203 research outputs found
Imaging Spectropolarimetry with IBIS: Evolution of Bright Points in the Quiet Sun
We present the results from first spectropolarimetric observations of the
solar photosphere acquired at the Dunn Solar Telescope with the Interferometric
Bidimensional Spectrometer. Full Stokes profiles were measured in the Fe I
630.15 nm and Fe I 630.25 nm lines with high spatial and spectral resolutions
for 53 minutes, with a Stokes V noise of 0.003 the continuum intensity level.
The dataset allows us to study the evolution of several magnetic features
associated with G-band bright points in the quiet Sun. Here we focus on the
analysis of three distinct processes, namely the coalescence, fragmentation and
cancellation of G-band bright points. Our analysis is based on a SIR inversion
of the Stokes I and V profiles of both Fe I lines. The high spatial resolution
of the G-band images combined with the inversion results helps to interpret the
undergoing physical processes. The appearance (dissolution) of high-contrast
G-band bright points is found to be related to the local increase (decrease) of
the magnetic filling factor, without appreciable changes in the field strength.
The cancellation of opposite-polarity bright points can be the signature of
either magnetic reconnection or the emergence/submergence of magnetic loops.Comment: 4 pages, 5 figures, accepted for publication in ApJ Letter
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
Inter-network regions of the Sun at millimetre wavelengths
The continuum intensity at wavelengths around 1 mm provides an excellent way
to probe the solar chromosphere. Future high-resolution millimetre arrays, such
as the Atacama Large Millimeter Array (ALMA), will thus produce valuable input
for the ongoing controversy on the thermal structure and the dynamics of this
layer. Synthetic brightness temperature maps are calculated on basis of
three-dimensional radiation (magneto-)hydrodynamic (MHD) simulations. While the
millimetre continuum at 0.3mm originates mainly from the upper photosphere, the
longer wavelengths considered here map the low and middle chromosphere. The
effective formation height increases generally with wavelength and also from
disk-centre towards the solar limb. The average intensity contribution
functions are usually rather broad and in some cases they are even
double-peaked as there are contributions from hot shock waves and cool
post-shock regions in the model chromosphere. Taking into account the
deviations from ionisation equilibrium for hydrogen gives a less strong
variation of the electron density and with it of the optical depth. The result
is a narrower formation height range. The average brightness temperature
increases with wavelength and towards the limb. The relative contrast depends
on wavelength in the same way as the average intensity but decreases towards
the limb. The dependence of the brightness temperature distribution on
wavelength and disk-position can be explained with the differences in formation
height and the variation of temperature fluctuations with height in the model
atmospheres.Comment: 15 pages, 10 figures, accepted for publication in A&A (15.05.07
CRISP Spectropolarimetric Imaging of Penumbral Fine Structure
We discuss penumbral fine structure in a small part of a pore, observed with
the CRISP imaging spectropolarimeter at the Swedish 1-m Solar Telescope (SST),
close to its diffraction limit of 0.16 arcsec. Milne-Eddington inversions
applied to these Stokes data reveal large variations of field strength and
inclination angle over dark-cored penumbral intrusions and a dark-cored light
bridge. The mid-outer part of this penumbra structure shows 0.3 arcsec wide
spines, separated by 1.6 arcsec (1200 km) and associated with 30 deg
inclination variations. Between these spines, there are no small-scale magnetic
structures that easily can be be identified with individual flux tubes. A
structure with nearly 10 deg more vertical and weaker magnetic field is seen
midways between two spines. This structure is co-spatial with the brightest
penumbral filament, possibly indicating the location of a convective upflow
from below.Comment: Accepted for publication in ApJL 17 Oct 2008. One Figure adde
Imaging Spectropolarimetry with IBIS II: on the fine structure of G-band bright features
We present new results from first observations of the quiet solar photosphere
performed through the Interferometric BIdimensional Spectrometer (IBIS) in
spectropolarimetric mode. IBIS allowed us to measure the four Stokes parameters
in the FeI 630.15 nm and FeI 630.25 nm lines with high spatial and spectral
resolutions for 53 minutes; the polarimetric sensitivity achieved by the
instrument is 0.003 the continuum intensity level. We focus on the correlation
which emerges between G-band bright feature brightness and magnetic filling
factor of ~ 1000 G (kG) fields derived by inverting Stokes I and V profiles.
