294 research outputs found
Where the granular flows bend
Based on IMaX/Sunrise data, we report on a previously undetected phenomenon
in solar granulation. We show that in a very narrow region separating granules
and intergranular lanes the spectral line width of the Fe I 5250.2 A line
becomes extremely small. We offer an explanation of this observation with the
help of magneto-convection simulations. These regions with extremely small line
widths correspond to the places where the granular flows bend from mainly
upflow in granules to downflow in intergranular lanes. We show that the
resolution and image stability achieved by IMaX/Sunrise are important
requisites to detect this interesting phenomenon.Comment: Accepted for the Sunrise Special Issue of ApJ
Discriminant analysis of solar bright points and faculae II. Contrast and morphology analysis
Taken at a high spatial resolution of 0.1 arcsec, Bright Points (BPs) are
found to coexist with faculae in images and the latter are often resolved as
adjacent striations. Understanding the properties of these different features
is fundamental to carrying out proxy magnetometry. To shed light on the
relationship between BPs and faculae, we studied them separately after the
application of a classification method, developed and described in a previous
paper) on active region images at various heliocentric angles. In this Paper,
we explore different aspects of the photometric properties of BPs and faculae,
namely their G-band contrast profiles, their peak contrast in G-band and
continuum, as well as morphological parameters. We find that: (1) the width of
the contrast profiles of the classified BPs and faculae are consistent with
studies of disk center BPs at and limb faculae, which indirectly confirms the
validity of our classification, (2) the profiles of limb faculae are limbward
skewed on average, while near disk center they exhibit both centerward and
limbward skewnesses due to the distribution of orientations of the faculae, (3)
the relation between the peak contrasts of BPs and faculae and their apparent
area discloses a trend reminiscent of magnetogram studies. The skewness of
facular profiles provides a novel constraint for 3D MHD models of faculae. As
suggested by the asymmetry and orientation of their contrast profiles, faculae
near disk center could be induced by inclined fields, while apparent BPs near
the limb seem to be in fact small faculae misidentified. The apparent area of
BPs and faculae could be possibly exploited for proxy magnetometry
Fine structures in the atmosphere above a sunspot umbra
We present simultaneous photospheric and chromospheric observations of the
trailing sunspot in NOAA 10904, obtained with the Swedish Solar Telescope (SST)
La Palma, Canary Islands. Time series of high resolution \ion{Ca}{ii}\,
images show transient jet-like structures in sunspot umbrae are elongated,
which we call umbral microjets. These jets are directed roughly parallel to
nearby penumbral microjets, suggesting that they are aligned with the
background magnetic field. In general, first a bright dot-like structure
appears, from which a jet later emerges, although some jets appear without an
associated chromospheric dot. Bright photospheric umbral dots are associated
with umbral microjets arising in the outer umbra. Nevertheless, a one-to-one
correspondence between jet-like events and underlying umbral dots is not seen.
They are typically less than 1\arcsec ~long and less than 0\farcs3 wide. The
typical lifetime of umbral microjets is around one minute. The brightness of
these structures increases from the center of the umbra towards the
umbra-penumbra boundary along with the brightness of the local background.Comment: 5 pages, 6 figures. Accepted for publication in A&A Lette
Three-dimensional magnetic structure of a sunspot: comparison of the photosphere and upper chromosphere
We investigate the magnetic field of a sunspot in the upper chromosphere and
compare it to the field's photospheric properties. We observed the main leading
sunspot of the active region NOAA 11124 on two days with the Tenrife Infrared
Polarimeter-2 (TIP-2) mounted at the German Vacuum Tower Telescope (VTT).
Through inversion of Stokes spectra of the He I triplet at 1083.0 nm, we
obtained the magnetic field vector of the upper chromosphere. For comparison
with the photosphere we applied height-depended inversions of the Si I 1082.71
nm and Ca I 1083.34 nm lines. We found that the umbral magnetic field strength
in the upper chromosphere is lower by a factor of 1.30-1.65 compared to the
photosphere. The magnetic field strength of the umbra decreases from the
photosphere towards the upper chromosphere by an average rate of 0.5-0.9 G
km. The difference in the magnetic field strength between both
atmospheric layers steadily decreases from the sunspot center to the outer
boundary of the sunspot, with the field (in particular its horizontal
component) being stronger in the chromopshere outside the spot, suggestive of a
magnetic canopy. The sunspot displays a twist that on average is similar in the
two layers. However, the differential twist between photosphere and
chromosphere increases rapidly towards the outer penumbral boundary. The
magnetic field vector is more horizontal with respect to the solar surface by
roughly 5-20 in the photosphere compared to the upper chromosphere.
