5,583 research outputs found
3D Temperature Mapping of Solar Photospheric Fine Structure Using Ca II H Filtergrams
Context. The wings of the Ca II H and K lines provide excellent photospheric
temperature diagnostics. At the Swedish 1-meter Solar Telescope the blue wing
of Ca II H is scanned with a narrowband interference filter mounted on a
rotation stage. This provides up to 0"10 spatial resolution filtergrams at high
cadence that are concurrent with other diagnostics at longer wavelengths. Aims.
The aim is to develop observational techniques that provide the photospheric
temperature stratification at the highest spatial resolution possible and use
those to compare simulations and observations at different heights. Methods. We
use filtergrams in the Ca II H blue wing obtained with a tiltable interference
filter at the SST. Synthetic observations are produced from 3D HD and 3D MHD
numerical simulations and degraded to match the observations. The temperature
structure obtained from applying the method to the synthetic data is compared
with the known structure in the simulated atmospheres and with observations of
an active region. Cross-correlation techniques using restored non-simultaneous
continuum images are used to reduce high-altitude, small-scale seeing signal
introduced from the non-simultaneity of the frames when differentiating data.
Results. Temperature extraction using high resolution filtergrams in the Ca II
H blue wing works reasonably well when tested with simulated 3D atmospheres.
The cross-correlation technique successfully compensates the problem of
small-scale seeing differences and provides a measure of the spurious signal
from this source in differentiated data. Synthesized data from the simulated
atmospheres (including pores) match well the observations morphologically at
different observed heights and in vertical temperature gradients.Comment: Accepted the 10/10/2012 for publication in Astronomy & Astrophysics
in Section 9, The Sun. Published the 03/12/2012 v1 to v2: changed submission
metadata v2 to v3: small changes to match published versio
Dynamics of internetwork chromospheric fibrils: Basic properties and MHD kink waves
Using the spectroscopic imaging capabilities of the Swedish Solar Telescope,
we aim to provide the first investigation on the nature and dynamics of
elongated absorption features (fibrils) observed in H in the
internetwork. We observe and identify a number of internetwork fibrils, which
form away from the kilogauss, network magnetic flux, and we provide a synoptic
view on their behaviour. The internetwork fibrils are found to support
wave-like behaviour, which we interpret as Magnetohydrodynamic (MHD) kink
waves. The properties of these waves, that is, amplitude, period, and
propagation speed, are measured from time-distance diagrams and we attempt to
exploit them via magneto-seismology in order to probe the variation of plasma
properties along the wave-guides. We found that the Internetwork (IN) fibrils
appear, disappear, and re-appear on timescales of tens of minutes, suggesting
that they are subject to repeated heating. No clear photospheric footpoints for
the fibrils are found in photospheric magnetograms or H wing images.
However, we suggest that they are magnetised features as the majority of them
show evidence of supporting propagating MHD kink waves, with a modal period of
~s. Additionally, one IN fibril is seen to support a flow directed along
its elongated axis, suggesting a guiding field. The wave motions are found to
propagate at speeds significantly greater than estimates for typical
chromospheric sound speeds. Through their interpretation as kink waves, the
measured speeds provide an estimate for local average Alfv\'en speeds.
Furthermore, the amplitudes of the waves are also found to vary as a function
of distance along the fibrils, which can be interpreted as evidence of
stratification of the plasma in the neighbourhood of the IN fibril.Comment: Accepted Astronomy & Astrophysic
Chromospheric Inversions of a Micro-flaring Region
We use spectropolarimetric observations of the Ca II 8542~\AA\ line, taken
from the Swedish 1-m Solar Telescope (SST), in an attempt to recover dynamic
activity in a micro-flaring region near a sunspot via inversions. These
inversions show localized mean temperature enhancements of 1000~K in the
chromosphere and upper photosphere, along with co-spatial bi-directional
Doppler shifting of 5 - 10 km s. This heating also extends along a
nearby chromospheric fibril, co-spatial to 10 - 15 km s down-flows.
Strong magnetic flux cancellation is also apparent in one of the footpoints,
concentrated in the chromosphere. This event more closely resembles that of an
Ellerman Bomb (EB), though placed slightly higher in the atmosphere than is
typically observed.Comment: 9 pages, 9 figures, accepted in ApJ. Movies are stored here:
https://star.pst.qub.ac.uk/webdav/public/areid/Microflare
The effects of transients on photospheric and chromospheric power distributions
We have observed a quiet Sun region with the Swedish 1-meter Solar Telescope
(SST) equipped with CRISP Imaging SpectroPolarimeter. High-resolution,
high-cadence, H line scanning images were taken to observe different
layers of the solar atmosphere from the photosphere to upper chromosphere. We
study the distribution of power in different period-bands at different heights.
Power maps of the upper photosphere and the lower chromosphere show suppressed
power surrounding the magnetic-network elements, known as "magnetic shadows".
These also show enhanced power close to the photosphere, traditionally referred
to as "power halos". The interaction between acoustic waves and inclined
magnetic fields is generally believed to be responsible for these two effects.
