117 research outputs found
Quasi-Static 3D-Magnetic Field Evolution in Solar Active Region NOAA 11166 Associated with X1.5 Flare
We study the quasi-static evolution of coronal magnetic fields constructed
from the Non Linear Force Free Field (NLFFF) approximation aiming to understand
the relation between the magnetic field topology and ribbon emission during an
X1.5 flare in active region (AR) NOAA 11166. The flare with a quasi-elliptical,
and two remote ribbons occurred on March 9, 2011 at 23:13UT over a positive
flux region surrounded by negative flux at the center of the bipolar AR. Our
analysis of the coronal magnetic structure with potential and NLFFF solutions
unveiled the existence of a single magnetic null point associated with a
fan-spine topology and is co-spatial with the hard X-ray source. The footpoints
of the fan separatrix surface agree with the inner edge of the quasi-elliptical
ribbon and the outer spine is linked to one of the remote ribbons. During the
evolution, the slow footpoint motions stressed the fieldlines along the
polarity inversion line and caused electric current layers in the corona around
the fan separatrix surface. These current layers trigger magnetic reconnection
as a consequence of dissipating currents, which are visible as cusped shape
structures at lower heights. The reconnection process reorganised the magnetic
field topology whose signatures are observed at the separatrices/QSL structure
both in the photosphere and corona during the pre-to-post flare evolution. In
agreement with previous numerical studies, our results suggest that the
line-tied footpoint motions perturb the fan-spine system and cause null point
reconnection, which eventually causes the flare emission at the footpoints of
the fieldlines.Comment: Accepted, 2014, The Astrophysical Journa
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
Magnetic structure of solar active region NOAA 11158
Magnetic fields in the solar corona are responsible for a wide range of
phenomena. However, any direct measurements of the coronal magnetic fields are
very difficult due to lack of suitable spectral lines, weak magnetic fields,
and high temperatures. Therefore, one extrapolates photospheric field
measurements into the corona. Owing to low coronal plasma , we can apply
a force-free model in lowest order to study the slow evolution of active region
(AR) magnetic fields. On applying these models to AR 11158 and compared with
coronal plasma tracers, we found that (1) the approximation of potential field
to coronal structures over large length scales is a reasonable one, 2) linear
force-free (LFF) assumption to AR coronal fields may not be applicable model as
it assumes uniform twist over the entire AR, and 3) for modeling fields at
sheared, stressed locations where energy release in the form of flares are
usually observed, non-linear force free fields (NLFFF) seem to provide a good
approximation. The maximum available free-energy profile shows step-wise
decrease that is sufficient to power an M-class flare as observed.Comment: To appear in BASI 2013, Bulletin of Astronomical Society of Indi
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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