63 research outputs found
Opposite magnetic polarity of two photospheric lines in single spectrum of the quiet Sun
We study the structure of the photospheric magnetic field of the quiet Sun by
investigating weak spectro-polarimetric signals. We took a sequence of Stokes
spectra of the Fe I 630.15 nm and 630.25 nm lines in a region of quiet Sun near
the disk center, using the POLIS spectro-polarimeter at the German VTT on
Tenerife. The line cores of these two lines form at different heights in the
atmosphere. The 3 noise level of the data is about 1.8 . We present co-temporal and co-spatial Stokes- profiles of the Fe I
630 nm line pair, where the two lines show opposite polarities in a single
spectrum. We compute synthetic line profiles and reproduce these spectra with a
two-component model atmosphere: a non-magnetic component and a magnetic
component. The magnetic component consists of two magnetic layers with opposite
polarity: the upper one moves upwards while the lower one moves downward.
In-between, there is a region of enhanced temperature. The Stokes- line pair
of opposite polarity in a single spectrum can be understood as a magnetic
reconnection event in the solar photosphere. We demonstrate that such a
scenario is realistic, but the solution may not be unique.Comment: 4 pages, 5 figures, accepted in Astronomy & Astrophysics Letter
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
Hanle effect in coherent backscattering
We study the shape of the coherent backscattering (CBS) cone obtained when
resonant light illuminates a thick cloud of laser-cooled rubidium atoms in
presence of a homogenous magnetic field. We observe new magnetic
field-dependent anisotropies in the CBS signal. We show that the observed
behavior is due to the modification of the atomic radiation pattern by the
magnetic field (Hanle effect in the excited state).Comment: 4 pages, 3 figure
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
Stokes Diagnostis of 2D MHD-simulated Solar Magnetogranulation
We study the properties of solar magnetic fields on scales less than the
spatial resolution of solar telescopes. A synthetic infrared
spectropolarimetric diagnostics based on a 2D MHD simulation of
magnetoconvection is used for this. We analyze two time sequences of snapshots
that likely represent two regions of the network fields with their immediate
surrounding on the solar surface with the unsigned magnetic flux density of 300
and 140 G. In the first region we find from probability density functions of
the magnetic field strength that the most probable field strength at logtau_5=0
is equal to 250 G. Weak fields (B < 500 G) occupy about 70% of the surface,
while stronger fields (B 1000 G) occupy only 9.7% of the surface. The magnetic
flux is -28 G and its imbalance is -0.04. In the second region, these
parameters are correspondingly equal to 150 G, 93.3 %, 0.3 %, -40 G, and -0.10.
We estimate the distribution of line-of-sight velocities on the surface of log
tau_5=-1. The mean velocity is equal to 0.4 km/s in the first simulated region.
The averaged velocity in the granules is -1.2 km/s and in the intergranules is
2.5 km/s. In the second region, the corresponding values of the mean velocities
are equal to 0, -1.8, 1.5 km/s. In addition we analyze the asymmetry of
synthetic Stokes-V profiles of the Fe I 1564.8 nm line. The mean values of the
amplitude and area asymmetry do not exceed 1%. The spatially smoothed amplitude
asymmetry is increased to 10% while the area asymmetry is only slightly varied.Comment: 24 pages, 12 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
Spectropolarimetric observations of an arch filament system with the GREGOR solar telescope
Arch filament systems occur in active sunspot groups, where a fibril
structure connects areas of opposite magnetic polarity, in contrast to active
region filaments that follow the polarity inversion line. We used the GREGOR
Infrared Spectrograph (GRIS) to obtain the full Stokes vector in the spectral
lines Si I 1082.7 nm, He I 1083.0 nm, and Ca I 1083.9 nm. We focus on the
near-infrared calcium line to investigate the photospheric magnetic field and
velocities, and use the line core intensities and velocities of the helium line
to study the chromospheric plasma. The individual fibrils of the arch filament
system connect the sunspot with patches of magnetic polarity opposite to that
of the spot. These patches do not necessarily coincide with pores, where the
magnetic field is strongest. Instead, areas are preferred not far from the
polarity inversion line. These areas exhibit photospheric downflows of moderate
velocity, but significantly higher downflows of up to 30 km/s in the
chromospheric helium line. Our findings can be explained with new emerging flux
where the matter flows downward along the fieldlines of rising flux tubes, in
agreement with earlier results.Comment: Proceedings 12th Potsdam Thinkshop to appear in Astronomische
Nachrichte
Diffusive spin transport
Information to be stored and transported requires physical carriers. The
quantum bit of information (qubit) can for instance be realised as the spin 1/2
degree of freedom of a massive particle like an electron or as the spin 1
polarisation of a massless photon. In this lecture, I first use irreducible
representations of the rotation group to characterise the spin dynamics in a
least redundant manner. Specifically, I describe the decoherence dynamics of an
arbitrary spin S coupled to a randomly fluctuating magnetic field in the
Liouville space formalism. Secondly, I discuss the diffusive dynamics of the
particle's position in space due to the presence of randomly placed impurities.
