387 research outputs found
Quiet Sun magnetic fields observed by Hinode: Support for a local dynamo
The Hinode mission has revealed copious amounts of horizontal flux covering
the quiet Sun. Local dynamo action has been proposed to explain the presence of
this flux. We sought to test whether the quiet Sun flux detected by Hinode is
due to a local or the global dynamo by studying long-term variations in the
polarisation signals detectable at the disc centre of the quiet Sun between
November 2006 and May 2012, with particular emphasis on weak signals in the
internetwork. The investigation focusses on line-integrated circular
polarisation V_tot and linear polarisation LP_tot profiles obtained from the Fe
I 6302.5 \AA absorption line in Hinode SOT/SP. Both circular and linear
polarisation signals show no overall variation in the fraction of selected
pixels from 2006 until 2012. There is also no variation in the magnetic flux in
this interval of time. The probability density functions (PDF) of the
line-of-sight magnetic flux can be fitted with a power law from 1.17 x 10^17 Mx
to 8.53 x 10^18 Mx with index \alpha=-1.82 \pm 0.02 in 2007. The variation of
\alpha 's across all years does not exceed a significance of 1\sigma. Linearly
polarised features are also fitted with a power law, with index \alpha=-2.60
\pm 0.06 in 2007. Indices derived from linear polarisation PDFs of other years
also show no significant variation. Our results show that the ubiquitous
horizontal polarisation on the edges of bright granules seen by Hinode are
invariant during the minimum of cycle 23. This supports the notion that the
weak circular and linear polarisation is primarily caused by an independent
local dynamo
The Height of Chromospheric Loops in an Emerging Flux Region
Context. The chromospheric layer observable with the He I 10830 {\AA} triplet
is strongly warped. The analysis of the magnetic morphology of this layer
therefore requires a reliable technique to determine the height at which the He
I absorption takes place.
Aims. The He I absorption signature connecting two pores of opposite polarity
in an emerging flux region is investigated. This signature is suggestive of a
loop system connecting the two pores. We aim to show that limits can be set on
the height of this chromospheric loop system.
Methods. The increasing anisotropy in the illumination of a thin, magnetic
structure intensifies the linear polarization signal observed in the He I
triplet with height. This signal is altered by the Hanle effect. We apply an
inversion technique incorporating the joint action of the Hanle and Zeeman
effects, with the absorption layer height being one of the free parameters.
Results. The observed linear polarization signal can be explained only if the
loop apex is higher than \approx5 Mm. Best agreement with the observations is
achieved for a height of 6.3 Mm.
Conclusions. The strength of the linear polarization signal in the loop apex
is inconsistent with the assumption of a He I absorption layer at a constant
height level. The determined height supports the earlier conclusion that dark
He 10830 {\AA} filaments in emerging flux regions trace emerging loops.Comment: 7 pages, 4 figure
Milne-Eddington inversions of the He I 10830 {\AA} Stokes profiles: Influence of the Paschen-Back effect
The Paschen-Back effect influences the Zeeman sublevels of the He I multiplet
at 10830 {\AA}, leading to changes in strength and in position of the Zeeman
components of these lines. We illustrate the relevance of this effect using
synthetic Stokes profiles of the He I 10830 {\AA} multiplet lines and
investigate its influence on the inversion of polarimetric data. We invert data
obtained with the Tenerife Infrared Polarimeter (TIP) at the German Vacuum
Tower Telescope (VTT). We compare the results of inversions based on synthetic
profiles calculated with and without the Paschen-Back effect being included. We
find that when taking into account the incomplete Paschen-Back effect, on
average 16% higher field strength values are obtained. We also show that this
effect is not the main cause for the area asymmetry exhibited by many He I
10830 Stokes V-profiles. This points to the importance of velocity and magnetic
field gradients over the formation height range of these lines.Comment: Accepted for publication in A&A on Jun 12th 200
Measuring the Wilson depression of sunspots using the divergence-free condition of the magnetic field vector
Context: The Wilson depression is the difference in geometric height of unit
continuum optical depth between the sunspot umbra and the quiet Sun. Measuring
the Wilson depression is important for understanding the geometry of sunspots.
