281 research outputs found
High Resolution Observations using Adaptive Optics: Achievements and Future Needs
Over the last few years, several interesting observations were obtained with
the help of solar Adaptive Optics (AO). In this paper, few observations made
using the solar AO are enlightened and briefly discussed. A list of
disadvantages with the current AO system are presented. With telescopes larger
than 1.5m are expected during the next decade, there is a need to develop the
existing AO technologies for large aperture telescopes. Some aspects of this
development are highlighted. Finally, the recent AO developments in India are
also presented
Convection and the origin of Evershed flows in sunspot penumbrae
We discuss a numerical 3D radiation-MHD simulation of penumbral fine
structure in a small sunspot. This simulation shows the development of short
filamentary structures with horizontal flows, similar to observed Evershed
flows, and an inward propagation of these structures at a speed compatible with
observations. We conclude that the Evershed flow represents the horizontal flow
component of overturning convection in gaps with strongly reduced field
strength. The top of the flow is always directed outward--away from the umbra--
because of the broken symmetry due to the inclined magnetic field. Upflows
occur in the inner parts of the gaps and most of the gas turns over radially
(outwards and sideways), and descends back down again. The ascending, cooling
and overturning flow tends to bend magnetic field lines down, forcing a
weakening of the field that makes it easier for gas located in an adjacent
layer--further in--to initiate a similar sequence of motion, aided by lateral
heating, thus causing the inward propagation of the filament.Comment: 4 pages, 1 figure. Submitted to ApJ
Striation and convection in penumbral filaments
Observations with the 1-m Swedish Solar Telescope of the flows seen in
penumbral filaments are presented. Time sequences of bright filaments show
overturning motions strikingly similar to those seen along the walls of small
isolated structures in the active regions. The filaments show outward
propagating striations with inclination angles suggesting that they are aligned
with the local magnetic field. We interpret it as the equivalent of the
striations seen in the walls of small isolated magnetic structures. Their
origin is then a corrugation of the boundary between an overturning convective
flow inside the filament and the magnetic field wrapping around it. The outward
propagation is a combination of a pattern motion due to the downflow observed
along the sides of bright filaments, and the Evershed flow. The observed short
wavelength of the striation argues against the existence of a dynamically
significant horizontal field inside the bright filaments. Its intensity
contrast is explained by the same physical effect that causes the dark cores of
filaments, light bridges and `canals'. In this way striation represents an
important clue to the physics of penumbral structure and its relation with
other magnetic structures on the solar surface. We put this in perspective with
results from the recent 3-D radiative hydrodynamic simulations.Comment: Accepted for publication in A&
Twist, Writhe & Helicity in the inner penumbra of a sunspot
The aim of this work is the determination of the twist, writhe, and self
magnetic helicity of penumbral filaments located in an inner Sunspot penumbra.
To this extent, we inverted data taken with the spectropolarimeter (SP) aboard
Hinode with the SIR (Stokes Inversion based on Response function) code. For the
construction of a 3D geometrical model we applied a genetic algorithm
minimizing the divergence of the magnetic field vector and the net
magnetohydrodynamic force, consequently a force-free solution would be reached
if possible. We estimated two proxies to the magnetic helicity frequently used
in literature: the force-free parameter and the current helicity term. We show
that both proxies are only qualitative indicators of the local twist as the
magnetic field in the area under study significantly departures from a
force-free configuration. The local twist shows significant values only at the
borders of bright penumbral filaments with opposite signs on each side. These
locations are precisely correlated to large electric currents. The average
twist (and writhe) of penumbral structures is very small. The spines (dark
filaments in the background) show a nearly zero writhe. The writhe per unit
length of the intraspines diminishes with increasing length of the tube axes.
