137 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
Penumbral fine structure and driving mechanisms of large-scale flows in simulated sunspots
We analyze in detail the penumbral structure found in a recent radiative MHD
simulation. Near tau=1, the simulation produces penumbral fine structure
consistent with the observationally inferred interlocking comb structure. Fast
outflows exceeding 8 km/s are present along almost horizontal stretches of the
magnetic field; in the outer half of the penumbra, we see opposite polarity
flux indicating flux returning beneath the surface. The bulk of the penumbral
brightness is maintained by small-scale motions turning over on scales shorter
than the length of a typical penumbral filament. The resulting vertical rms
velocity at tau=1 is about half of that found in the quiet Sun. Radial outflows
in the sunspot penumbra have two components. In the uppermost few 100 km, fast
outflows are driven primarily through the horizontal component of the Lorentz
force, which is confined to narrow boundary layers beneath tau=1, while the
contribution from horizontal pressure gradients is reduced in comparison to
granulation as a consequence of anisotropy. The resulting Evershed flow reaches
its peak velocity near tau=1 and falls off rapidly with height. Outflows
present in deeper layers result primarily from a preferred ring-like alignment
of convection cells surrounding the sunspot. These flows reach amplitudes of
about 50% of the convective rms velocity rather independent of depth. A
preference for the outflow results from a combination of Lorentz force and
pressure driving. While the Evershed flow dominates by velocity amplitude, most
of the mass flux is present in deeper layers and likely related to a
large-scale moat flow.Comment: 24 pages, 21 figures, accepted by Ap
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&
Enhanced Joule Heating in Umbral Dots
We present a study of magnetic profiles of umbral dots (UDs) and its
consequences on the Joule heating mechanisms. Hamedivafa (2003) studied Joule
heating using vertical component of magnetic field. In this paper UDs magnetic
profile has been investigated including the new azimuthal component of magnetic
field which might explain the relatively larger enhancement of Joule heating
causing more brightness near circumference of UD.Comment: 8 pages, 1 figure, accepted in Solar Physic
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&
Brightness, distribution, and evolution of sunspot umbral dots
We present a 106-minute TiO (705.7nm) time series of high spatial and
temporal resolution that contains thousands of umbral dots (UDs) in a mature
sunspot in the active region NOAA 10667 at =0.95. The data were acquired
with the 1-m Swedish Solar Telescope on La Palma. With the help of a multilevel
tracking (MLT) algorithm the sizes, brightnesses, and trajectories of 12836
umbral dots were found and analyzed. The MLT allows UDs with very low contrast
to be reliably identified. Inside the umbra we determine a UD filling factor of
11%. The histogram of UD lifetimes is monotonic, i.e. a UD does not have a
typical lifetime. Three quarters of the UDs lived for less than 150s and showed
no or little motion. The histogram of the UD diameters exhibits a maximum at
225km, i.e. most of the UDs are spatially resolved. UDs display a typical
horizontal velocity of 420m/s and a typical peak intensity of 51% of the mean
intensity of the quiet photosphere, making them on average 20% brighter than
the local umbral background. Almost all mobile UDs (large birth-death distance)
were born close to the umbra-penumbra boundary, move towards the umbral center,
and are brighter than average. Notably bright and mobile UDs were also observed
along a prominent UD chain, both ends of which are located at the
umbra-penumbra boundary. Their motion started primarily at either of the ends
of the chain, continued along the chain, and ended near the chain's center. We
observed the splitting and merging of UDs and the temporal succession of both.
