Modeling Repeatedly FlaringδSunspots

Abstract

Active regions (AR) appearing on the surface of the Sun are classified into α, β, γ, and δ by the rules of the Mount Wilson Observatory, California on the basis of their topological complexity. Amongst these, the δ-sunspots are known to be super-active and produce the most X-ray flares. Here, we present results from a simulation of the Sun by mimicking the upper layers and the corona, but starting at a more primitive stage than any earlier treatment. We find that this initial state consisting of only a thin sub-photospheric magnetic sheet breaks into multiple flux-tubes which evolve into a colliding-merging system of spots of opposite polarity upon surface emergence, similar to those often seen on the Sun. The simulation goes on to produce many exotic δ-sunspot associated phenomena: repeated flaring in the range of typical solar flare energy release and ejective helical flux ropes with embedded cool-dense plasma filaments resembling solar coronal mass ejections.Delta-sunspots are formed when two sunspots of opposite polarity magnetic field appear very close to each other and reside in the same penumbra, the radial filamentary structure outside the umbral region of the strongest magnetic fields. Strong shear and horizontal magnetic fields often exist at the polarity-inversion line separating the two polarities [1]. The subsurface processes which form the δ-sunspots are still debated. Early observational studies [2,3] propose that δ-sunspots form from collision-merging of topologically separate dipoles, while numerical simulations by [4,5] show that kink unstable magnetic flux-tube -helical field lines winding around a central axis -emerging from the subsurface can have a δ-sunspot like structure. More recently, attempts to model the δ-spot in the NOAA AR 11158 utilized a uniformly twisted sub-surface flux-tube initially buoyant in two adjacent regions along its length [6,7]. Also, [8] found a magnetic flux concentration resembling a δ sunspot in their stratified helical dynamo simulation. These studies did not report any flaring activity. On the other hand [9] initialized their simulation with two parallel flux-tubes each lying at a different depth from the surface and with a different value of the initial magnetic twist which later evolved into a δ-sunspot like structure and powered multiple reconnection events. Delta-sunspots are highly flare-productive -95% of the strongest (X-class) X-ray flares originate from these regions [3]. Using a realistic numerical simulation [10] showed that interaction between adjacent expanding magnetic bipoles pressing against each other can lead to the formation of strong current layers in the atmosphere which in turn lead to repeated flaring in the region. Here, we report on a three-dimensional magneto-hydrodynamic (MHD) simulation of the formation of a δ-sunpot like region as a result of the break-up of a cool magnetic layer embedded in the upper solar convection zone into several flux-tubes due to the growth of three-dimensional unstable modes excited in the layer. This instab...

Magnetic Properties of Solar Active Regions That Govern Large Solar Flares and Eruptions

“…Therefore, we need a systematic survey using flux emergence simulations to model these types of ARs (Toriumi et al 2014a;Fang & Fan 2015;Takasao et al 2015;Chatterjee et al 2016) and investigate their formation processes as well as the storage of magnetic energy (amount, place, etc.). In Figure 4, we found that S ribbon /S spot and |Φ| ribbon /|Φ| AR are larger for the CME-eruptive events, which may indicate the importance of the relative magnetic structure of the flaring region and the entire AR.…”

Applying the Weighted Horizontal Magnetic Gradient Method to a Simulated Flaring Active Region

“…Our analysis is centered on the numerical case study reported in Chatterjee et al (2016). For completeness, we briefly describe the salient points of the model setup here.…”

“…We test the concept of the weighted horizontal magnetic gradient (WG M ) method proposed by Korsós et al (2015, hereafter K15) by analyzing the emerging magnetic flux that generates a series of flares in the simulation, first reported in Chatterjee et al (2016).I nS e c t i o n2,w eb r i e fly outline the simulation setup and the numerical code used. In Section 3, we perform a detailed analysis of the flaring regions of the simulation in terms of ohmic heating and temperature increase for comparison of the findings with the WG M method given in Section 4.…”

“…In order to differentiate the flare onset signal from other fluctuations, we combine the information on change of slope of energy versus time with the first appearance of the flashes of high temperature in the accompanying animation files at three different heights. Also, we use the information from the time of occurrence of the bipolar reconnection jets in Figure 4(b) of Chatterjee et al (2016) for the B 1 , C 1 , and C 2 flares, which matches with the time from the energy curves in Figure 4(a) of the same paper. The magnetic energy, B  d , released during the B 2 flare is calculated to be 2×10 29 erg.…”

“…The magnetic energy, B  d , released during the B 2 flare is calculated to be 2×10 29 erg. The B  d values for the B 1 and C 1 flares were given in Chatterjee et al (2016), as well as in Table 1 for completeness. The Poynting flux into the area surrounding the flare also decreases rapidly after t = 215.03 minutes and becomes close to zero.…”