Current methods suffer from systematic effects or need to make assumptions on
the geometry of the magnetic field. This leads to large systematic
uncertainties of the derived Wilson depressions.
Aims: We aim at developing a robust method for deriving the Wilson depression
that only requires the information about the magnetic field that is accessible
from spectropolarimetry, and that does not rely on assumptions on the geometry
of sunspots or on their magnetic field.
Methods: Our method is based on minimizing the divergence of the magnetic
field vector derived from spectropolarimetric observations. We focus on large
spatial scales only in order to reduce the number of free parameters.
Results: We test the performance of our method using synthetic Hinode data
derived from two sunspot simulations. We find that the maximum and the umbral
averaged Wilson depression for both spots determined with our method typically
lies within 100 km of the true value obtained from the simulations. In
addition, we apply the method to Hinode observations of a sunspot. The derived
Wilson depression (about 600 km) is consistent with results typically obtained
from the Wilson effect. We also find that the Wilson depression obtained from
using horizontal force balance gives 110 - 180 km smaller Wilson depressions
than both, what we find and what we deduce directly from the simulations. This
suggests that the magnetic pressure and the magnetic curvature force contribute
to the Wilson depression by a similar amount.Comment: 12 pages, 8 figures. Accepted for publication in Astronomy &
Astrophysic
Migration of Ca II H bright points in the internetwork
The migration of magnetic bright point-like features (MBP) in the lower solar
atmosphere reflects the dispersal of magnetic flux as well as the horizontal
flows of the atmospheric layer they are embedded in. We analyse trajectories of
the proper motion of intrinsically magnetic, isolated internetwork Ca II H MBPs
(mean lifetime 461 +- 9 s) to obtain their diffusivity behaviour. We use
seeing-free high spatial and temporal resolution image sequences of quiet-Sun,
disc-centre observations obtained in the Ca II H 3968 {\AA} passband of the
Sunrise Filter Imager (SuFI) onboard the Sunrise balloon-borne solar
observatory. Small MBPs in the internetwork are automatically tracked. The
trajectory of each MBP is then calculated and described by a diffusion index
({\gamma}) and a diffusion coefficient (D). We further explore the distribution
of the diffusion indices with the help of a Monte Carlo simulation. We find
{\gamma} = 1.69 +- 0.08 and D = 257 +- 32 km^2/s averaged over all MBPs.
Trajectories of most MBPs are classified as super-diffusive, i.e., {\gamma} >
1, with the determined {\gamma} being to our knowledge the largest obtained so
far. A direct correlation between D and time-scale ({\tau}) determined from
trajectories of all MBPs is also obtained. We discuss a simple scenario to
explain the diffusivity of the observed, relatively short-lived MBPs while they
migrate within a small area in a supergranule (i.e., an internetwork area). We
show that the scatter in the {\gamma} values obtained for individual MBPs is
due to their limited lifetimes. The super-diffusive MBPs can be well-described
as random walkers (due to granular evolution and intergranular turbu- lence)
superposed on a large systematic (background) velocity, caused by granular,
mesogranular and supergranular flows.Comment: 10 pages, 7 figures, 3 table
Probing deep photospheric layers of the quiet Sun with high magnetic sensitivity
Context. Investigations of the magnetism of the quiet Sun are hindered by extremely weak polarization signals in Fraunhofer spectral lines. Photon noise, straylight, and the systematically different sensitivity of the Zeeman effect to longitudinal and transversal magnetic fields result in controversial results in terms of the strength and angular distribution of the magnetic field vector. Aims. The information content of Stokes measurements close to the diffraction limit of the 1.5 m GREGOR telescope is analyzed. We took the effects of spatial straylight and photon noise into account. Methods. Highly sensitive full Stokes measurements of a quiet-Sun region at disk center in the deep photospheric Fe i lines in the 1.56 μm region were obtained with the infrared spectropolarimeter GRIS at the GREGOR telescope. Noise statistics and Stokes V asymmetries were analyzed and compared to a similar data set of the Hinode spectropolarimeter (SOT/SP). Simple diagnostics based directly on the shape and strength of the profiles were applied to the GRIS data. We made use of the magnetic line ratio technique, which was tested against realistic magneto-hydrodynamic simulations (MURaM). Results. About 80% of the GRIS spectra of a very quiet solar region show polarimetric signals above a 3σ level. Area and amplitude asymmetries agree well with small-scale surface dynamo-magneto hydrodynamic simulations. The magnetic line ratio analysis reveals ubiquitous magnetic regions in the ten to hundred Gauss range with some concentrations of kilo-Gauss fields. Conclusions. The GRIS spectropolarimetric data at a spatial resolution of ≈ 0.