Prominence eruption initiated by helical kink-instability of an embedded flux rope

We study the triggering mechanism of a limb-prominence eruption and the associated coronal mass ejection near AR 12342 using SDO and LASCO/SOHO observations. The prominence is seen with an embedded flux thread (FT) at one end and bifurcates from the middle to a different footpoint location. The morphological evolution of the FT is similar to an unstable flux rope (FR), which we regard as prominence embedded FR. The FR twist exceeds the critical value. In addition, the morphology of the prominence plasma in 304\AA~images marks the helical nature of the magnetic skeleton with a total of 2.96 turns along arc length. The potential field extrapolation model indicates that the critical height of the background magnetic field gradient falls within the inner corona (105Mm) consistent with the extent of coronal plasma loops. These results suggest that the helical kink instability in the embedded FR caused the slow rise of the prominence to a height of the torus instability domain. Moreover, the differential emission measure analysis unveils heating of the prominence plasma to coronal temperatures during eruption, suggesting a reconnection-related heating underneath the upward rising embedded FR. The prominence starts with a slow rise motion of 10km/s, followed by fast and slow acceleration phases having an average acceleration of $28.9m/s^2$, $2.4m/s^2$ in C2, C3 field of view respectively. As predicted by previous numerical simulations, the observed synchronous kinematic profiles of the CME leading edge and the core supports the involved FR instability in the prominence initiation.Comment: Accepted in ApJ, 13 pages, 9 figure

Electronic structure and optical band gap of CoFe2O4 thin films

Electronic structure and optical band gap of CoFe2O4 thin films grown on (001) oriented LaAlO3 have been investigated. Surprisingly, these films show additional Raman modes at room temperature as compared to a bulk spinel structure. The splitting of Raman modes is explained by considering the short-range ordering of Co and Fe cations in octahedral site of spinel structure. In addition, an expansion of band-gap is observed with the reduction of film thickness, which is explained by the quantum size effect and misfit dislocation. Such results provide interesting insights for the growth of spinel phases.Comment: 15 pages, 5 figures, and 1 table; Accepted and to be published/appeared in APL soo

Atomically thin group-V elemental films: theoretical investigations of antimonene allotropes

Group-V elemental monolayers including phosphorene are emerging as promising 2D materials with semiconducting electronic properties. Here, we present the results of first principles calculations on stability, mechanical and electronic properties of 2D antimony (Sb), antimonene. Our calculations show that free-standing {\alpha} and \b{eta} allotropes of antimonene are stable and semiconducting. The {\alpha}-Sb has a puckered structure with two atomic sub-layers and \b{eta}-Sb has a buckled hexagonal lattice. The calculated Raman spectra and STM images have distinct features thus facilitating characterization of both allotropes. The \b{eta}-Sb has nearly isotropic mechanical properties while {\alpha}-Sb shows strongly anisotropic characteristics. An indirect-direct band gap transition is expected with moderate tensile strains applied to the monolayers, which opens up the possibility of their applications in optoelectronics

Statistical study of magnetic non-potential measures in confined and eruptive flares

Using the HMI/SDO vector magnetic field observations, we studied the relation of degree of magnetic non-potentiality with the observed flare/CME in active regions. From a sample of 77 flare/CME cases, we found a general relation that degree of non-potentiality is positively correlated with the flare strength and the associated CME speeds. Since the magnetic flux in the flare-ribbon area is more related to the reconnection, we trace the strong gradient polarity inversion line (SGPIL), Schrijver's R value manually along the flare-ribbon extent. Manually detected SGPIL length and R values show higher correlation with the flare strength and CME speed than the automatically traced values without flare-ribbon information. It highlights the difficulty of predicting the flare strength and CME speed a priori from the pre-flare magnetograms used in flare prediction models. Although the total, potential magnetic energy proxies show weak positive correlation, the decrease in free energy exhibits higher correlation (0.56) with the flare strength and CME speed. Moreover, the eruptive flares have threshold of SGPIL length (31Mm), R value ($1.6\times10^{19}$Mx), free-energy decrease ($2\times10^{31}$erg) compared to confined ones. In 90\% eruptive flares, the decay-index curve is steeper reaching $n_{crit}=1.5$ within 42Mm, whereas it is beyond 42Mm in $>70$% confined flares. While indicating the improved statistics in the predictive capability of the AR eruptive behavior with the flare-ribbon information, our study provides threshold magnetic properties for a flare to be eruptive.Comment: 12 pages, 9 figures, accepted in Ap

