2,662 research outputs found
Short dynamic fibrils in sunspot chromospheres
Sunspot chromospheres display vigorous oscillatory signature when observed in
chromospheric diagnostics like the strong Ca II lines and H-alpha. New
high-resolution sunspot observations from the Swedish 1-m Solar Telescope show
the ubiquitous presence of small-scale periodic jet-like features that move up
and down. This phenomenon has not been described before. Their typical width is
about 0.3 arcsec and they display clear parabolic trajectories in space-time
diagrams. The maximum extension of the top of the jets is lowest in the umbra,
a few 100 km, and progressively longer further away from the umbra in the
penumbra, with the longest more than 1000 km. These jets resemble dynamic
fibrils found in plage regions but at smaller extensions. LTE inversion of
spectro-polarimetric Ca II 8542 observations enabled for a comparison of the
magnetic field inclination and the properties of these short jets. We find that
the most extended of these jets also have longer periods and tend to be located
in regions with more horizontal magnetic fields. This is a direct observational
confirmation of the mechanism of long-period waves propagating along inclined
magnetic fields into the solar chromosphere. This mechanism was identified
earlier as the driver of dynamic fibrils in plage, part of the mottles in quiet
Sun, and type I spicules at the limb. The sunspot dynamic fibrils that we
report here represent a new class of manifestation of this mechanism. They are
not the same as the transient penumbral and umbral micro-jets reported earlier.Comment: animations of figures can be found at
http://folk.uio.no/rouppe/dfsunspot
Is the sky the limit? Performance of the revamped Swedish 1-m Solar Telescope and its blue- and red-beam re-imaging systems
We demonstrate that for data recorded with a solar telescope that uses
adaptive optics and/or post-processing to compensate for many low- and
high-order aberrations, the RMS granulation contrast is directly proportional
to the Strehl ratio calculated from the residual (small-scale) wavefront error.
We demonstrate that the wings of the high-order compensated PSF for SST are
likely to extend to a radius of not more than about 2 arcsec, consistent with
earlier conclusions drawn from straylight compensation of sunspot images. We
report on simultaneous measurements of seeing and solar granulation contrast
averaged over 2 sec time intervals at several wavelengths from 525 nm to 853.6
nm on the red-beam (CRISP beam) and wavelengths from 395 nm to 484 nm on the
blue-beam (CHROMIS beam). These data were recorded with the Swedish 1-m Solar
Telescope (SST) that has been revamped with an 85-electrode adaptive mirror and
a new tip-tilt mirror, both of which were polished to exceptionally high
optical quality. The highest 2-sec average image contrast measured in April
2015 through 0.3-0.9 nm interference filters at 525 nm, 557 nm, 630 nm and
853.5 nm with compensation only for the diffraction limited point spread
function of SST is 11.8%, 11.8%, 10.2% and 7.2% respectively. Similarly, the
highest 2-sec contrast measured at 395 nm, 400 nm and 484 nm in May 2016
through 0.37-1.3 nm filters is 16%, 16% and 12.5% respectively. The granulation
contrast observed with SST compares favorably with that of other telescopes.
Simultaneously with the above wideband red-beam data, we also recorded
narrow-band continuum images with the CRISP imaging spectropolarimeter. We find
that contrasts measured with CRISP are entirely consistent with the
corresponding wide-band contrasts, demonstrating that any additional image
degradation by the CRISP etalons and telecentric optical system is marginal or
even insignificant.Comment: In press in Astronomy & Astrophysic
Chromospheric condensations and magnetic field in a C3.6-class flare studied via He I D3 spectro-polarimetry
Context. Magnetic reconnection during flares takes place in the corona but a
substantial part of flare energy is deposited in the chromosphere. However,
high-resolution spectro-polarimetric chromospheric observations of flares are
very rare. The most used observables are Ca II 8542 {\AA} and He I 10830 {\AA}.