In this study we explore if small-scale transients can influence the
distribution of power at different heights. We show that the presence of
transients, like mottles, Rapid Blueshifted Excursions (RBEs) and Rapid
Redshifted Excursions (RREs), can strongly influence the power-maps. The short
and finite lifetime of these events strongly affects all powermaps, potentially
influencing the observed power distribution. We show that Doppler-shifted
transients like RBEs and RREs that occur ubiquitously, can have a dominant
effect on the formation of the power halos in the quiet Sun. For magnetic
shadows, transients like mottles do not seem to have a significant effect in
the power suppression around 3 minutes and wave interaction may play a key role
here. Our high cadence observations reveal that flows, waves and shocks
manifest in presence of magnetic fields to form a non-linear
magnetohydrodynamic system.Comment: 11 pages, 11 Figures, 4 movies (will be available online in ApJ). ApJ
(accepted
Superlattice Magnetophonon Resonances in Strongly Coupled InAs/GaSb Superlattices
We report an experimental study of miniband magnetoconduction in
semiconducting InAs/GaSb superlattices. For samples with miniband widths below
the longitudinal optical phonon energy we identify a new superlattice
magnetophonon resonance (SLMPR) caused by resonant scattering of electrons
across the mini-Brillouin zone. This new resonant feature arises directly from
the drift velocity characteristics of the superlattice dispersion and total
magnetic quantisation of the superlattice Landau level minibands.Comment: 9 pages, 8 figures, submitted to Phys. Rev.
Opposite polarity field with convective downflow and its relation to magnetic spines in a sunspot penumbra
We discuss NICOLE inversions of Fe I 630.15 nm and 630.25 nm Stokes spectra
from a sunspot penumbra recorded with the CRISP imaging spectropolarimeter on
the Swedish 1-m Solar Telescope at a spatial resolution close to 0.15". We
report on narrow radially extended lanes of opposite polarity field, located at
the boundaries between areas of relatively horizontal magnetic field (the
intra-spines) and much more vertical field (the spines). These lanes harbor
convective downflows of about 1 km/s. The locations of these downflows close to
the spines agree with predictions from the convective gap model (the "gappy
penumbra") proposed six years ago, and more recent 3D MHD simulations. We also
confirm the existence of strong convective flows throughout the entire
penumbra, showing the expected correlation between temperature and vertical
velocity, and having vertical RMS velocities of about 1.2 km/s.Comment: Accepted for publication in A&A (06-March-2013). Minor corrections
made in this version
Beyond Correlation Filters: Learning Continuous Convolution Operators for Visual Tracking
Discriminative Correlation Filters (DCF) have demonstrated excellent
performance for visual object tracking. The key to their success is the ability
to efficiently exploit available negative data by including all shifted
versions of a training sample. However, the underlying DCF formulation is
restricted to single-resolution feature maps, significantly limiting its
potential. In this paper, we go beyond the conventional DCF framework and
introduce a novel formulation for training continuous convolution filters. We
employ an implicit interpolation model to pose the learning problem in the
continuous spatial domain. Our proposed formulation enables efficient
integration of multi-resolution deep feature maps, leading to superior results
on three object tracking benchmarks: OTB-2015 (+5.1% in mean OP), Temple-Color
(+4.6% in mean OP), and VOT2015 (20% relative reduction in failure rate).
Additionally, our approach is capable of sub-pixel localization, crucial for
the task of accurate feature point tracking. We also demonstrate the
effectiveness of our learning formulation in extensive feature point tracking
experiments. Code and supplementary material are available at
http://www.cvl.isy.liu.se/research/objrec/visualtracking/conttrack/index.html.Comment: Accepted at ECCV 201
Stable Umbral Chromospheric Structures
Aims. To understand the morphology of the chromosphere in sunspot umbra. We
investigate if the horizontal structures observed in the spectral core of the
Ca II H line are ephemeral visuals caused by the shock dynamics of more stable
structures, and examine their relationship with observables in the H-alpha
line. Methods. Filtergrams in the core of the Ca II H and H-alpha lines as
observed with the Swedish 1-m Solar Telescope are employed. We utilise a
technique that creates composite images and tracks the flash propagation
horizontally. Results. We find 0"15 wide horizontal structures, in all of the
three target sunspots, for every flash where the seeing was moderate to good.
Discrete dark structures are identified that are stable for at least two umbral
flashes, as well as systems of structures that live for up to 24 minutes. We
find cases of extremely extended structures with similar stability, with one
such structure showing an extent of 5". Some of these structures have a
correspondence in H-alpha but we were unable to find a one to one
correspondence for every occurrence. If the dark streaks are formed at the same
heights as umbral flashes then there are systems of structures with strong
departures from the vertical for all three analysed sunspots. Conclusions.
Long-lived Ca II H filamentary horizontal structures are a common and likely
ever-present feature in the umbra of sunspots. If the magnetic field in the
chromosphere of the umbra is indeed aligned with the structures, then the
present theoretical understanding of the typical umbra needs to be revisited.Comment: Accepted to Astronomy and Astrophysics. Online material (Fig3.mov and
Fig4.mov) will be available at A&
Automata, Groups, Limit Spaces, and Tilings
We explore the connections between automata, groups, limit spaces of
self-similar actions, and tilings. In particular, we show how a group acting
``nicely'' on a tree gives rise to a self-covering of a topological groupoid,
and how the group can be reconstructed from the groupoid and its covering. The
connection is via finite-state automata. These define decomposition rules, or
self-similar tilings, on leaves of the solenoid associated with the covering.Comment: to appear in J. Algebr
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