51 research outputs found
Evershed clouds as precursors of moving magnetic features around sunspots
The relation between the Evershed flow and moving magnetic features (MMFs) is
studied using high-cadence, simultaneous spectropolarimetric measurements of a
sunspot in visible (630.2 nm) and near-infrared (1565 nm) lines. Doppler
velocities, magnetograms, and total linear polarization maps are calculated
from the observed Stokes profiles. We follow the temporal evolution of two
Evershed clouds that move radially outward along the same penumbral filament.
Eventually, the clouds cross the visible border of the spot and enter the moat
region, where they become MMFs. The flux patch farther from the sunspot has the
same polarity of the spot, while the MMF closer to it has opposite polarity and
exhibits abnormal circular polarization profiles. Our results provide strong
evidence that at least some MMFs are the continuation of the penumbral Evershed
flow into the moat. This, in turn, suggests that MMFs are magnetically
connected to sunspots.Comment: To appear in ApJ Letters, Vol 649, 2006 September 20 issu
Temporal evolution of the Evershed flow in sunspots. I. Observational characterization of Evershed clouds
[Abridged] The magnetic and kinematic properties of the photospheric Evershed
flow are relatively well known, but we are still far from a complete
understanding of its nature. The evolution of the flow with time, which is
mainly due to appearance of velocity packets called Evershed clouds (ECs), may
provide information to further constrain its origin. Here we undertake a
detailed analysis of the evolution of the Evershed flow by studying the
properties of ECs. In this first paper we determine the sizes, proper motions,
location in the penumbra, and frequency of appearance of ECs, as well as their
typical Doppler velocities, linear and circular polarization signals, Stokes V
area asymmetries, and continuum intensities. High-cadence, high-resolution,
full vector spectropolarimetric measurements in visible and infrared lines are
used to derive these parameters. We find that ECs appear in the mid penumbra
and propage outward along filaments with large linear polarization signals and
enhanced Evershed flows. The frequency of appearance of ECs varies between 15
and 40 minutes in different filaments. ECs exhibit the largest Doppler
velocities and linear-to-circular polarization ratios of the whole penumbra. In
addition, lines formed deeper in the atmosphere show larger Doppler velocities,
much in the same way as the ''quiescent'' Evershed flow. According to our
observations, ECs can be classified in two groups: type I ECs, which vanish in
the outer penumbra, and type II ECs, which cross the outer penumbral boundary
and enter the sunspot moat. Most of the observed ECs belong to type I. On
average, type II ECs can be detected as velocity structures outside of the spot
for only about 14 min. Their proper motions in the moat are significantly
reduced with respect to the ones they had in the penumbra.Comment: Accepted for publication in A&
Disintegration of Magnetic Flux in Decaying Sunspots as Observed with the Hinode SOT
Continuous observations of sunspot penumbrae with the Solar Optical Telescope
aboard \textit{Hinode} clearly show that the outer boundary of the penumbra
fluctuates around its averaged position. The penumbral outer boundary moves
inward when granules appear in the outer penumbra. We discover that such
granules appear one after another while moving magnetic features (MMFs) are
separating from the penumbral ``spines'' (penumbral features that have stronger
and more vertical fields than those of their surroundings). These granules that
appear in the outer penumbra often merge with bright features inside the
penumbra that move with the spines as they elongate toward the moat region.
