126 research outputs found
Localization of Matter Waves in 2D-Disordered Optical Potentials
We consider ultracold atoms in 2D-disordered optical potentials and calculate
microscopic quantities characterizing matter wave quantum transport in the
non-interacting regime. We derive the diffusion constant as function of all
relevant microscopic parameters and show that coherent multiple scattering
induces significant weak localization effects. In particular, we find that even
the strong localization regime is accessible with current experimental
techniques and calculate the corresponding localization length.Comment: 4 pages, 3 figures, figures changed, references update
Magnetic properties of photospheric regions having very low magnetic flux
The magnetic properties of the quiet Sun are investigated using a novel
inversion code, FATIMA, based on the Principal Component Analysis of the
observed Stokes profiles. The stability and relatively low noise sensitivity of
this inversion procedure allows for the systematic inversion of large data sets
with very weak polarization signal. Its application to quiet Sun observations
of network and internetwork regions reveals that a significant fraction of the
quiet Sun contains kilogauss fields (usually with very small filling factors)
and confirms that the pixels with weak polarization account for most of the
magnetic flux. Mixed polarities in the resolution element are also found to
occur more likely as the polarization weakens.Comment: To apapear in ApJ. 39 pages, 12 figures (2 of them are color figures
Stokes diagnostics of simulated solar magneto-convection
We present results of synthetic spectro-polarimetric diagnostics of radiative
MHD simulations of solar surface convection with magnetic fields. Stokes
profiles of Zeeman-sensitive lines of neutral iron in the visible and infrared
spectral ranges emerging from the simulated atmosphere have been calculated in
order to study their relation to the relevant physical quantities and compare
with observational results. We have analyzed the dependence of the Stokes-I
line strength and width as well as of the Stokes-V signal and asymmetries on
the magnetic field strength. Furthermore, we have evaluated the correspondence
between the actual velocities in the simulation with values determined from the
Stokes-I (Doppler shift of the centre of gravity) and Stokes-V profiles
(zero-crossing shift). We confirm that the line weakening in strong magnetic
fields results from a higher temperature (at equal optical depth) in the
magnetic flux concentrations. We also confirm that considerable Stokes-V
asymmetries originate in the peripheral parts of strong magnetic flux
concentrations, where the line of sight cuts through the magnetopause of the
expanding flux concentration into the surrounding convective donwflow.Comment: Astronomy & Astrophysics, in pres
Quiet Sun magnetic fields from simultaneous inversions of visible and infrared spectropolarimetric observations
We study the quiet Sun magnetic fields using spectropolarimetric observations
of the infrared and visible Fe I lines at 6301.5, 6302.5, 15648 and 15653 A.
Magnetic field strengths and filling factors are inferred by the simultaneous
fit of the observed Stokes profiles under the MISMA hypothesis. The
observations cover an intra-network region at the solar disk center. We analyze
2280 Stokes profiles whose polarization signals are above noise in the two
spectral ranges, which correspond to 40% of the field of view. Most of these
profiles can be reproduced only with a model atmosphere including 3 magnetic
components with very different field strengths, which indicates the
co-existence of kG and sub-kG fields in our 1.5" resolution elements. We
measure an unsigned magnetic flux density of 9.6 G considering the full field
of view. Half of the pixels present magnetic fields with mixed polarities in
the resolution element. The fraction of mixed polarities increases as the
polarization weakens. We compute the probability density function of finding
each magnetic field strength. It has a significant contribution of kG field
strengths, which concentrates most of the observed magnetic flux and energy.
