189 research outputs found
Stratification of sunspot umbral dots from inversion of Stokes profiles recorded by Hinode
This work aims to constrain the physical nature of umbral dots (UDs) using
high-resolution spectropolarimetry. Full Stokes spectra recorded by the
spectropolarimeter on Hinode of 51 UDs in a sunspot close to the disk center
are analyzed. The height dependence of the temperature, magnetic field vector,
and line-of-sight velocity across each UD is obtained from an inversion of the
Stokes vectors of the two FeI lines at 630 nm. No difference is found at higher
altitudes (-3 <= log(tau) <= -2) between the UDs and the diffuse umbral
background. Below that level the difference rapidly increases, so that at the
continuum formation level (log(tau) = 0) we find on average a temperature
enhancement of 570 K, a magnetic field weakening of 510 G, and upflows of 800
m/s for peripheral UDs, whereas central UDs display an excess temperature of on
average 550 K, a field weakening of 480 G, and no significant upflows. The
results for, in particular, the peripheral UDs, including cuts of magnetic
vector and velocity through them, look remarkably similar to the output of
recent radiation MHD simulations. They strongly suggest that UDs are produced
by convective upwellings
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
Magnetic fields in solar plage regions: insights from high-sensitivity spectropolarimetry
Plage regions are patches of concentrated magnetic field in the Sun's
atmosphere where hot coronal loops are rooted. While previous studies have shed
light on the properties of plage magnetic fields in the photosphere, there are
still challenges in measuring the overlying chromospheric magnetic fields,
which are crucial to understanding the overall heating and dynamics. Here, we
utilize high-sensitivity, spectropolarimetric data obtained by the four-meter
Daniel K. Inouye Solar Telescope (DKIST) to investigate the dynamic environment
and magnetic field stratification of an extended, decaying plage region. The
data show strong circular polarization signals in both plage cores and
surrounding fibrils. Notably, weak linear polarization signals clearly
differentiate between plage patches and the fibril canopy, where they are
relatively stronger. Inversions of the Ca II 8542 spectra show
an imprint of the fibrils in the chromospheric magnetic field, with typical
field strength values ranging from 200-300 G in fibrils. We confirm the
weak correlation between field strength and cooling rates in the lower
chromosphere. Additionally, we observe supersonic downflows and strong velocity
gradients in the plage periphery, indicating dynamical processes occurring in
the chromosphere. These findings contribute to our understanding of the
magnetic field and dynamics within plages, emphasizing the need for further
research to explore the expansion of magnetic fields with height and the
three-dimensional distribution of heating rates in the lower chromosphere.Comment: 17 pages, 8 figures, accepted for publication in ApJ
Bright fibrils in Ca II K
Context: Except for the Ca II resonance lines, fibrils are ubiquitously
present in most high-resolution observations of chromospheric lines. Aims: We
show that fibrils are also a prevailing feature in Ca II K, provided the
spatial-resolution is sufficiently high. Methods: We present high spatial
resolution observations of an active region in the Ca I} K line from the
Swedish Solar Telescope. Through a comparison between photospheric intensity
and magnetic field data, we study the connection between bright chromospheric
fibrils and photospheric structures. Additionally, using Fourier analysis we
study how the fibrils are linked to the observed dynamics. Results: We find
that very narrow, bright fibrils are a prevailing feature over large portions
of the observed field. We also find a clear connection between the fibril
footpoints and photospheric magnetic features. We show that the fibrils play
two distinct roles in the observed dynamics: depending on their location they
can act as a canopy suppressing oscillations or they can channel low-frequency
oscillations into the chromosphere. Conclusions: The Ca II K fibrils share many
characteristics with fibrils observed in other chromospheric lines, but some
features, such as the very small widths, are unique to these observations.Comment: To be published in A&A. High resolution version can be downloaded
from: http://www.mps.mpg.de/homes/pietarila/fibrils.pd
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
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