Sunspot Rotation as a Driver of Major Solar Eruptions in NOAA Active Region 12158

We studied the developing conditions of sigmoid structure under the influence of magnetic non-potential characteristics of a rotating sunspot in the active region (AR) 12158. Vector magnetic field measurements from Helioseismic Magnetic Imager and coronal EUV observations from Atmospheric Imaging Assembly reveal that the erupting inverse-S sigmoid had roots in the location of the rotating sunspot. Sunspot rotates at a rate of 0-5deg/h with increasing trend in the first half followed by a decrease. Time evolution of many non-potential parameters had a well correspondence with the sunspot rotation. The evolution of the AR magnetic structure is approximated by a time series of force free equilibria. The NLFFF magnetic structure around the sunspot manifests the observed sigmoid structure. Field lines from the sunspot periphery constitute the body of the sigmoid and those from interior overly the sigmoid similar to a fluxrope structure. While the sunspot is being rotating, two major CME eruptions occurred in the AR. During the first (second) event, the coronal current concentrations enhanced (degraded) consistent with the photospheric net vertical current, however the magnetic energy is released during both the cases. The analysis results suggest that the magnetic connections of the sigmoid are driven by slow motion of sunspot rotation, which transforms to a highly twisted flux rope structure in a dynamical scenario. An exceeding critical twist in the flux rope probably leads to the loss of equilibrium and thus triggering the onset of two eruptions.Comment: 13pages, 11 figures, Accepted in Ap

Effects of extrinsic point defects in phosphorene: B, C, N, O and F Adatoms

Phosphorene is emerging as a promising 2D semiconducting material with a direct band gap and high carrier mobility. In this paper, we examine the role of the extrinsic point defects including surface adatoms in modifying the electronic properties of phosphorene using density functional theory. The surface adatoms considered are B, C, N, O and F with a [He] core electronic configuration. Our calculations show that B and C, with electronegativity close to P, prefer to break the sp3 bonds of phosphorene, and reside at the interstitial sites in the 2D lattice by forming sp2 bonds with the native atoms. On the other hand, N, O and F, which are more electronegative than P, prefer the surface sites by attracting the lone pairs of phosphorene. B, N and F adsorption will also introduce local magnetic moment to the lattice. Moreover, B, C, N and F adatoms will modify the band gap of phosphorene yielding metallic transverse tunneling characters. Oxygen does not modify the band gap of phosphorene, and a diode like tunneling behavior is observed. Our results therefore offer a possible route to tailor the electronic and magnetic properties of phosphorene by the adatom functionalization, and provide the physical insights of the environmental sensitivity of phosphorene, which will be helpful to experimentalists in evaluating the performance and aging effects of phosphorene-based electronic devices

Tunnelling Characteristics of Stone-Wales Defects in Monolayers of Sn and Group-V Elements

Topological defects in ultrathin layers are often formed during synthesis and processing, thereby, strongly influencing their electronic properties . In this paper, we investigate the role of Stone-Wales (SW) defects in modifying the electronic properties of the monolayers of Sn and group-V elements. The calculated results find the electronic properties of stanene (monolayer of Sn atoms) to be strongly dependent on the concentration of SW-defects e.g., defective stanene has nearly zero band gap (~ 0.03 eV) for the defect concentration of 2.2 x 10^13 cm^-2 which opens up to 0.2 eV for the defect concentration of 3.7 x 10^13 cm^-2. In contrast, SW-defects appear to induce conduction states in the semiconducting monolayers of group-V elements. These conduction states act as channels for electron tunnelling, and the calculated tunnelling characteristics show the highest differential conductance for the negative bias with the asymmetric current-voltage characteristics. On the other hand, the highest differential conductance was found for the positive bias in stanene. Simulated STM topographical images of stanene and group-V monolayers show distinctly different features in terms of their cross-sectional views and distance-height profiles which can serve as fingerprints to identify the topological defects in the monolayers of group-IV and group-V elements in experiments.Comment: 18 pages, 5 figures, 1 tabl