Thus, the axes of tubes related to intraspines are less wrung when the tubes
are more horizontal. As the writhe of the spines is very small, we can conclude
that the writhe reaches only significant values when the tube includes the
border of a intraspine.Comment: 7 pages, 4 figures; Astrophysical Journal, in pres
Morphology and evolution of umbral dots and their substructures
Substructures - dark lanes and tails - of umbral dots (UDs) were predicted by
numerical simulations of magnetoconvection. We analyse a 6 h 23 min time series
of broadband images of a large umbra in the active region NOAA 10634, acquired
with the 1-m Swedish Solar Telescope, in the wavelength band around 602 nm. A
43 min part of this series was reconstructed with the MFBD method, reaching a
spatial resolution of 0.14". We measure brightness, size, lifetime, and
horizontal velocities of various umbral structures. Most (90 %) of UDs and
bright point-like features in faint LBs split and merge, and their median
lifetimes are 3.5 or 5.7 min, depending on whether the split or merge event is
considered as the end of their life. Both UDs and features in faint LBs that do
not split or merge are clearly smaller (0.15") than the average size (0.17") of
all features. Horizontal motions of umbral bright small-scale features are
directed either into the umbra or along faint LBs with mean horizontal
velocities of 0.34 km/s. Features faster than 0.4 km/s appear mostly at the
periphery of the umbra. The intensity of dark lanes, measured in four bright
central UDs (CUDs), is by a factor 0.8 lower than the peak intensity of CUDs.
The width of dark lanes is probably less than the resolution limit 0.14". The
characteristic time of substructure changes of UDs is ~4 min. We observe narrow
(0.14") bright and dark filaments connected with PUDs. Usually one dark and two
bright filaments form a 0.4" wide tail attached to one PUD, resembling a short
dark-cored penumbral filament. Our results indicate the similarity between PUDs
and PGs located at the tips of bright penumbral filaments. The features seen in
numerical MHD simulations are consistent with our observations of dark lanes in
CUDs and tails attached to PUDs.Comment: 8 pages, 8 figures, Astronomy & Astrophysics, in pres
Solar Atmospheric Oscillations and the Chromospheric Magnetic Topology
We investigate the oscillatory properties of the quiet solar chromosphere in
relation to the underlying photosphere, with particular regard to the effects
of the magnetic topology. We perform a Fourier analysis on a sequence of
line-of-sight velocities measured simultaneously in a photospheric (Fe I 709.0
nm) and a chromospheric line (Ca II 854.2 nm). The velocities were obtained
from full spectroscopic data acquired at high spatial resolution with the
Interferometric BIdimensional Spectrometer (IBIS). The field of view
encompasses a full supergranular cell, allowing us to discriminate between
areas with different magnetic characteristics. We show that waves with
frequencies above the acoustic cut-off propagate from the photosphere to upper
layers only in restricted areas of the quiet Sun. A large fraction of the quiet
chromosphere is in fact occupied by ``magnetic shadows'', surrounding network
regions, that we identify as originating from fibril-like structures observed
in the core intensity of the Ca II line. We show that a large fraction of the
chromospheric acoustic power at frequencies below the acoustic cut-off,
residing in the proximity of the magnetic network elements, directly propagates
from the underlying photosphere. This supports recent results arguing that
network magnetic elements can channel low-frequency photospheric oscillations
into the chromosphere, thus providing a way to input mechanical energy in the
upper layers.Comment: 4 pages, 3 figure, A&A Letters in pres
Small-scale convection signatures associated with strong plage solar magnetic field
In this work, we study and quantify properties of strong-field small-scale
convection and compare observed properties with those predicted by numerical
simulations. We analyze spectropolarimetric 630.25 nm data from a unipolar
ephemeral region near sun center. We use line-of-sight velocities and magnetic
field measurements obtained with Milne-Eddington inversion techniques along
with measured continuum intensities and Stokes V amplitude asymmetry at a
spatial resolution of 0.15 arcseconds to establish statistical relations
between the measured quantities. We also study these properties for different
types of distinct magnetic features, such as micropores, bright points,
ribbons, flowers and strings. We present the first direct observations of a
small-scale granular magneto-convection pattern within extended regions of
strong (more than 600 G average) magnetic field. Along the boundaries of the
