158,934 research outputs found
Non-potential field formation in the X-shaped quadrupole magnetic field configuration
Some types of solar flares are observed in X-shaped quadrupolar field
configuration. To understand the magnetic energy storage in such a region, we
studied non-potential field formation in an X-shaped quadrupolar field region
formed in the active region NOAA 11967, which produced three X-shaped M-class
flares on February 2, 2014. Nonlinear force-free field modeling was applied to
a time series of vector magnetic field maps from the Solar Optical Telescope on
board Hinode and Helioseismic and Magnetic Imager on board Solar Dynamics
Observatory. Our analysis of the temporal three-dimensional magnetic field
evolution shows that the sufficient free energy had already been stored more
than 10 hours before the occurrence of the first M-class flare and that the
storage was observed in a localized region. In this localized region,
quasi-separatrix layers (QSLs) started to develop gradually from 9 hours before
the first M-class flare. One of the flare ribbons that appeared in the first
M-class flare was co-spatial with the location of the QSLs, suggesting that the
formation of the QSLs is important in the process of energy release. These QSLs
do not appear in the potential field calculation, indicating that they were
created by the non-potential field. The formation of the QSLs was associated
with the transverse photospheric motion of the pre-emerged flux and the
emergence of a new flux. This observation indicates that the occurrence of the
flares requires the formation of QSLs in the non-potential field in which free
magnetic energy is stored in advance.Comment: Accepted for publication in Ap
A ribbon of twins for extracting vessel boundaries
This paper presents an efficient model for automatic detection and extraction of blood vessels in ocular fundus images. The model is formed using a combination of the concept of ribbon snakes and twin snakes. On each edge, the twin concept is introduced by using two snakes, one inside and one outside the boundary. The ribbon concept integrates the pair of twins on the two vessel edges into a single ribbon. The twins maintain the consistency of the vessel width, particularly on very blurred, thin and noisy vessels. The model exhibits excellent performance in extracting the boundaries of vessels, with improved robustness compared to alternative models in the presence of occlusion, poor contrast or noise. Results are presented which demonstrate the performance of the discussed edge extraction method, and show a significant improvement compared to classical snake formulations
Flare induced penumbra formation in the sunspot of NOAA 10838
We have observed formation of penumbrae on a pore in the active region
NOAA10838 using Dunn Solar Telescope at NSO,Sunpot,USA. Simultaneous
observations using different instruments (DLSP,UBF,Gband and CaK) provide us
with vector magnetic field at photosphere, intensity images and Doppler
velocity at different heights from photosphere to chromosphere. Results from
our analysis of this particular data-set suggests that penumbrae are formed as
a result of relaxation of magnetic field due to a flare happening at the same
time. Images in \Halpha\ show the flare (C 2.9 as per GOES) and vector magnetic
fields show a re-orientation and reduction in the global value (a
measure of twist). We feel such relaxation of loop structures due to
reconnections or flare could be one of the way by which field lines fall back
to the photosphere to form penumbrae.Comment: 4 pages, Presented at IAU symposium 273- Physics of Sun and Starspot
Remnant radio-loud AGN in the Herschel-ATLAS field
Only a small fraction of observed active galactic nuclei (AGN) display large-scale radio emission associated with jets, yet these radio-loud AGN have become increasingly important in models of galaxy evolution. In determining the dynamics and energetics of the radio sources over cosmic time, a key question concerns what happens when their jets switch off. The resulting ‘remnant' radio-loud AGN have been surprisingly evasive in past radio surveys, and therefore statistical information on the population of radio-loud AGN in their dying phase is limited. In this paper, with the recent developments of Low-Frequency Array (LOFAR) and the Very Large Array, we are able to provide a systematically selected sample of remnant radio-loud AGN in the Herschel-ATLAS field. Using a simple core-detection method, we constrain the upper limit on the fraction of remnants in our radio-loud AGN sample to 9 per cent, implying that the extended lobe emission fades rapidly once the core/jets turn off. We also find that our remnant sample has a wide range of spectral indices (−1.5 ⩽ α1400150 ⩽ −0.5), confirming that the lobes of some remnants may possess flat spectra at low frequencies just as active sources do. We suggest that, even with the unprecedented sensitivity of LOFAR, our sample may still only contain the youngest of the remnant population
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