated ribbon (R4) located between R2 and R3. The methods of correlation, Fourier, wavelet, and empirical mode decomposition are used. QPPs in Halpha emission are compared with those in microwave and X-ray emission. We found multi-wavelength QPPs with periods around 150 s, 125 s, and 190 s. The 150-s period is seen to co-exist in Halpha, hard X-rays, and microwave emissions, that allowed us to connect it with flare kernels R1 and R2. These kernels spatially coincide with the site of the primary flare energy release. The 125-s period is found in the Halpha emission of the elongated ribbon R4 and the microwave emission at 5.7 GHz during the decay phase. The 190-s period is present in the emission during all flare phases in the Halpha emission of both the remote source R3 and the elongated ribbon R4, in soft X-rays, and microwaves at 4--8 GHz. We connected the dominant 150-s QPPs with the slipping reconnection mechanism occurring in the fan. We suggested that the period of 125 s in the elongated ribbon can be caused by a kink oscillation of the outer spine connecting the primary reconnection site with the remote footpoint. The period of 190 s is associated with the 3-min sunspot oscillations.
We report our multiwavelength observations of two homologous circular-ribbon flares (CRFs) in active region 11991 on 2014 March 5, focusing on the transverse oscillations of an extreme-ultraviolet (EUV) loop excited by the flares. The transverse oscillations are of fast standing kink-mode. The first-stage oscillation triggered by the C2.8 flare is decayless with lower amplitudes (310$-$510 km). The periods (115$-$118 s) in different wavelengths are nearly the same, indicating coherent oscillations. The magnetic field of the loop is estimated to be 65$-$78 G. The second-stage oscillation triggered by the M1.0 flare is decaying with larger amplitudes (1250$-$1280 km). The periods decreases from 117 s in 211 {AA} to 70 s in 171 {AA}, implying a decrease of loop length or an implosion after a gradual expansion. The damping time, being 147$-$315 s, increases with the period, so that the values of $tau/P$ are close to each other in different wavelengths. The thickness of the inhomogeneous layer is estimated to be $sim$0farcs45 under the assumption of resonant absorption. This is the first observation of the excitation of two kink-mode loop oscillations by two sympathetic flares. The results are important for understanding of the excitation of kink oscillations of coronal loops and hence the energy balance in the solar corona. Our findings also validate the prevalence of significantly amplified amplitudes of oscillations by successive drivers.
Magnetic flux ropes play a key role in triggering solar flares in the solar atmosphere. In this paper, we investigate the evolution of active region NOAA 12268 within 36 hours from 2015 January 29 to 30, during which a flux rope was formed and three M-class and three C-class flares were triggered without coronal mass ejections. During the evolution of the active region, the flare emission seen in the H$alpha$ and ultraviolet wavebands changed from a circular shape (plus an adjacent conjugated ribbon and a remote ribbon) to three relatively straight and parallel ribbons. Based on a series of reconstructed nonlinear force-free fields, we find sheared or twisted magnetic field lines and a large-scale quasi-separatrix layer (QSL) associated with 3D null points in a quadrupolar magnetic field. These features always existed and constantly evolved during the two days. The twist of the flux rope was gradually accumulated that eventually led to its instability. Around the flux rope, there were some topological structures, including a bald patch, a hyperbolic flux tube and a torus QSL. We discuss how the particular magnetic structure and its evolution produce the flare emission. In particular, the bifurcation of the flux rope can explain the transition of the flares from circular to parallel ribbons. We propose a two-stage evolution of the magnetic structure and its associated flares. In the first stage, sheared arcades under the dome-like large-scale QSL were gradually transformed into a flux rope through magnetic reconnection, which produced the circular ribbon flare. In the second stage, the flux rope bifurcated to form the three relatively straight and parallel flare ribbons.
Continuum emission, also called white-light emission (WLE), and permanent changes of the magnetic field ($Delta{B}_{{rm{LOS}}}$) are often observed during solar flares. But their relation and their precise mechanisms are still unknown. We study statistically the relationship between $Delta{B}_{{rm{LOS}}}$ and WLE during 75 solar flares of different strengths and locations on the solar disk. We analyze SDO/HMI data and determine for each pixel in each flare if it exhibited WLE and/or $Delta{B}_{{rm{LOS}}}$. We then investigate the occurrence, strength, and spatial size of the WLE, its dependence on flare energy, and its correlation to the occurrence of $Delta{B}_{{rm{LOS}}}$. We detected WLE in 44/75 flares and $Delta{B}_{{rm{LOS}}}$ in 59/75 flares. We find that WLE and $Delta{B}_{{rm{LOS}}}$ are related, and their locations often overlap between 0-60%. Not all locations coincide, thus potentially indicating differences in their origin. We find that the WL area is related to the flare class by a power law and extend the findings of previous studies, that the WLE is related to the flare class by a power law, to also be valid for C-class flares. To compare unresolved (Sun-as-a-star) WL measurements to our data, we derive a method to calculate temperatures and areas of such data under the black-body assumption. The calculated unresolved WLE areas improve, but still differ to the resolved flaring area by about a factor of 5-10 (previously 10-20), which could be explained by various physical or instrumental causes. This method could also be applied to stellar flares to determine their temperatures and areas independently.
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