1,027 research outputs found
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
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
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
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 conformal measures and harmonic functions for group extensions
We prove a Perron-Frobenius-Ruelle theorem for group extensions of
topological Markov chains based on a construction of -finite conformal
measures and give applications to the construction of harmonic functions.Comment: To appear in Proceedings of "New Trends in Onedimensional Dynamics,
celebrating the 70th birthday of Welington de Melo
Synoptic Hα Full-Disk Observations of the Sun from Big Bear Solar Observatory – I. Instrumentation, Image Processing, Data Products, and First Results
The Big Bear Solar Observatory (BBSO) has a long tradition of synoptic full-disk observations. Synoptic observations of contrast enhanced full-disk images in the Ca ii K-line have been used with great success to reproduce the H i Lα irradiance variability observed with the Upper Atmosphere Research Satellite (UARS). Recent improvements in data calibration procedures and image- processing techniques enable us now to provide contrast enhanced Hα full-disk images with a spatial resolution of approximately 2′′ and a temporal resolution of up to 3 frames min−1. In this first paper in a series, we describe the instruments, the data calibration procedures, and the image-processing techniques used to obtain our daily Hα full-disk observations. We also present the final data products such as low- and high-contrast images, and Carrington rotation charts. A time series of an erupting mini-filament further illustrates the quality of our Hα full-disk observations and motivate one of the future research projects. This lays a solid foundation for our subsequent studies of solar activity and chromospheric fine structures. The high quality and the sunrise- to-sunset operation of the Hα full-disk observations presented in this paper make them an ideal choice to study statistical properties of mini-filament eruptions, chromospheric differential rotation, and meridional flows within the chromosphere, as well as the evolution of active regions, filaments, flares, and prominences
Equilibrium states for potentials with \sup\phi - \inf\phi < \htop(f)
In the context of smooth interval maps, we study an inducing scheme approach
to prove existence and uniqueness of equilibrium states for potentials
with he `bounded range' condition \sup \phi - \inf \phi < \htop, first used
by Hofbauer and Keller. We compare our results to Hofbauer and Keller's use of
Perron-Frobenius operators. We demonstrate that this `bounded range' condition
on the potential is important even if the potential is H\"older continuous. We
also prove analyticity of the pressure in this context.Comment: Added Lemma 6 to deal with the disparity between leading eigenvalues
and operator norms. Added extra references and corrected some typo
- …