We investigate the evolution of NOAA Active Region 11817 during 2013 August 10-12, when it developed a complex field configuration and produced four confined, followed by two eruptive, flares. These C-and-above flares are all associated with a magnetic flux rope (MFR) located along the major polarity inversion line, where shearing and converging photospheric flows are present. Aided by the nonlinear force-free field modeling, we identify the MFR through mapping magnetic connectivities and computing the twist number T w for each individual field line. The MFR is moderately twisted (|T w | < 2) and has a well-defined boundary of high squashing factor Q. We found that the field line with the extremum |T w | is a reliable proxy of the rope axis, and that the MFR's peak |T w | temporarily increases within half an hour before each flare while it decreases after the flare peak for both confined and eruptive flares. This pre-flare increase 9 Max-Planck-Institut für Sonnensystemforschung, Justus-von-Liebig-Weg 3, 37077 Göttingen, Germany -2 -in |T w | has little effect on the active region's free magnetic energy or any other parameters derived for the whole region, due to its moderate amount and the MFR's relatively small volume, while its decrease after flares is clearly associated with the stepwise decrease in the whole region's free magnetic energy due to the flare. We suggest that T w may serve as a useful parameter in forewarning the onset of eruption, and therefore, the consequent space weather effects. The helical kink instability is identified as the prime candidate onset mechanism for the considered flares.
We studied the background field for 60 two-ribbon flares of M-and-above classes during 2011-2015. These flares are categorized into two groups, i.e., eruptive and confined flares, based on whether a flare is associated with a coronal mass ejection or not. The background field of source active regions is approximated by a potential field extrapolated from the B z component of vector magnetograms provided by the Helioseismic and Magnetic Imager. We calculated the decay index n of the background field above the flaring polarity inversion line, and defined a critical height h crit corresponding to the theoretical threshold (n crit = 1.5) of the torus instability. We found that h crit is approximately half of the distance between the centroids of opposite polarities in active regions and that the distribution of h crit is bimodal: it is significantly higher for confined flares than for eruptive ones. The decay index increases monotonously with increasing height for 86% (84%) of the eruptive (confined) flares but displays a saddle-like profile for the rest, 14% (16%), which are found exclusively in active regions of multipolar field configuration. Moreover, n at the saddle bottom is significantly smaller in confined flares than that in eruptive ones. These results highlight the critical role of background field in regulating the eruptive behavior of two-ribbon flares.
Solar active regions (ARs) are the major sources of two of the most violent solar eruptions, namely flares and coronal mass ejections (CMEs). The largest AR in the past 24 years, NOAA AR 12192, which crossed the visible disk from 2014 October 17 to 30, unusually produced more than one hundred flares, including 32 M-class and 6 X-class ones, but only one small CME. Flares and CMEs are believed to be two phenomena in the same eruptive process. Why is such a flare-rich AR so CME-poor? We compared this AR with other four ARs; two were productive in both and two were inert. The investigation of the photospheric parameters based on the SDO/HMI vector magnetogram reveals that the flare-rich AR 12192, as with the other two productive ARs, has larger magnetic flux, current, and free magnetic energy than the two inert ARs but, in contrast to the two productive ARs, it has no strong, concentrated current helicity along both sides of the flaring neutral line, indicating the absence of a mature magnetic structure consisting of highly sheared or twisted field lines. Furthermore, the decay index above the AR 12192 is relatively low, showing strong constraint. These results suggest that productive ARs are always large and have enough current and free energy to power flares, but whether or not a flare is accompanied by a CME is seemingly related to (1) the presence of a mature sheared or twisted core field serving as the seed of the CME, or (2) a weak enough constraint of the overlying arcades.
Typical solar flares display two quasi-parallel, bright ribbons on the chromosphere. In between is the polarity inversion line (PIL) separating concentrated magnetic fluxes of opposite polarity in active regions (ARs). Intriguingly a series of flares exhibiting X-shaped ribbons occurred at the similar location on the outskirts of NOAA AR 11967, where magnetic fluxes were scattered, yet three of them were alarmingly energetic. The X shape, whose center coincided with hard X-ray emission, was similar in UV/EUV, which cannot be accommodated in the standard flare model. Mapping out magnetic connectivities in potential fields, we found that the X morphology was dictated by the intersection of two quasi-separatrix layers, i.e., a hyperbolic flux tube (HFT), within which a separator connecting a double null was embedded. This topology was not purely local but regulated by fluxes and flows over the whole AR. The nonlinear force-free field model suggested the formation of a current layer at the HFT, where the current dissipation can be mapped to the X-shaped ribbons via field-aligned heat conduction. These results highlight the critical role of HFTs in 3D magnetic reconnection and have important implications for astrophysical and laboratory plasmas.
A methylene blue modi®ed graphite electrode was ®rst prepared by adsorbing the molecules on a spectrographic graphite disk electrode saturated with wax. Then, a novel hydrogen peroxide biosensor was fabricated based on immobilization of horseradish peroxidase on the methylene blue modi®ed graphite electrode by cross-linking with glutaraldehyde. This electrode exhibited excellent electrochemical activity in aqueous solutions, and the electron transfer rate constant for the adsorbed methylene blue was determined as 0.12 s 71. It was demonstrated that the modi®ed methylene blue molecules could shuttle electrons between immobilized HRP and graphite substrate very well for H 2 O 2 sensing. The characteristics of this sensor were evaluated with respect to applied potential, pH and temperature. A detection limit of 3.0610 76 molaL hydrogen peroxide was examined at simple experimental conditions. Dopamine and ascorbic acid did not interfere with the determination. This sensor was also stable, reproducible and outstanding for long-term use.
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