1,251 research outputs found
Evaluating local correlation tracking using CO5BOLD simulations of solar granulation
Flows on the solar surface are linked to solar activity, and LCT is one of
the standard techniques for capturing the dynamics of these processes by
cross-correlating solar images. However, the link between contrast variations
in successive images to the underlying plasma motions has to be quantitatively
confirmed. Radiation hydrodynamics simulations of solar granulation
(e.g.,CO5BOLD) provide access to both the wavelength-integrated, emergent
continuum intensity and the 3D velocity field at various heights in the solar
atmosphere. Thus, applying LCT to continuum images yields horizontal proper
motions, which are then compared to the velocity field of the simulated
(non-magnetic) granulation. In this study, we evaluate the performance of an
LCT algorithm previously developed for bulk-processing Hinode G-band images,
establish it as a quantitative tool for measuring horizontal proper motions,
and clearly work out the limitations of LCT or similar techniques designed to
track optical flows. Horizontal flow maps and frequency distributions of the
flow speed were computed for a variety of LCT input parameters including the
spatial resolution, the width of the sampling window, the time cadence of
successive images, and the averaging time used to determine persistent flow
properties. Smoothed velocity fields from the hydrodynamics simulation at three
atmospheric layers (log tau=-1,0,and +1) served as a point of reference for the
LCT results. LCT recovers many of the granulation properties, e.g.,the shape of
the flow speed distributions, the relationship between mean flow speed and
averaging time, and also--with significant smoothing of the simulated velocity
field--morphological features of the flow and divergence maps. However, the
horizontal proper motions are grossly underestimated by as much as a factor of
three. The LCT flows match best the flows deeper in the atmosphere at log
tau=+1.Comment: 11 pages, 16 figures, accepted for publication in Astronomy and
Astrophysic
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
High-resolution imaging spectroscopy of two micro-pores and an arch filament system in a small emerging-flux region
Aims. The purpose of this investigation is to characterize the temporal
evolution of an emerging flux region, the associated photospheric and
chromospheric flow fields, and the properties of the accompanying arch filament
system. Methods. This study is based on imaging spectroscopy with the
G\"ottingen Fabry-P\'erot Interferometer at the Vacuum Tower Telescope, on 2008
August 7. Cloud model (CM) inversions of line scans in the strong chromospheric
absorption H line yielded CM parameters, which describe the cool plasma
contained in the arch filament system. Results. The observations cover the
decay and convergence of two micro-pores with diameters of less than one
arcsecond and provide decay rates for intensity and area. The photospheric
horizontal flow speed is suppressed near the two micro-pores indicating that
the magnetic field is sufficiently strong to affect the convective energy
transport. The micro-pores are accompanied by an arch filament system, where
small-scale loops connect two regions with H line-core brightenings
containing an emerging flux region with opposite polarities. The chromospheric
velocity of the cloud material is predominantly directed downwards near the
footpoints of the loops with velocities of up to 12 km/s, whereas loop tops
show upward motions of about 3 km/s. Conclusions. Micro-pores are the smallest
magnetic field concentrations leaving a photometric signature in the
photosphere. In the observed case, they are accompanied by a miniature arch
filament system indicative of newly emerging flux in the form of
-loops. Flux emergence and decay take place on a time-scale of about
two days, whereas the photometric decay of the micro-pores is much more rapid
(a few hours), which is consistent with the incipient submergence of
-loops. The results are representative for the smallest emerging flux
regions still recognizable as such.Comment: 15 pages, 16 figures, 3 tables, published in A&
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
The Thermal Environment of the Fiber Glass Dome for the New Solar Telescope at Big Bear Solar Observatory
The New Solar Telescope (NST) is a 1.6-meter off-axis Gregory-type telescope
with an equatorial mount and an open optical support structure. To mitigate the
temperature fluctuations along the exposed optical path, the effects of
local/dome-related seeing have to be minimized. To accomplish this, NST will be
housed in a 5/8-sphere fiberglass dome that is outfitted with 14 active vents
evenly spaced around its perimeter. The 14 vents house louvers that open and
close independently of one another to regulate and direct the passage of air
through the dome. In January 2006, 16 thermal probes were installed throughout
the dome and the temperature distribution was measured. The measurements
confirmed the existence of a strong thermal gradient on the order of 5 degree
Celsius inside the dome. In December 2006, a second set of temperature
measurements were made using different louver configurations. In this study, we
present the results of these measurements along with their integration into the
thermal control system (ThCS) and the overall telescope control system (TCS).Comment: 12 pages, 11 figures, submitted to SPIE Optics+Photonics, San Diego,
U.S.A., 26-30 August 2007, Conference: Solar Physics and Space Weather
Instrumentation II, Proceedings of SPIE Volume 6689, Paper #2
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