Above a lightbridge, the chromospheric magnetic field is equally strong as that
in the umbra, whereas the lightbridge's field is weaker than its surroundings
in the photosphere by roughly 1 kG. This suggests a cusp-like magnetic field
structure above the lightbridge.Comment: 12 pages, 15 figures, accepted for publication in A&
Vertical magnetic field gradient in the photospheric layers of sunspots
We investigate the vertical gradient of the magnetic field of sunspots in the
photospheric layer. Independent observations were obtained with the SOT/SP
onboard the Hinode spacecraft and with the TIP-2 mounted at the VTT. We apply
state-of-the-art inversion techniques to both data sets to retrieve the
magnetic field and the corresponding vertical gradient. In the sunspot
penumbrae we detected patches of negative vertical gradients of the magnetic
field strength, i.e.,the magnetic field strength decreases with optical depth
in the photosphere. The negative gradient patches are located in the inner and
partly in the middle penumbrae in both data sets. From the SOT/SP observations,
we found that the negative gradient patches are restricted mainly to the deep
photospheric layers and are concentrated near the edges of the penumbral
filaments. MHD simulations also show negative gradients in the inner penumbrae,
also at the locations of filaments. Both in the observations and simulation
negative gradients of the magnetic field vs. optical depth dominate at some
radial distances in the penumbra. The negative gradient with respect to optical
depth in the inner penumbrae persists even after averaging in the azimuthal
direction, both in the observations and, to a lesser extent, also in MHD
simulations. We interpret the observed localized presence of the negative
vertical gradient of the magnetic field strength in the observations as a
consequence of stronger field from spines expanding with height and closing
above the weaker field inter-spines. The presence of the negative gradients
with respect to optical depth after azimuthal averaging can be explained by two
different mechanisms: the high corrugation of equal optical depth surfaces and
the cancellation of polarized signal due to the presence of unresolved opposite
polarity patches in the deeper layers of the penumbra.Comment: 17 pages, 25 figures, accepted for publication in A&
Waves as the source of apparent twisting motions in sunspot penumbrae
The motion of dark striations across bright filaments in a sunspot penumbra
has become an important new diagnostic of convective gas flows in penumbral
filaments. The nature of these striations has, however, remained unclear. Here
we present an analysis of small scale motions in penumbral filaments in both
simulations and observations. The simulations, when viewed from above, show
fine structure with dark lanes running outwards from the dark core of the
penumbral filaments. The dark lanes either occur preferentially on one side or
alternate between both sides of the filament. We identify this fine structure
with transverse (kink) oscillations of the filament, corresponding to a
sideways swaying of the filament. These oscillations have periods in the range
of 5-7 min and propagate outward and downward along the filament. Similar
features are found in observed G-band intensity time series of penumbral
filaments in a sunspot located near disk center obtained by the Broadband
Filter Imager (BFI) on board {\it Hinode}. We also find that some filaments
show dark striations moving to both sides of the filaments. Based on the
agreement between simulations and observations we conclude that the motions of
these striations are caused by transverse oscillations of the underlying bright
filaments.Comment: Accepted for publication in Astrophysical Journal on 8th April 201
Search for short-period acoustic waves with high resolution 2D-spectra
The data for this contributionw ere takeninAugust 2000 with the “G¨ottingen” two-dimensional spectrometer in the VTT on Tenerife. Our spectrometer is based on two scanning
Fabry-Perot Interferometers (FPIs). The non-magnetic Fe I 5434˚A line was observed in the quiet Sun at disk center. Time sequences of 41 min duration and with 25 s cadence were taken. Strictly simultaneously with the narrow-band FPI images (32m˚A FWHM), broad-band images were recorded. The latter were reconstructed with speckle methods. With the known “true” broad-band object we are able to restore the narrow-band images as well. Our aim is to find
propagating acoustic waves at short periods and small scales. The results obtained so far are presented and the prospects are discussed
Time series of high resolution photospheric spectra in a quiet region of the Sun. I. Analysis of global and spatial variations of line parameters
A 50 min time series of one-dimensional slit-spectrograms, taken in quiet sun
at disk center, observed at the German Vacuum Tower Telescope (Observatorio del
Teide), was used to study the global and spatial variations of different line
parameters. In order to determine the vertical structure of the photosphere two
lines with well separated formation heights have been considered. The data have
been filtered of p-modes to isolate the pure convective phenomenon. From our
studies of global correlation coefficients and coherence and phase shift
analyzes between the several line parameters, the following results can be
reported. The convective velocity pattern preserves structures larger than 1.0"
up to the highest layers of the photosphere (~ 435 km). However, at these
layers, in the intensity pattern only structures larger than 2.0" are still
connected with those at the continuum level although showing inverted
brightness contrast. This confirms an inversion of temperature that we have
found at a height of ~140 km. A possible evidence of gravity waves superimposed
to the convective motions is derived from the phase shift analysis. We
interpret the behavior of the full width at half maximum and the equivalent
width as a function of the distance to the granular borders, as a consequence
of enhanced turbulence and/or strong velocity gradients in the intergranular
lanes.Comment: 16 pages, 15 figures, 5 tables; Astronomy & Astrophysics, Volume 408,
p.363-378, 200
Detection of Small-Scale Granular Structures in the Quiet Sun with the New Solar Telescope
Results of a statistical analysis of solar granulation are presented. A data
set of 36 images of a quiet Sun area on the solar disk center was used. The
data were obtained with the 1.6 m clear aperture New Solar Telescope (NST) at
Big Bear Solar Observatory (BBSO) and with a broad-band filter centered at the
TiO (705.7 nm) spectral line. The very high spatial resolution of the data
(diffraction limit of 77 km and pixel scale of 0.0375) augmented by the very
high image contrast (15.50.6%) allowed us to detect for the first time a
distinct subpopulation of mini-granular structures. These structures are
dominant on spatial scales below 600 km. Their size is distributed as a power
law with an index of -1.8 (which is close to the Kolmogorov's -5/3 law) and no
predominant scale. The regular granules display a Gaussian (normal) size
distribution with a mean diameter of 1050 km. Mini-granular structures
contribute significantly to the total granular area. They are predominantly
confined to the wide dark lanes between regular granules and often form chains
and clusters, but different from magnetic bright points. A multi-fractality
test reveals that the structures smaller than 600 km represent a multi-fractal,
whereas on larger scales the granulation pattern shows no multi-fractality and
can be considered as a Gaussian random field. The origin, properties and role
of the newly discovered population of mini-granular structures in the solar
magneto-convection are yet to be explored.Comment: 13 pages, 5 figure
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