flux concentrations we see mostly downflows and asymmetric Stokes V profiles,
consistent with synthetic line profiles calculated from MHD simulations. We
note the frequent occurrence of bright downflows along these boundaries. In the
interior of the flux concentrations, we observe an up/down flow pattern that we
identify as small-scale magnetoconvection, appearing similar to that of
field-free granulation but with scales 4 times smaller. Measured RMS velocities
are 70% of those of nearby field-free granulation, even though the average
radiative flux is not reduced. The interiors of these flux concentrations are
dominated by upflows.Comment: Accepted for publication in Astronomy and Astrophysic
Transient downflows associated with the intensification of small-scale magnetic features and bright point formation
Small-scale magnetic features are present everywhere in the solar
photosphere. Theoretical models, numerical calculations, and simulations
describing the formation of these features have existed for a few decades, but
there are only a few observational studies in direct support of the
simulations. In this study we present the evolution of small-scale magnetic
features with a spatial resolution close to 0.15 arcsecond and compare these
observations with those predicted by numerical simulations and also with
previous observational work of a similar nature. We analyze a 40 min time
sequence of full Stokes spectropolarimetric 630.25 nm data from a plage region
near the Sun center. We use line-of-sight velocities and magnetic field
measurements obtained using Milne-Eddington inversion techniques with and
without stray-light compensation along with measured continuum and line minimum
intensities. We discuss the results in relation to earlier observations and
simulations. We present eight cases involving strong downflows and magnetic
field intensification. All cases studied are associated with the formation of a
bright point in the continuum. In three out of the eight cases we find the
presence of weak opposite polarity field in close proximity to the downflow.
Our data are consistent with earlier simulations describing flux tube collapse,
but the transition to a state with stronger field appears transient and
short-lived, rather than resulting in a permanent field intensification. Three
cases of weak opposite polarity field found adjacent to the downflows do not
appear related to reconnection but may be related to overturning convection
pulling down some field lines and leading to up/down "serpentine" field, as
seen in some simulations.Comment: Accepted for publication in Astronomy & Astrophysic
Microscopic Model for Granular Stratification and Segregation
We study segregation and stratification of mixtures of grains differing in
size, shape and material properties poured in two-dimensional silos using a
microscopic lattice model for surface flows of grains. The model incorporates
the dissipation of energy in collisions between rolling and static grains and
an energy barrier describing the geometrical asperities of the grains. We study
the phase diagram of the different morphologies predicted by the model as a
function of the two parameters. We find regions of segregation and
stratification, in agreement with experimental finding, as well as a region of
total mixing.Comment: 4 pages, 7 figures, http://polymer.bu.edu/~hmakse/Home.htm
Fine structure, magnetic field and heating of sunspot penumbrae
We interpret penumbral filaments as due to convection in field-free, radially
aligned gaps just below the visible surface of the penumbra, intruding into a
nearly potential field above. This solves the classical discrepancy between the
large heat flux and the low vertical velocities observed in the penumbra. The
presence of the gaps causes strong small-scale fluctuations in inclination,
azimuth angle and field strength, but without strong forces acting on the gas.
The field is nearly horizontal in a region around the cusp-shaped top of the
gap, thereby providing an environment for Evershed flows. We identify this
region with the recently discovered dark penumbral cores. Its darkness has the
same cause as the dark lanes in umbral light-bridges, reproduced in numerical
simulations by Nordlund and Stein (2005). We predict that the large vertical
and horizontal gradients of the magnetic field inclination and azimuth in the
potential field model will produce the net circular polarization seen in
observations. The model also explains the significant elevation of bright
filaments above their surroundings. It predicts that dark areas in the penumbra
are of two different kinds: dark filament cores containing the most inclined
(horizontal) fields, and regions between bright filaments, containing the least
inclined field lines.Comment: submitted to A&
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