Aims. We aim to study the chromosphere during a C3.6 class flare via
spectro-polarimetric observations of the He I D3 line. Methods. We present the
first SST/CRISP spectro-polarimetric observations of He I D3. We analyze the
data using the inversion code HAZEL, and estimate the LOS velocity and the
magnetic field vector. Results. Strong He I D3 emission at the flare
footpoints, as well as strong He I D3 absorption profiles tracing the flaring
loops are observed during the flare. The He I D3 traveling emission kernels at
the flare footpoints exhibit strong chromospheric condensations of up to
60 km/s at their leading edge. Our observations suggest that such condensations
result in shocking the deep chromosphere, causing broad and modestly
blueshifted He I D3 profiles indicating subsequent upflows. A strong and rather
vertical magnetic field of up to 2500 G is measured in the flare
footpoints, confirming that the He I D3 line is likely formed in the deep
chromosphere at those locations. We provide chromospheric line-of-sight
velocity and magnetic field maps obtained via He I D3 inversions. We propose a
fan-spine configuration as the flare magnetic field topology. Conclusions. The
He I D3 line is an excellent diagnostic to study the chromosphere during
flares. The impact of strong condensations on the deep chromosphere has been
observed. Detailed maps of the flare dynamics and the magnetic field are
obtained.Comment: Accepted for publication in A&
Fan-shaped jets above the light bridge of a sunspot driven by reconnection
We report on a fan-shaped set of high-speed jets above a strongly magnetized
light bridge (LB) of a sunspot observed in the H line. We study the
origin, dynamics and thermal properties of the jets using high-resolution
imaging spectroscopy in H from the Swedish 1-m Solar Telescope and data
from the Solar Dynamics Observatory and Hinode. The H jets have lengths
of 7-38 Mm, are impulsively accelerated to a speed of km/s close to
photospheric footpoints in the LB, and exhibit a constant deceleration
consistent with solar effective gravity. They are predominantly launched from
one edge of the light bridge, and their footpoints appear bright in the
H wings. AIA data indicates elongated brightenings that are nearly
co-spatial with the H jets. We interpret them as jets of at least
transition region temperatures. The magnetic field in the light bridge has a
strength of kG and it is nearly horizontal. All jet properties are
consistent with magnetic reconnection as the driver.Comment: Accepted for publication in A&
A Hot Downflowing Model Atmosphere For Umbral Flashes And The Physical Properties Of Their Dark Fibrils
We perform NLTE inversions in a large set of umbral flashes, including the
dark fibrils visible within them, and in the quiescent umbra by using the
inversion code NICOLE on a set of full Stokes high-resolution Ca II 8542 A
observations of a sunspot at disk center. We find that the dark structures have
Stokes profiles that are distinct from those of the quiescent and flashed
regions. They are best reproduced by atmospheres that are more similar to the
flashed atmosphere in terms of velocities, even if with reduced amplitudes. We
also find two sets of solutions that finely fit the flashed profiles: a set
that is upflowing, featuring a transition region that is deeper than in the
quiescent case and preceded by a slight dip in temperature, and a second
solution with a hotter atmosphere in the chromosphere but featuring downflows
close to the speed of sound at such heights. Such downflows may be related, or
even dependent, on the presence of coronal loops, rooted in the umbra of
sunspots, as is the case in the region analyzed. Similar loops have been
recently observed to have supersonic downflows in the transition region and are
consistent with the earlier "sunspot plumes" which were invariably found to
display strong downflows in sunspots. Finally we find, on average, a magnetic
field reduction in the flashed areas, suggesting that the shock pressure is
moving field lines in the upper layers.Comment: Accepted in June for publication at ApJ. Comments to
[email protected] or [email protected]
Spatial resolution effects on the solar open flux estimates
Spectropolarimetric observations used to infer the solar magnetic fields are
obtained with a limited spatial resolution. The effects of this limited
resolution on the inference of the open flux over the observed region have not
been extensively studied. We aim to characterize the biases that arise in the
inference of the mean flux density by performing an end-to-end study that
involves the generation of synthetic data, its interpretation (inversion), and
a comparison of the results with the original model. We synthesized polarized
spectra of the two magnetically sensitive lines of neutral iron around 630\,nm
from a state-of-the-art numerical simulation of the solar photosphere. We then
performed data degradation to simulate the effect of the telescope with a
limited angular resolution and interpreted (inverted) the data using a
Milne-Eddington spectropolarimetric inversion code. We then studied the
dependence of the inferred parameters on the telescope resolution. The results
show a significant decrease in the mean magnetic flux density -- related to the
open flux observed at the disk center -- with decreasing telescope resolution.
The original net magnetic field flux is fully resolved by a 1m telescope, but a
20\,cm aperture telescope yields a 30\% smaller value. Even in the fully
resolved case, the result is still biased due to the corrugation of the
photospheric surface. Even the spatially averaged quantities, such as the open
magnetic flux in the observed region, are underestimated when the magnetic
structures are unresolved. The reason for this is the presence of
nonlinearities in the magnetic field inference process. This effect might have
implications for the modeling of large-scale solar magnetic fields; for
example, those corresponding to the coronal holes, or the polar magnetic
fields, which are relevant to our understanding of the solar cycle.Comment: To be published in A&
CRISPRED: A data pipeline for the CRISP imaging spectropolarimeter
The production of science-ready data from major solar telescopes requires expertise beyond that of the typical observer. This is a consequence of the increasing complexity of instruments and observing sequences, which require calibrations and corrections for instrumental and seeing effects that are not only difficult to measure, but are also coupled in ways that require careful analysis in the design of the correction procedures. Modern space-based telescopes have data-processing pipelines capable of routinely producing well-characterized data products. High resolution imaging spectropolarimeters at ground-based telescopes need similar data pipelines.We present new methods for flat-fielding spectropolarimetric data acquired with telecentric Fabry-Perot instruments and a new approach for accurate camera co-alignment for image restoration. We document a procedure that forms the basis of current state-of-the-art processing of data from the CRISP imaging spectropolarimeter at the Swedish 1 m Solar Telescope (SST). By collecting, implementing, and testing a suite of computer programs, we have defined a data reduction pipeline for this instrument. This pipeline, CRISPRED, streamlines the process of making science-ready data.It is implemented and operated in IDL, with time-consuming steps delegated to C.CRISPRED will also be the basis for the data pipeline of the forthcoming CHROMIS instrument.
© ESO, 201
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