This kG contribution has a preferred magnetic polarity, while the polarity of
the weak fields is balanced.Comment: 16 pages and 14 figure
On the validity of the 630 nm Fe I nm lines for the magnetometry of the internetwork quiet Sun
The purpose of this work is to analyze the reliability of the magnetic field
strengths inferred from the 630 nm pair of Fe I lines at internetwork quiet Sun
regions. Some numerical experiments have been performed that demonstrate the
inability of these lines to recover the magnetic field strength in such low
flux solar regions. It is shown how different model atmospheres, with magnetic
field strengths ranging from few hundred Gauss to kiloGauss, give rise to
Stokes profiles that can not be distinguished. The reasons for this degeneracy
are discussed.Comment: Accepted for publication in A&
Simulation of a flux emergence event and comparison with observations by Hinode
We study the observational signature of flux emergence in the photosphere
using synthetic data from a 3D MHD simulation of the emergence of a twisted
flux tube. Several stages in the emergence process are considered. At every
stage we compute synthetic Stokes spectra of the two iron lines Fe I 6301.5
{\AA} and Fe I 6302.5 {\AA} and degrade the data to the spatial and spectral
resolution of Hinode's SOT/SP. Then, following observational practice, we apply
Milne-Eddington-type inversions to the synthetic spectra in order to retrieve
various atmospheric parameters and compare the results with recent Hinode
observations. During the emergence sequence, the spectral lines sample
different parts of the rising flux tube, revealing its twisted structure. The
horizontal component of the magnetic field retrieved from the simulations is
close to the observed values. The flattening of the flux tube in the
photosphere is caused by radiative cooling, which slows down the ascent of the
tube to the upper solar atmosphere. Consistent with the observations, the
rising magnetized plasma produces a blue shift of the spectral lines during a
large part of the emergence sequence.Comment: A&A Letter, 3 figure
Magnetic fields of opposite polarity in sunspot penumbrae
Context. A significant part of the penumbral magnetic field returns below the
surface in the very deep photosphere. For lines in the visible, a large portion
of this return field can only be detected indirectly by studying its imprints
on strongly asymmetric and three-lobed Stokes V profiles. Infrared lines probe
a narrow layer in the very deep photosphere, providing the possibility of
directly measuring the orientation of magnetic fields close to the solar
surface.
Aims. We study the topology of the penumbral magnetic field in the lower
photosphere, focusing on regions where it returns below the surface.
Methods. We analyzed 71 spectropolarimetric datasets from Hinode and from the
GREGOR infrared spectrograph. We inferred the quality and polarimetric accuracy
of the infrared data after applying several reduction steps. Techniques of
spectral inversion and forward synthesis were used to test the detection
algorithm. We compared the morphology and the fractional penumbral area covered
by reversed-polarity and three-lobed Stokes V profiles for sunspots at disk
center. We determined the amount of reversed-polarity and three-lobed Stokes V
profiles in visible and infrared data of sunspots at various heliocentric
angles. From the results, we computed center-to-limb variation curves, which
were interpreted in the context of existing penumbral models.
Results. Observations in visible and near-infrared spectral lines yield a
significant difference in the penumbral area covered by magnetic fields of
opposite polarity. In the infrared, the number of reversed-polarity Stokes V
profiles is smaller by a factor of two than in the visible. For three-lobed
Stokes V profiles the numbers differ by up to an order of magnitude.Comment: 11 pages 10 figures plus appendix (2 pages 3 figures). Accepted as
part of the A&A special issue on the GREGOR solar telescop
The relationship between auroral hiss at high altitudes over the polar caps and the substorm dynamics of aurora
Strong variations of intensity and cutoff frequency of the auroral hiss were observed by INTERBALL-2 and POLAR satellites at high altitudes, poleward from the auroral oval. The hiss intensifications are correlated with the auroral activations during substorms and/or pseudo-breakups. The low cutoff frequency of auroral hiss increases with the distance between the aurora and the satellite footprint. Multicomponent wave measurements of the hiss emissions on board the POLAR spacecraft show that the horizontal component of the Poynting flux of auroral hiss changes its direction in good accordance with longitudinal displacements of the bright auroras. The vertical component of the Poynting flux is directed upward from the aurora region, indicating that hiss could be generated by upgoing electron beams. This relationship between hiss and the aurora dynamics means that the upgoing electron beams are closely related to downgoing electron beams which produce the aurora. During the auroral activations the upgoing and downgoing beams move and change their intensities simultaneously.<br><br> <b>Keywords.</b> Magnetospheric physics (Auroral phenomena; Plasma waves and instabilities; Storms and substorms
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