For the first time the evolution of brightness, size, and horizontal speed of a
typical UD could be determined in a statistically significant way. Considerable
differences between the evolution of central and peripheral UDs are found,
which point to a difference in origin
Theoretical Models of Sunspot Structure and Dynamics
Recent progress in theoretical modeling of a sunspot is reviewed. The
observed properties of umbral dots are well reproduced by realistic simulations
of magnetoconvection in a vertical, monolithic magnetic field. To understand
the penumbra, it is useful to distinguish between the inner penumbra, dominated
by bright filaments containing slender dark cores, and the outer penumbra, made
up of dark and bright filaments of comparable width with corresponding magnetic
fields differing in inclination by some 30 degrees and strong Evershed flows in
the dark filaments along nearly horizontal or downward-plunging magnetic
fields. The role of magnetic flux pumping in submerging magnetic flux in the
outer penumbra is examined through numerical experiments, and different
geometric models of the penumbral magnetic field are discussed in the light of
high-resolution observations. Recent, realistic numerical MHD simulations of an
entire sunspot have succeeded in reproducing the salient features of the
convective pattern in the umbra and the inner penumbra. The siphon-flow
mechanism still provides the best explanation of the Evershed flow,
particularly in the outer penumbra where it often consists of cool, supersonic
downflows.Comment: To appear in "Magnetic Coupling between the Interior and the
Atmosphere of the Sun", eds. S.S. Hasan and R.J. Rutten, Astrophysics and
Space Science Proceedings, Springer-Verlag, Heidelberg, Berlin, 200
Discovery of inward moving magnetic enhancements in sunspot penumbrae
Sunspot penumbrae show a fine structure in continuum intensity that displays
considerable dynamics. The magnetic field, in contrast, although also highly
structured, has appeared to be relatively static. Here we report the discovery
of inward moving magnetic enhancements in the penumbrae of two regular sunspots
based on time series of SOHO/MDI magnetograms. Local enhancements of the LOS
component of the magnetic field in the inner part of the penumbral region move
inward to the umbra-penumbra boundary with a radial speed of about 0.3 km
s. These local inward-moving enhancements of the LOS component of the
magnetic fields appear to be relatively common. They are associated with dark
structures and tend to display downflows relatively to the penumbral
background. Possible explanations are discussed.Comment: 4 pages, 4 figures, submitted to ApJ Letter
Downward pumping of magnetic flux as the cause of filamentary structures in sunspot penumbrae
The structure of a sunspot is determined by the local interaction between magnetic fields and convection near the Sun's surface. The dark central umbra is surrounded by a filamentary penumbra, whose complicated fine structure has only recently been revealed by high-resolution observations. The penumbral magnetic field has an intricate and unexpected interlocking-comb structure and some field lines, with associated outflows of gas, dive back down below the solar surface at the outer edge of the spot. These field lines might be expected to float quickly back to the surface because of magnetic buoyancy, but they remain submerged. Here we show that the field lines are kept submerged outside the spot by turbulent, compressible convection, which is dominated by strong, coherent, descending plumes. Moreover, this downward pumping of magnetic flux explains the origin of the interlocking-comb structure of the penumbral magnetic field, and the behaviour of other magnetic features near the sunspot
The Source of Three-minute Magneto-acoustic Oscillations in Coronal Fans
We use images of high spatial, spectral and temporal resolution, obtained
using both ground- and space-based instrumentation, to investigate the coupling
between wave phenomena observed at numerous heights in the solar atmosphere.
Intensity oscillations of 3 minutes are observed to encompass photospheric
umbral dot structures, with power at least three orders-of-magnitude higher
than the surrounding umbra. Simultaneous chromospheric velocity and intensity
time series reveal an 87 \pm 8 degree out-of-phase behavior, implying the
presence of standing modes created as a result of partial wave reflection at
the transition region boundary. An average blue-shifted Doppler velocity of
~1.5 km/s, in addition to a time lag between photospheric and chromospheric
oscillatory phenomena, confirms the presence of upwardly-propagating slow-mode
waves in the lower solar atmosphere. Propagating oscillations in EUV intensity
are detected in simultaneous coronal fan structures, with a periodicity of 172
\pm 17 s and a propagation velocity of 45 \pm 7 km/s. Numerical simulations
reveal that the damping of the magneto-acoustic wave trains is dominated by
thermal conduction. The coronal fans are seen to anchor into the photosphere in
locations where large-amplitude umbral dot oscillations manifest. Derived
kinetic temperature and emission measure time-series display prominent
out-of-phase characteristics, and when combined with the previously established
sub-sonic wave speeds, we conclude that the observed EUV waves are the coronal
counterparts of the upwardly-propagating magneto-acoustic slow-modes detected
in the lower solar atmosphere. Thus, for the first time, we reveal how the
propagation of 3 minute magneto-acoustic waves in solar coronal structures is a
direct result of amplitude enhancements occurring in photospheric umbral dots.Comment: Accepted into ApJ (13 pages and 10 figures
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