798 research outputs found
Lateral Chirality-sorting Optical Spin Forces in Evanescent Fields
The transverse component of the spin angular momentum of evanescent waves
gives rise to lateral optical forces on chiral particles, which have the
unusual property of acting in a direction in which there is neither a field
gradient nor wave propagation. As their direction and strength depends on the
chiral polarizability of the particle, they act as chirality-sorting and may
offer a mechanism for passive chirality spectroscopy. The absolute strength of
the forces also substantially exceeds that of other recently predicted sideways
optical forces, such that they may more readily offer an experimental
confirmation of the phenomenon.Comment: 7 pages, 2 Figure
Properties of the inner penumbral boundary and temporal evolution of a decaying sunspot
It was empirically determined that the umbra-penumbra boundaries of stable
sunspots are characterized by a constant value of the vertical magnetic field.
We analyzed the evolution of the photospheric magnetic field properties of a
decaying sunspot belonging to NOAA 11277 between August 28 - September 3, 2011.
The observations were acquired with the spectropolarimeter on-board of the
Hinode satellite. We aim to proof the validity of the constant vertical
magnetic-field boundary between the umbra and penumbra in decaying sunspots. A
spectral-line inversion technique was used to infer the magnetic field vector
from the full-Stokes profiles. In total, eight maps were inverted and the
variation of the magnetic properties in time were quantified using linear or
quadratic fits. We found a linear decay of the umbral vertical magnetic field,
magnetic flux, and area. The penumbra showed a linear increase of the vertical
magnetic field and a sharp decay of the magnetic flux. In addition, the
penumbral area quadratically decayed. The vertical component of the magnetic
field is weaker on the umbra-penumbra boundary of the studied decaying sunspot
compared to stable sunspots. Its value seem to be steadily decreasing during
the decay phase. Moreover, at any time of the shown sunspot decay, the inner
penumbra boundary does not match with a constant value of the vertical magnetic
field, contrary to what was seen in stable sunspots. During the decaying phase
of the studied sunspot, the umbra does not have a sufficiently strong vertical
component of the magnetic field and is thus unstable and prone to be
disintegrated by convection or magnetic diffusion. No constant value of the
vertical magnetic field was found for the inner penumbral boundary.Comment: Accepted for publication in Astronomy & Astrophysics, 6 pages, 7
figure
Magnetic loop emergence within a granule
We investigate the temporal evolution of magnetic flux emerging within a
granule in the quiet-Sun internetwork at disk center. We combined IR
spectropolarimetry performed in two Fe I lines at 1565 nm with
speckle-reconstructed G-band imaging. We determined the magnetic field
parameters by a LTE inversion of the full Stokes vector using the SIR code, and
followed their evolution in time. To interpret the observations, we created a
geometrical model of a rising loop in 3D. The relevant parameters of the loop
were matched to the observations where possible. We then synthesized spectra
from the 3D model for a comparison to the observations. We found signatures of
magnetic flux emergence within a growing granule. In the early phases, a
horizontal magnetic field with a distinct linear polarization signal dominated
the emerging flux. Later on, two patches of opposite circular polarization
signal appeared symmetrically on either side of the linear polarization patch,
indicating a small loop-like structure. The mean magnetic flux density of this
loop was roughly 450 G, with a total magnetic flux of around 3x10^17 Mx. During
the ~12 min episode of loop occurrence, the spatial extent of the loop
increased from about 1 to 2 arcsec. The middle part of the appearing feature
was blueshifted during its occurrence, supporting the scenario of an emerging
loop. The temporal evolution of the observed spectra is reproduced to first
order by the spectra derived from the geometrical model. The observed event can
be explained as a case of flux emergence in the shape of a small-scale loop.Comment: 10 pages, 13 figures; accepted for Astronomy and Astrophysics; ps and
eps figures in full resolution are available at
http://www.astro.sk/~koza/figures/aa2009_loop
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
Multiepoch Radial Velocity Observations of L Dwarfs
We report on the development of a technique for precise radial-velocity
measurements of cool stars and brown dwarfs in the near infrared. Our technique
is analogous to the Iodine (I2) absorption cell method that has proven so
successful in the optical regime. We rely on telluric CH4 absorption features
to serve as a wavelength reference, relative to which we measure Doppler shifts
of the CO and H2O features in the spectra of our targets. We apply this
technique to high-resolution (R~50,000) spectra near 2.3 micron of nine L
dwarfs taken with the Phoenix instrument on Gemini-South and demonstrate a
typical precision of 300 m/s. We conduct simulations to estimate our expected
precision and show our performance is currently limited by the signal-to-noise
of our data. We present estimates of the rotational velocities and systemic
velocities of our targets. With our current data, we are sensitive to
companions with M sin i > 2MJ in orbits with periods less than three days. We
identify no companions in our current data set. Future observations with
improved signal-to-noise should result in radial-velocity precision of 100 m/s
for L dwarfs.Comment: Accepted for publication in ApJ, 24 pages, 7 figure
Flare-induced changes of the photospheric magnetic field in a -spot deduced from ground-based observations
Aims: Changes of the magnetic field and the line-of-sight velocities in the
photosphere are being reported for an M-class flare that originated at a
-spot belonging to active region NOAA 11865.
