3,277 research outputs found
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 ,
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
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
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
(Mx), free-energy decrease (erg) compared to
confined ones. In 90\% eruptive flares, the decay-index curve is steeper
reaching within 42Mm, whereas it is beyond 42Mm in %
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
Formation and eruption of sigmoidal structure from a weak field region of NOAA 11942
Using observations from Solar Dynamics Observatory, we studied an interesting
example of a sigmoid formation and eruption from small-scale flux canceling
regions of active region (AR) 11942. Analysis of HMI and AIA observations infer
that initially the AR is compact and bipolar in nature, evolved to sheared
configuration consisting of inverse J-shaped loops hosting a filament channel
over a couple of days. By tracking the photospheric magnetic features, shearing
and converging motions are observed to play a prime role in the development of
S-shaped loops and further flux cancellation leads to tether-cutting
reconnection of J-loops. This phase is co-temporal with the filament rise
motion followed by sigmoid eruption at 21:32 UT on January 6. The flux rope
rises in phases of slow (v = 26 km~s) and fast (a= 55
ms) rise motion categorizing the CME as slow with an associated weak
C1.0 class X-ray flare. The flare ribbon separation velocity peaks at around
peak time of the flare at which maximum reconnection rate (2.14 Vcm)
occurs. Further, the EUV light-curves of 131, 171\AA~have delayed peaks of 130
minutes compared to 94\AA~and is explained by differential emission measure.
Our analysis suggests that the energy release is proceeded in a much long time
duration, manifesting the onset of filament rise and eventual eruption driven
by converging and canceling flux in the photosphere. Unlike strong eruption
events, the observed slow CME and weak flare are indications of slow runway
tether-cutting reconnection where most of the sheared arcade is relaxed during
the extended post phase of the eruption.Comment: Accepted for Publication in The Astrophysical journal on 19 February,
2019. It has 17 pages including 12 figure
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