This suggests that convective motions around the penumbral outer boundary are
related to the disintegration of magnetic flux in the sunspot. We also find
that dark penumbral filaments frequently elongate into the moat region in the
vicinity of MMFs that detach from penumbral spines. Such elongating dark
penumbral filaments correspond to nearly horizontal fields extending from the
penumbra. Pairs of MMFs with positive and negative polarities are sometimes
observed along the elongating dark penumbral filaments. This strongly supports
the notion that such elongating dark penumbral filaments have magnetic fields
with a ``sea serpent''-like structure. Evershed flows, which are associated
with the penumbral horizontal fields, may be related to the detachment of the
MMFs from the penumbral spines, as well as to the formation of the MMFs along
the dark penumbral filaments that elongate into the moat region.Comment: Accepted for publication in Ap
Temporal evolution of the Evershed flow in sunspots. II. Physical properties and nature of Evershed clouds
Context: Evershed clouds (ECs) represent the most conspicuous variation of
the Evershed flow in sunspot penumbrae. Aims: We determine the physical
properties of ECs from high spatial and temporal resolution spectropolarimetric
measurements. Methods: The Stokes profiles of four visible and three infrared
spectral lines are subject to inversions based on simple one-component models
as well as more sophisticated realizations of penumbral flux tubes embedded in
a static ambient field (uncombed models). Results: According to the
one-component inversions, the EC phenomenon can be understood as a perturbation
of the magnetic and dynamic configuration of the penumbral filaments along
which these structures move. The uncombed inversions, on the other hand,
suggest that ECs are the result of enhancements in the visibility of penumbral
flux tubes. We conjecture that the enhancements are caused by a perturbation of
the thermodynamic properties of the tubes, rather than by changes in the vector
magnetic field. The feasibility of this mechanism is investigated performing
numerical experiments of thick penumbral tubes in mechanical equilibrium with a
background field. Conclusions: While the one-component inversions confirm many
of the properties indicated by a simple line parameter analysis (Paper I of
this series), we tend to give more credit to the results of the uncombed
inversions because they take into account, at least in an approximate manner,
the fine structure of the penumbra.Comment: Accepted for publication in A&
Models and Observations of Sunspot Penumbrae
The mysteries of sunspot penumbrae have been under an intense scrutiny for
the past 10 years. During this time, some models have been proposed and
refuted, while the surviving ones had to be modified, adapted and evolved to
explain the ever-increasing array of observational constraints. In this
contribution I will review two of the present models, emphasizing their
contributions to this field, but also pinpointing some of their inadequacies to
explain a number of recent observations at very high spatial resolution. To
help explaining these new observations I propose some modifications to each of
them. These modifications bring those two seemingly opposite models closer
together into a general picture that agrees well with recent 3D
magneto-hydrodynamic simulations.Comment: 9 pages, 1 color figure. Review talk to appear in the proceedings of
the International Workshop of 2008 Solar Total Eclipse: Solar Magnetism,
Corona and Space Weather--Chinese Space Solar Telescope Scienc
Stray-light contamination and spatial deconvolution of slit-spectrograph observations
Stray light caused by scattering on optical surfaces and in the Earth's
atmosphere degrades the spatial resolution of observations. We study the
contribution of stray light to the two channels of POLIS. We test the
performance of different methods of stray-light correction and spatial
deconvolution to improve the spatial resolution post-facto. We model the stray
light as having two components: a spectrally dispersed component and a
component of parasitic light caused by scattering inside the spectrograph. We
use several measurements to estimate the two contributions: observations with a
(partly) blocked FOV, a convolution of the FTS spectral atlas, imaging in the
pupil plane, umbral profiles, and spurious polarization signal in telluric
lines. The measurements allow us to estimate the spatial PSF of POLIS and the
main spectrograph of the German VTT. We use the PSF for a deconvolution of both
spectropolarimetric data and investigate the effect on the spectra. The
parasitic contribution can be directly and accurately determined for POLIS,
amounting to about 5%. We estimate a lower limit of about 10% across the full
FOV for the dispersed stray light. In quiet Sun regions, the stray-light level
from the close surroundings (d< 2") of a given spatial point is about 20%. The
stray light reduces to below 2% at a distance of 20" from a lit area for both