Methods: High-resolution ground-based near-infrared spectropolarimetric
observations were acquired simultaneously in two photospheric spectral lines,
Fe I 10783 \AA\ and Si I 10786 \AA, with the Tenerife Infrared Polarimeter at
the Vacuum Tower Telescope (VTT) in Tenerife on 2013 October 15. The
observations covered several stages of the M-class flare. Inversions of the
full-Stokes vector of both lines were carried out and the results were put into
context using (extreme)-ultraviolet filtergrams from the Solar Dynamics
Observatory (SDO).
Results: The active region showed high flaring activity during the whole
observing period. After the M-class flare, the longitudinal magnetic field did
not show significant changes along the polarity inversion line (PIL). However,
an enhancement of the transverse magnetic field of approximately 550 G was
found that bridges the PIL and connects umbrae of opposite polarities in the
-spot. At the same time, a newly formed system of loops appeared
co-spatially in the corona as seen in 171 \AA\ filtergrams of the Atmospheric
Imaging Assembly (AIA) on board SDO. However, we cannot exclude that the
magnetic connection between the umbrae already existed in the upper atmosphere
before the M-class flare and became visible only later when it was filled with
hot plasma. The photospheric Doppler velocities show a persistent upflow
pattern along the PIL without significant changes due to the flare.
Conclusions: The increase of the transverse component of the magnetic field
after the flare together with the newly formed loop system in the corona
support recent predictions of flare models and flare observations.Comment: 8 pages, 9 figures. Accepted for publication in Astronomy &
Astrophysic
Temporal evolution of arch filaments as seen in He I 10830 \r{A}
We study the evolution of an arch filament system (AFS) and of its individual
arch filaments to learn about the processes occurring in them. We observed the
AFS at the GREGOR solar telescope on Tenerife at high cadence with the very
fast spectroscopic mode of the GREGOR Infrared Spectrograph (GRIS) in the He I
10830 \AA\ spectral range. The He I triplet profiles were fitted with analytic
functions to infer line-of-sight (LOS) velocities to follow plasma motions
within the AFS. We tracked the temporal evolution of an individual arch
filament over its entire lifetime, as seen in the He I 10830 \AA\ triplet. The
arch filament expanded in height and extended in length from 13" to 21". The
lifetime of this arch filament is about 30 min. About 11 min after the arch
filament is seen in He I, the loop top starts to rise with an average Doppler
velocity of 6 km/s. Only two minutes later, plasma drains down with supersonic
velocities towards the footpoints reaching a peak velocity of up to 40 km/s in
the chromosphere. The temporal evolution of He I 10830 \AA\ profiles near the
leading pore showed almost ubiquitous dual red components of the He I triplet,
indicating strong downflows, along with material nearly at rest within the same
resolution element during the whole observing time. We followed the arch
filament as it carried plasma during its rise from the photosphere to the
corona. The material then drained toward the photosphere, reaching supersonic
velocities, along the legs of the arch filament. Our observational results
support theoretical AFS models and aids in improving future models.Comment: Accepted for publication in Astronomy & Astrophysics, 12 pages, 15
figures, 1 online movi
A new dawn? The Roman Catholic Church and environmental issues
This is a PDF version of an article published in New Blackfriars© 1997. The definitive version is available at www.blackwell-synergy.com.This article discusses the stance of the Roman Catholic Church on environmental issues and argues that the Church tends to stay on the fringe rather than get involved. Some of the ways in which Roman Catholic theologians have incorporated environmental issues into theological reflection is discussed, as are environmental challenges facing the Church in Britain (conservation, resources, biodiversity, animal welfare, biotechnology, cooperate/individual ethics, environmental justice, economics/policy development, and global issues)
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