192 research outputs found
Properties of Umbral Dots as Measured from the New Solar Telescope Data and MHD Simulations
We studied bright umbral dots (UDs) detected in a moderate size sunspot and
compared their statistical properties to recent MHD models. The study is based
on high resolution data recorded by the New Solar Telescope at the Big Bear
Solar Observatory and 3D MHD simulations of sunspots. Observed UDs, living
longer than 150 s, were detected and tracked in a 46 min long data set, using
an automatic detection code. Total 1553 (620) UDs were detected in the
photospheric (low chromospheric) data. Our main findings are: i) none of the
analyzed UDs is precisely circular, ii) the diameter-intensity relationship
only holds in bright umbral areas, and iii) UD velocities are inversely related
to their lifetime. While nearly all photospheric UDs can be identified in the
low chromospheric images, some small closely spaced UDs appear in the low
chromosphere as a single cluster. Slow moving and long living UDs seem to exist
in both the low chromosphere and photosphere, while fast moving and short
living UDs are mainly detected in the photospheric images. Comparison to the 3D
MHD simulations showed that both types of UDs display, on average, very similar
statistical characteristics. However, i) the average number of observed UDs per
unit area is smaller than that of the model UDs, and ii) on average, the
diameter of model UDs is slightly larger than that of observed ones.Comment: Accepted by the AP
Aneurysm of internal carotid artery, which simulated paratonsillary abscess (clinical case)
This article describes a rare case of paratonsillar abscess imitation among the patients with the aneurysm of ICA. The purpose of the study is to describe the methods of this disease diagnosing using the example of a clinical case, to give a general description of this nosology occurrence frequency, and to recommend a detailed approach to the differential diagnosis among this category of patient
Distinction of representations via Bruhat-Tits buildings of p-adic groups
Introductory and pedagogical treatmeant of the article : P. Broussous
"Distinction of the Steinberg representation", with an appendix by Fran\c{c}ois
Court\`es, IMRN 2014, no 11, 3140-3157. To appear in Proceedings of Chaire Jean
Morlet, Dipendra Prasad, Volker Heiermann Ed. 2017. Contains modified and
simplified proofs of loc. cit. This article is written in memory of
Fran\c{c}ois Court\`es who passed away in september 2016.Comment: 33 pages, 4 figure
Turbulence in the Solar Atmosphere: Manifestations and Diagnostics via Solar Image Processing
Intermittent magnetohydrodynamical turbulence is most likely at work in the
magnetized solar atmosphere. As a result, an array of scaling and multi-scaling
image-processing techniques can be used to measure the expected
self-organization of solar magnetic fields. While these techniques advance our
understanding of the physical system at work, it is unclear whether they can be
used to predict solar eruptions, thus obtaining a practical significance for
space weather. We address part of this problem by focusing on solar active
regions and by investigating the usefulness of scaling and multi-scaling
image-processing techniques in solar flare prediction. Since solar flares
exhibit spatial and temporal intermittency, we suggest that they are the
products of instabilities subject to a critical threshold in a turbulent
magnetic configuration. The identification of this threshold in scaling and
multi-scaling spectra would then contribute meaningfully to the prediction of
solar flares. We find that the fractal dimension of solar magnetic fields and
their multi-fractal spectrum of generalized correlation dimensions do not have
significant predictive ability. The respective multi-fractal structure
functions and their inertial-range scaling exponents, however, probably provide
some statistical distinguishing features between flaring and non-flaring active
regions. More importantly, the temporal evolution of the above scaling
exponents in flaring active regions probably shows a distinct behavior starting
a few hours prior to a flare and therefore this temporal behavior may be
practically useful in flare prediction. The results of this study need to be
validated by more comprehensive works over a large number of solar active
regions.Comment: 26 pages, 7 figure
Generators for the hyperelliptic Torelli group and the kernel of the Burau representation at t = -1
We prove that the hyperelliptic Torelli group is generated by Dehn twists about
separating curves that are preserved by the hyperelliptic involution. This verifies a
conjecture of Hain. The hyperelliptic Torelli group can be identified with the kernel
of the Burau representation evaluated at t = −1 and also the fundamental group of
the branch locus of the period mapping, and so we obtain analogous generating sets
for those. One application is that each component in Torelli space of the locus of
hyperelliptic curves becomes simply connected when curves of compact type are added
Analysis of cancellation in two-dimensional magnetohydrodynamic turbulence
A signed measure analysis of two-dimensional intermittent magnetohydrodynamic
turbulence is presented. This kind of analysis is performed to characterize the
scaling behavior of the sign-oscillating flow structures, and their geometrical
properties. In particular, it is observed that cancellations between positive
and negative contributions of the field inside structures, are inhibited for
scales smaller than the Taylor microscale, and stop near the dissipative scale.
