1,262 research outputs found
Near-infrared spectropolarimetry of a delta-spot
Sunspots harboring umbrae of both magnetic polarities within a common
penumbra (delta-spots) are often but not always related to flares. We present
first near-infrared (NIR) observations (Fe I 1078.3 nm and Si I 1078.6 nm
spectra) obtained with the Tenerife Infrared Polarimeter (TIP) at the Vacuum
Tower Telescope (VTT) in Tenerife on 2012 June 17, which afford accurate and
sensitive diagnostics to scrutinize the complex fields along the magnetic
neutral line of a delta-spot within active region NOAA 11504. We examine the
vector magnetic field, line-of-sight (LOS) velocities, and horizontal proper
motions of this rather inactive delta-spot. We find a smooth transition of the
magnetic vector field from the main umbra to that of opposite polarity
(delta-umbra), but a discontinuity of the horizontal magnetic field at some
distance from the delta-umbra on the polarity inversion line. The magnetic
field decreases faster with height by a factor of two above the delta-umbra.
The latter is surrounded by its own Evershed flow. The Evershed flow coming
from the main umbra ends at a line dividing the spot into two parts. This line
is marked by the occurrence of central emission in the Ca II 854.2 nm line.
Along this line, high chromospheric LOS-velocities of both signs appear. We
detect a shear flow within the horizontal flux transport velocities parallel to
the dividing line.Comment: 4 pages, will appear as Letter in Astronomy & Astrophysic
Two-Dimensional Spectroscopy of Photospheric Shear Flows in a Small delta Spot
In recent high-resolution observations of complex active regions,
long-lasting and well-defined regions of strong flows were identified in major
flares and associated with bright kernels of visible, near-infrared, and X-ray
radiation. These flows, which occurred in the proximity of the magnetic neutral
line, significantly contributed to the generation of magnetic shear. Signatures
of these shear flows are strongly curved penumbral filaments, which are almost
tangential to sunspot umbrae rather than exhibiting the typical radial
filamentary structure. Solar active region NOAA 10756 was a moderately complex,
beta-delta sunspot group, which provided an opportunity to extend previous
studies of such shear flows to quieter settings. We conclude that shear flows
are a common phenomenon in complex active regions and delta spots. However,
they are not necessarily a prerequisite condition for flaring. Indeed, in the
present observations, the photospheric shear flows along the magnetic neutral
line are not related to any change of the local magnetic shear. We present
high-resolution observations of NOAA 10756 obtained with the 65-cm vacuum
reflector at Big Bear Solar Observatory (BBSO). Time series of
speckle-reconstructed white-light images and two-dimensional spectroscopic data
were combined to study the temporal evolution of the three-dimensional vector
flow field in the beta-delta sunspot group. An hour-long data set of consistent
high quality was obtained, which had a cadence of better than 30 seconds and
sub-arcsecond spatial resolution.Comment: 23 pages, 6 gray-scale figures, 4 color figures, 2 tables, submitted
to Solar Physic
Utilizing weak pump depletion to stabilize squeezed vacuum states
We propose and demonstrate a pump-phase locking technique that makes use of
weak pump depletion (WPD) - an unavoidable effect that is usually neglected -
in a sub-threshold optical parametric oscillator (OPO). We show that the phase
difference between seed and pump beam is imprinted on both light fields by the
non-linear interaction in the crystal and can be read out without disturbing
the squeezed output. Our new locking technique allows for the first
experimental realization of a pump-phase lock by reading out the pre-existing
phase information in the pump field. There is no degradation of the detected
squeezed states required to implement this scheme.Comment: 11 pages, 7 figure
Ca II 8542 \AA\ brightenings induced by a solar microflare
We study small-scale brightenings in Ca II 8542 \AA\ line-core images to
determine their nature and effect on localized heating and mass transfer in
active regions. High-resolution 2D spectroscopic observations of an active
region in the Ca II 8542 \AA\ line were acquired with the GFPI attached to the
1.5-meter GREGOR telescope. Inversions of the spectra were carried out using
NICOLE. We identified three brightenings of sizes up to 2"x2". We found
evidence that the brightenings belonged to the footpoints of a microflare (MF).
The properties of the observed brightenings disqualified the scenarios of
Ellerman bombs or IRIS bombs. However, this MF shared some common properties
with flaring active-region fibrils or flaring arch filaments (FAFs): (1) FAFs
and MFs are both apparent in chromospheric and coronal layers according to the
AIA channels, and (2) both show flaring arches with lifetimes of about 3.0-3.5
min and lengths of about 20". The inversions revealed heating by 600 K at the
footpoint location in the ambient chromosphere during the impulsive phase.