POLIS and the main spectrograph. A two-component model of the stray-light
contributions seems to be sufficient for a basic correction of observed
spectra. The instrumental PSF obtained can be used to model the off-limb stray
light, to determine the stray-light contamination accurately for observation
targets with large spatial intensity gradients such as sunspots, and also
allows one to improve the spatial resolution of observations post-facto.Comment: 14 pages, 16 figures, accepted by A&A. Version V2 revised for
language editin
The Effects of Atmospheric Dispersion on High-Resolution Solar Spectroscopy
We investigate the effects of atmospheric dispersion on observations of the
Sun at the ever-higher spatial resolutions afforded by increased apertures and
improved techniques. The problems induced by atmospheric refraction are
particularly significant for solar physics because the Sun is often best
observed at low elevations, and the effect of the image displacement is not
merely a loss of efficiency, but the mixing of information originating from
different points on the solar surface. We calculate the magnitude of the
atmospheric dispersion for the Sun during the year and examine the problems
produced by this dispersion in both spectrographic and filter observations. We
describe an observing technique for scanning spectrograph observations that
minimizes the effects of the atmospheric dispersion while maintaining a regular
scanning geometry. Such an approach could be useful for the new class of
high-resolution solar spectrographs, such as SPINOR, POLIS, TRIPPEL, and ViSP
High cadence spectropolarimetry of moving magnetic features observed around a pore
Moving magnetic features (MMFs) are small-size magnetic elements that are
seen to stream out from sunspots, generally during their decay phase. Several
observational results presented in the literature suggest them to be closely
related to magnetic filaments that extend from the penumbra of the parent spot.
Nevertheless, few observations of MMFs streaming out from spots without
penumbra have been reported. The literature still lacks of analyses of the
physical properties of these features.
We investigate physical properties of monopolar MMFs observed around a small
pore that had developed penumbra in the days preceding our observations and
compare our results with those reported in the literature for features observed
around sunspots. We analyzed NOAA 11005 during its decay phase with data
acquired at the Dunn Solar Telescope in the FeI 617.3
nm spectral lines with IBIS, and in the G-band. The field of view showed
monopolar MMFs of both polarities streaming out from the leading negative
polarity pore of the observed active region. Combining different analyses of
the data, we investigated the temporal evolution of the relevant physical
quantities associated with the MMFs as well as the photospheric and
chromospheric signatures of these features.
We show that the characteristics of the investigated MMFs agree with those
reported in the literature for MMFs that stream out from spots with penumbrae.
Moreover, observations of at least two of the observed features suggest them to
be manifestations of emerging magnetic arches.Comment: Accepted by A&
The Evershed Flow and the Brightness of the Penumbra
The Evershed flow is a systematic motion of gas that occurs in the penumbra
of all sunspots. Discovered in 1909, it still lacks a satisfactory explanation.
We know that the flow is magnetized, often supersonic, and that it shows
conspicuous fine structure on spatial scales of 0.2"-0.3", but its origin
remains unclear. The hope is that a good observational understanding of the
relation between the flow and the penumbral magnetic field will help us
determine its nature. Here I review advances in the characterization of the
Evershed flow and sunspot magnetic fields from high-resolution spectroscopic
and spectropolarimetric measurements. Using this information as input for 2D
heat transfer simulations, it has been demonstrated that hot Evershed upflows
along nearly horizontal field lines are capable of explaining one of the most
intriguing aspects of sunspots: the surplus brightness of the penumbra relative
to the umbra. They also explain the existence of penumbral filaments with dark
cores. These results support the idea that the Evershed flow is largely
responsible for the transport of energy in the penumbra.Comment: 18 pages, 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, Heidelberg, 200
Sunspots: from small-scale inhomogeneities towards a global theory
The penumbra of a sunspot is a fascinating phenomenon featuring complex
velocity and magnetic fields. It challenges both our understanding of radiative
magneto-convection and our means to measure and derive the actual geometry of
the magnetic and velocity fields. In this contribution we attempt to summarize
the present state-of-the-art from an observational and a theoretical
perspective.Comment: Accepted for publication in Space Science Review
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