Moreover, from a simple geometrical argument, the relationship between the
cancellation exponent and the typical fractal dimension of the structures in
the flow is obtained.Comment: 21 pages, 5 figures (3 .jpg not included in the latex file
Multiresolution analysis of active region magnetic structure and its correlation with the Mt. Wilson classification and flaring activity
Two different multi-resolution analyses are used to decompose the structure
of active region magnetic flux into concentrations of different size scales.
Lines separating these opposite polarity regions of flux at each size scale are
found. These lines are used as a mask on a map of the magnetic field gradient
to sample the local gradient between opposite polarity regions of given scale
sizes. It is shown that the maximum, average and standard deviation of the
magnetic flux gradient for alpha, beta, beta-gamma and beta-gamma-delta active
regions increase in the order listed, and that the order is maintained over all
length-scales. This study demonstrates that, on average, the Mt. Wilson
classification encodes the notion of activity over all length-scales in the
active region, and not just those length-scales at which the strongest flux
gradients are found. Further, it is also shown that the average gradients in
the field, and the average length-scale at which they occur, also increase in
the same order. Finally, there are significant differences in the gradient
distribution, between flaring and non-flaring active regions, which are
maintained over all length-scales. It is also shown that the average gradient
content of active regions that have large flares (GOES class 'M' and above) is
larger than that for active regions containing flares of all flare sizes; this
difference is also maintained at all length-scales.Comment: Accepted for publication in Solar Physic
Are Solar Active Regions with Major Flares More Fractal, Multifractal, or Turbulent than Others?
Multiple recent investigations of solar magnetic field measurements have
raised claims that the scale-free (fractal) or multiscale (multifractal)
parameters inferred from the studied magnetograms may help assess the eruptive
potential of solar active regions, or may even help predict major flaring
activity stemming from these regions. We investigate these claims here, by
testing three widely used scale-free and multiscale parameters, namely, the
fractal dimension, the multifractal structure function and its inertial-range
exponent, and the turbulent power spectrum and its power-law index, on a
comprehensive data set of 370 timeseries of active-region magnetograms (17,733
magnetograms in total) observed by SOHO's Michelson Doppler Imager (MDI) over
the entire Solar Cycle 23. We find that both flaring and non-flaring active
regions exhibit significant fractality, multifractality, and non-Kolmogorov
turbulence but none of the three tested parameters manages to distinguish
active regions with major flares from flare-quiet ones. We also find that the
multiscale parameters, but not the scale-free fractal dimension, depend
sensitively on the spatial resolution and perhaps the observational
characteristics of the studied magnetograms. Extending previous works, we
attribute the flare-forecasting inability of fractal and multifractal
parameters to i) a widespread multiscale complexity caused by a possible
underlying self-organization in turbulent solar magnetic structures, flaring
and non-flaring alike, and ii) a lack of correlation between the fractal
properties of the photosphere and overlying layers, where solar eruptions
occur. However useful for understanding solar magnetism, therefore, scale-free
and multiscale measures may not be optimal tools for active-region
characterization in terms of eruptive ability or, ultimately,for major
solar-flare prediction.Comment: 25 pages, 7 figures, 2 tables, Solar Phys., in pres
A Nanoflare Distribution Generated by Repeated Relaxations Triggered by Kink Instability
Context: It is thought likely that vast numbers of nanoflares are responsible
for the corona having a temperature of millions of degrees. Current
observational technologies lack the resolving power to confirm the nanoflare
hypothesis. An alternative approach is to construct a magnetohydrodynamic
coronal loop model that has the ability to predict nanoflare energy
distributions.
Aims: This paper presents the initial results generated by such a model. It
predicts heating events with a range of sizes, depending on where the
instability threshold for linear kink modes is encountered. The aims are to
calculate the distribution of event energies and to investigate whether kink
instability can be predicted from a single parameter.
Methods: The loop is represented as a straight line-tied cylinder. The
twisting caused by random photospheric motions is captured by two parameters,
representing the ratio of current density to field strength for specific
regions of the loop. Dissipation of the loop's magnetic energy begins during
the nonlinear stage of the instability, which develops as a consequence of
current sheet reconnection. After flaring, the loop evolves to the state of
lowest energy where, in accordance with relaxation theory, the ratio of current
to field is constant throughout the loop and helicity is conserved.
Results: The results suggest that instability cannot be predicted by any
simple twist-derived property reaching a critical value. The model is applied
such that the loop undergoes repeated episodes of instability followed by
energy-releasing relaxation. Hence, an energy distribution of the nanoflares
produced is collated.
Conclusions: The final energy distribution features two nanoflare populations
that follow different power laws. The power law index for the higher energy
population is more than sufficient for coronal heating.Comment: 13 pages, 18 figure
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