Connecting the footpoints, a dark filamentary structure appeared in the Ca II
line-core images. Before the start of the MF, the spectra of this structure
already indicated average blueshifts, meaning upward motions of the plasma
along the LOS. During the impulsive phase, these velocities increased up to
-2.2 km/s. Downflows dominated at the footpoints. However, in the upper
photosphere, slight upflows occurred during the impulsive phase. Hence,
bidirectional flows are present in the footpoints of the MF. Conclusions: We
detected Ca II brightenings that coincided with the footpoint location of an
MF. The MF event led to a rise of plasma in the upper photosphere, both before
and during the impulsive phase. Excess mass, previously raised to at most
chromospheric layers, slowly drained downward along arches toward the
footpoints of the MF.Comment: Accepted for publication in Astronomy & Astrophysics, 13 pages, 6
figures, 1 online movi
Limit theorems for von Mises statistics of a measure preserving transformation
For a measure preserving transformation of a probability space
we investigate almost sure and distributional convergence
of random variables of the form where (called the \emph{kernel})
is a function from to and are appropriate normalizing
constants. We observe that the above random variables are well defined and
belong to provided that the kernel is chosen from the projective
tensor product with We establish a form of the individual ergodic theorem for such
sequences. Next, we give a martingale approximation argument to derive a
central limit theorem in the non-degenerate case (in the sense of the classical
Hoeffding's decomposition). Furthermore, for and a wide class of
canonical kernels we also show that the convergence holds in distribution
towards a quadratic form in independent
standard Gaussian variables . Our results on the
distributional convergence use a --\,invariant filtration as a prerequisite
and are derived from uni- and multivariate martingale approximations
High-resolution imaging and near-infrared spectroscopy of penumbral decay
Combining high-resolution spectropolarimetric and imaging data is key to
understanding the decay process of sunspots as it allows us scrutinizing the
velocity and magnetic fields of sunspots and their surroundings. Active region
NOAA 12597 was observed on 24/09/2016 with the 1.5-m GREGOR solar telescope
using high-spatial resolution imaging as well as imaging spectroscopy and
near-infrared (NIR) spectropolarimetry. Horizontal proper motions were
estimated with LCT, whereas LOS velocities were computed with spectral line
fitting methods. The magnetic field properties were inferred with the SIR code
for the Si I and Ca I NIR lines. At the time of the GREGOR observations, the
leading sunspot had two light-bridges indicating the onset of its decay. One of
the light-bridges disappeared, and an elongated, dark umbral core at its edge
appeared in a decaying penumbral sector facing the newly emerging flux. The
flow and magnetic field properties of this penumbral sector exhibited weak
Evershed flow, moat flow, and horizontal magnetic field. The penumbral gap
adjacent to the elongated umbral core and the penumbra in that penumbral sector
displayed LOS velocities similar to granulation. The separating polarities of a
new flux system interacted with the leading and central part of the already
established active region. As a consequence, the leading spot rotated 55-degree
in clockwise direction over 12 hours. In the high-resolution observations of a
decaying sunspot, the penumbral filaments facing flux emergence site contained
a darkened area resembling an umbral core filled with umbral dots. This umbral
core had velocity and magnetic field properties similar to the sunspot umbra.
This implies that the horizontal magnetic fields in the decaying penumbra
became vertical as observed in flare-induced rapid penumbral decay, but on a
very different time-scale.Comment: 14 pages, 11 figures, Accepted to be published in Astronomy and
Astrophysic
Horizontal flow fields observed in Hinode G-band images II. Flow fields in the final stages of sunspot decay
We present a subset of multi-wavelengths observations obtained with the
Japanese Hinode mission, the Solar Dynamics Observatory (SDO), and the Vacuum
Tower Telescope (VTT) at Observatorio del Teide, Tenerife, Spain during the
time period from 2010 November 18-23. Horizontal proper motions were derived
from G-band and Ca II H images, whereas line-of-sight velocities were extracted
from VTT Echelle H-alpha 656.28 nm spectra and Fe I 630.25 nm spectral data of
the Hinode/Spectro-Polarimeter, which also provided three-dimensional magnetic
field information. The Helioseismic and Magnetic Imager on board SDO provided
continuum images and line-of-sight magnetograms as context for the
high-resolution observations for the entire disk passage of the active region.
We have performed a quantitative study of photospheric and chromospheric flow
fields in and around decaying sunspots. In one of the trailing sunspots of
active region NOAA 11126, we observed moat flow and moving magnetic features
(MMFs), even after its penumbra had decayed. We also noticed a superpenumbral
structure around this pore. MMFs follow well-defined, radial paths from the
spot all the way to the border of a supergranular cell surrounding the spot. In
contrast, flux emergence near the other sunspot prevented it from establishing
such well ordered flow patterns, which could even be observed around a tiny
pore of just 2 Mm diameter. After the disappearance of the sunspots/pores a
coherent patch of abnormal granulation remained at their location, which was
characterized by more uniform horizontal proper motions, low divergence values,
and diminished photospheric Doppler velocities. This region, thus, differs
significantly from granulation and other areas covered by G-band bright points.
We conclude that this peculiar flow pattern is a signature of sunspot decay and
the dispersal of magnetic flux.Comment: 13 pages, 11 figures, accepted for publication in Astronomy and
Astrophysic
On random topological Markov chains with big images and preimages
We introduce a relative notion of the 'big images and preimages'-property for
random topological Markov chains. This then implies that a relative version of
the Ruelle-Perron-Frobenius theorem holds with respect to summable and locally
Hoelder continuous potentials.Comment: Corrected and extended version of the article published in
Stochastics and Dynamics 201
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