Combining these two dynamics yields a coherent, unified picture of diffusive
spin transport, as applicable to mesoscopic electronic devices or photons
propagating in cold atomic clouds.Comment: Lecture notes, published in A. Buchleitner, C. Viviescas, and M.
Tiersch (Eds.), "Entanglement and Decoherence. Foundations and Modern
Trends", Lecture Notes in Physics 768, Springer, Berlin (2009
Photospheric Magnetic Fields of the Trailing Sunspots in Active Region NOAA 12396
The solar magnetic field is responsible for all aspects of solar activity.
Sunspots are the main manifestation of the ensuing solar activity. Combining
high-resolution and synoptic observations has the ambition to provide a
comprehensive description of the sunspot growth and decay processes. Active
region NOAA 12396 emerged on 2015 August 3 and was observed three days later
with the 1.5-meter GREGOR solar telescope on 2015 August 6. High-resolution
spectropolarimetric data from the GREGOR Infrared Spectrograph (GRIS) are
obtained in the photospheric Si I 1082.7 nm and Ca I 1083.9
nm lines, together with the chromospheric He I 1083.0 nm triplet.
These near-infrared spectropolarimetric observations were complemented by
synoptic line-of-sight magnetograms and continuum images of the Helioseismic
and Magnetic Imager (HMI) and EUV images of the Atmospheric Imaging Assembly
(AIA) on board the Solar Dynamics Observatory (SDO).Comment: 4 pages, 2 figures, to be published in "Solar Polarization Workshop
8", ASP Proceedings, Luca Belluzzi (eds.
The distribution of Quiet Sun magnetic field strengths from 0 to 1800 G
The quiet Sun photospheric plasma has a variety of magnetic field strengths
going from zero to 1800 G. The empirical characterization of these field
strengths requires a probability density function (PDF), i.e., a function P(B)
describing the fraction of quiet Sun occupied by each field strength B. We show
how to combine magnetic field strength measurements based on the Zeeman effect
and the Hanle effect to estimate an unbiased P(B). The application of the
method to real observations renders a set of possible PDFs, which outline the
general characteristics of the quiet Sun magnetic fields. Their most probable
field strength differs from zero. The magnetic energy density is a significant
fraction of the kinetic energy of the granular motions at the base of the
photosphere (larger than 15% or larger than 2 10^{3} erg cm^{-3}). The unsigned
flux density (or mean magnetic field strength) has to be between 130 G and 190
G. A significant part of the unsigned flux (between 10% and 50%) and of the
magnetic energy (between 45% and 85%) are provided by the field strengths
larger than 500 G which, however, occupy only a small fraction of the surface
(between 1% and 10%). The fraction of kG fields in the quiet Sun is even
smaller, but they are important for a number of reasons. The kG fields still
trace a significant fraction of the total magnetic energy, they reach the high
photosphere, and they appear in unpolarized light images. The quiet Sun
photosphere has far more unsigned magnetic flux and magnetic energy than the
active regions and the network all together.Comment: To appear in ApJ. 14 pages, 9 figure
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