More in detail, we present the correlation first in a pixel-by-pixel study of
an approximatively 3 arcsec wide bright feature (a small network patch) and
then we show that such a result can be extended to all the bright features
found in the dataset at any instant of the time sequence. The higher the kG
filling factor associated to a feature the higher the brightness of the feature
itself. Filling factors up to about 35 % are obtained for the brightest
features. Considering the values of the filling factors derived from the
inversion analysis of spectropolarimetric data and the brightness variation
observed in G-band data we put forward an upper limit for the smallest scale
over which magnetic flux concentrations in intergranular lanes produce a G-band
brightness enhancement (~ 0.1''). Moreover, the brightness saturation observed
for feature sizes comparable to the resolution of the observations is
compatible with large G-band bright features being clusters of sub-arcsecond
bright points. This conclusion deserves to be confirmed by forthcoming
spectropolarimetric observations at higher spatial resolution.Comment: 10 pages, 7 figures, 1 table - Accepted for publication on Ap
Properties of simulated sunspot umbral dots
Realistic 3D radiative MHD simulations reveal the magneto-convective
processes underlying the formation of the photospheric fine structure of
sunspots, including penumbral filaments and umbral dots. Here we provide
results from a statistical analysis of simulated umbral dots and compare them
with reports from high-resolution observations. A multi-level segmentation and
tracking algorithm has been used to isolate the bright structures in synthetic
bolometric and continuum brightness images. Areas, brightness, and lifetimes of
the resulting set of umbral dots are found to be correlated: larger umbral dots
tend to be brighter and live longer. The magnetic field strength and velocity
structure of umbral dots on surfaces of constant optical depth in the continuum
at 630 nm indicate that the strong field reduction and high velocities in the
upper parts of the upflow plumes underlying umbral dots are largely hidden from
spectro-polarimetric observations. The properties of the simulated umbral dots
are generally consistent with the results of recent high-resolution
observations. However, the observed population of small, short-lived umbral
dots is not reproduced by the simulations, possibly owing to insufficient
spatial resolution.Comment: Accepted for publication in A&
SST/CRISP Observations of Convective Flows in a Sunspot Penumbra
Context. Recent discoveries of intensity correlated downflows in the interior
of a sunspot penumbra provide direct evidence for overturning convection,
adding to earlier strong indications of convection from filament dynamics
observed far from solar disk center, and supporting recent simulations of
sunspots.
Aims. Using spectropolarimetric observations obtained at a spatial resolution
approaching 0'.'1 with the Swedish 1-m Solar Telescope (SST) and its
spectropolarimeter CRISP, we investigate whether the convective downflows
recently discovered in the C i line at 538.03 nm can also be detected in the
wings of the Fe i line at 630.15 nm
Methods. We make azimuthal fits of the measured LOS velocities in the core
and wings of the 538 nm and 630 nm lines to disentangle the vertical and
horizontal flows. To investigate how these depend on the continuum intensity,
the azimuthal fits are made separately for each intensity bin. By using
spatially high-pass filtered measurements of the LOS component of the magnetic
field, the flow properties are determined separately for magnetic spines
(relatively strong and vertical field) and inter-spines (weaker and more
horizontal field).
Results. The dark convective downflows discovered recently in the 538.03 nm
line are evident also in the 630.15 nm line, and have similar strength. This
convective signature is the same in spines and inter-spines. However, the
strong radial (Evershed) outflows are found only in the inter-spines.
Conclusions. At the spatial resolution of the present SST/CRISP data, the
small-scale intensity pattern seen in continuum images is strongly related to a
convective up/down flow pattern that exists everywhere in the penumbra. Earlier
failures to detect the dark convective downflows in the interior penumbra can
be explained by inadequate spatial resolution in the observed data.Comment: Revised and expanded by 2.5 pages. Fig. 14 adde
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
DKIST unveils the serpentine topology of quiet Sun magnetism in the photosphere
We present the first quiet Sun spectropolarimetric observations obtained with
the Visible SpectroPolarimeter (ViSP) at the m Daniel K. Inouye Solar
Telescope (DKIST). We recorded observations in a wavelength range that includes
the magnetically sensitive Fe I doublet. With an
estimated spatial resolution of 0.08'', this represents the highest spatial
resolution full-vector spectropolarimetric observations ever obtained of the
quiet Sun. We identified small-scale magnetic elements, including
magnetic loops and unipolar magnetic patches, with linear and circular
polarisation detected in all of them. Of particular interest is a magnetic
element in which the polarity of the magnetic vector appears to change three
times in only km and which has linear polarisation signals throughout. We
find complex Stokes profiles at the polarity inversion lines of magnetic
loops and discover degenerate solutions, as we are unable to conclusively
determine whether these arise due to gradients in the atmospheric parameters or
smearing of opposite polarity signals. We analyse a granule which notably has
linear and circular polarisation signals throughout, providing an opportunity
to explore its magnetic properties. On this small scale we see the magnetic
field strength range from G at the granular boundary to kG in the
intergranular lane (IGL), and sanity check the values with the weak and strong
field approximations. A value of kG in the IGL is among the highest
measurements ever recorded for the internetwork.Comment: Accepted for publication in ApJ
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