″ 4 are so far unique in the combination of high spatial resolution scans and high magnetic field sensitivity. Nevertheless, the unavoidable effect of spatial straylight and the resulting dilution of the weak Stokes profiles means that inversion techniques still bear a high risk of misinterpretating the data. © ESO, 2016.This work was partly supported by the BK21 plus program through the National Research Foundation (NRF) funded by the Ministry of Education of Korea. This study is supported by the European Commissions FP7 Capacities Programme under the Grant Agreement number 312495. The GRIS instrument was developed thanks to the support by the Spanish Ministry of Economy and Competitiveness through the project AYA2010-18029 (Solar Magnetism and Astrophysical Spectropolarimetry).Peer Reviewe
Multicomponent He I 10830 {\AA} profiles in an active filament
We present new spectropolarimetric observations of the chromospheric He I
10830 {\AA} multiplet observed in a filament during its phase of activity. The
data were recorded with the new Tenerife Infrared Polarimeter (TIP-II) at the
German Vacuum Tower Telescope (VTT) on 2005 May 18. We inverted the He Stokes
profiles using multiple atmospheric components. The observed He Stokes profiles
display a remarkably wide variety of shapes. Most of the profiles show very
broad Stokes I absorptions and complex and spatially variable Stokes V
signatures. The inversion of the profiles shows evidence of different
atmospheric blue- and redshifted components of the He I lines within the
resolution element (1 arcsec), with supersonic velocities of up to 100 km/s. Up
to five different atmospheric components are found in the same profile. We show
that even these complex profiles can be reliably inverted.Comment: Accepted for publication in Astronomy and Astrophysics on September
27, 201
Inclinations of small quiet-Sun magnetic features based on a new geometric approach
High levels of horizontal magnetic flux have been reported in the quiet-Sun
internetwork, often based on Stokes profile inversions. Here we introduce a new
method for deducing the inclination of magnetic elements and use it to test
magnetic field inclinations from inversions. We determine accurate positions of
a set of small, bright magnetic elements in high spatial resolution images
sampling different photospheric heights obtained by the Sunrise balloon-borne
solar observatory. Together with estimates of the formation heights of the
employed spectral bands, these provide us with the inclinations of the magnetic
features. We also compute the magnetic inclination angle of the same magnetic
features from the inversion of simultaneously recorded Stokes parameters. Our
new, geometric method returns nearly vertical fields (average inclination of
around 14 deg with a relatively narrow distribution having a standard deviation
of 6 deg). In strong contrast to this, the traditionally used inversions give
almost horizontal fields (average inclination of 75+-8 deg) for the same small
magnetic features, whose linearly polarised Stokes profiles are adversely
affected by noise. The almost vertical field of bright magnetic features from
our geometric method is clearly incompatible with the nearly horizontal
magnetic fields obtained from the inversions. This indicates that the amount of
magnetic flux in horizontal fields deduced from inversions is overestimated in
the presence of weak Stokes signals, in particular if Stokes Q and U are close
to or under the noise level. By combining the proposed method with inversions
we are not just improving the inclination, but also the field strength. This
technique allows us to analyse features that are not reliably treated by
inversions, thus greatly extending our capability to study the complete
magnetic field of the quiet Sun.Comment: 12 pages, 9 figures, 1 table; Accepted for publication in Astronomy &
Astrophysic
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