251 research outputs found
Multiple scattering of light in cold atomic clouds with a magnetic field
Starting from a microscopic theory for atomic scatterers, we describe the
scattering of light by a single atom and study the coherent propagation of
light in a cold atomic cloud in the presence of a magnetic field B in the
mesoscopic regime. Non-pertubative expressions in B are given for the
magneto-optical effects and optical anisotropy. We then consider the multiple
scattering regime and address the fate of the coherent backscattering (CBS)
effect. We show that, for atoms with nonzero spin in their ground state, the
CBS interference contrast can be increased compared to its value when B=0, a
result at variance with classical samples. We validate our theoretical results
by a quantitative comparison with experimental data.Comment: 16 pages, 7 figure
Resolving the internal magnetic structure of the solar network
We analyze the spectral asymmetry of Stokes V (circularly polarized) profiles
of an individual network patch in the quiet Sun observed by Sunrise/IMaX. At a
spatial resolution of 0.15"-0.18", the network elements contain substructure
which is revealed by the spatial distribution of Stokes V asymmetries. The area
asymmetry between the red and blue lobes of Stokes V increases from nearly zero
at the core of the structure to values close to unity at its edges (one-lobed
profiles). Such a distribution of the area asymmetry is consistent with
magnetic fields expanding with height, i.e., an expanding magnetic canopy
(which is required to fulfill pressure balance and flux conservation in the
solar atmosphere). Inversion of the Stokes I and V profiles of the patch
confirms this picture, revealing a decreasing field strength and increasing
height of the canopy base from the core to the periphery of the network patch.
However, the non-roundish shape of the structure and the presence of negative
area and amplitude asymmetries reveal that the scenario is more complex than a
canonical flux tube expanding with height surrounded by downflows.Comment: accepted for publication in ApJ Letter
Magnetic Field Enhanced Coherence Length in Cold Atomic Gases
We study the effect of an external magnetic field on coherent backscattering
of light from a cool rubidium vapor. We observe that the backscattering
enhancement factor can be {\it increased} with . This surprising behavior
shows that the coherence length of the system can be increased by applying a
magnetic field, in sharp contrast with ususal situations. This is mainly due to
the lifting of the degeneracy between Zeeman sublevels. We find good agreement
between our experimental data and a full Monte-Carlosimulation, taking into
account the magneto-optical effects and the geometry of the atomic cloud
Photonic Hall effect in cold atomic clouds
On the basis of exact numerical simulations and analytical calculations, we
describe qualitatively and quantitatively the interference processes at the
origin of the photonic Hall effect for resonant Rayleigh (point-dipole)
scatterers in a magnetic field. For resonant incoming light, the induced giant
magneto-optical effects result in relative Hall currents in the percent range,
three orders of magnitude larger than with classical scatterers. This suggests
that the observation of the photonic Hall effect in cold atomic vapors is
within experimental reach.Comment: 4 pages 4 figure
Dynamic Shear Rheometer Testing of Asphalt Binder using 4 mm parallel plate geometry at low temperature: 2nd Interlaboratory Study including test results of 16 participating laboratories from Austria, Belgium, Czech Republic, France, Germany, Italy, Lithuania, Poland, and Sweden ; Final Report
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
Are there optical differences between storm-time substorms and isolated substorms?
We have performed an extensive analysis of auroral optical events
(substorms) that occurred during the development of the main phase of
magnetic storms. Using images from the Earth Camera on the Polar spacecraft
(Frank et al., 1995), we compared the optical emission features of substorms
occurring during 16 expansion phases of magnetic storms with the features of
isolated substorms occurring during non-storm times. The comparison used two
techniques, visual inspection and statistical comparisons. The comparisons
were based on the common characteristics seen in isolated substorms that
were initially identified by Akasofu (1964) and quantified by Gjerloev et
al. (2008). We find that when auroral activity does occur during main phase
development the characteristics of the aurora are very dissimilar to those
of the classical isolated substorm. The primary differences include the lack
of a surge/bulge, lack of bifurcation of the aurora, much shorter expansion
phases, and greater intensities.
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Since a surge/bulge and bifurcation of the aurora are characteristics of the
existence of a substorm current wedge, a key component of the
magnetosphere-ionosphere current system during substorms, the lack of this
component would indicate that the classical substorm model does not apply to
the storm time magnetosphere-ionosphere current system. Rather several of
the analyses suggest that the storm-time substorms are associated more
closely with the auroral oval, at least spatially, and, therefore, probably
with the plasma sheet dynamics during the main phase development. These
results then must call into question the widely held assumption that there
is no intrinsic difference between storm-time substorms and classical
isolated substorms
On the Doppler Shift and Asymmetry of Stokes Profiles of Photospheric FeI and Chromospheric MgI Lines
We analyzed the full Stokes spectra using simultaneous measurements of the
photospheric (FeI 630.15 and 630.25 nm) and chromospheric (MgI b2 517.27 nm)
lines. The data were obtained with the HAO/NSO Advanced Stokes Polarimeter,
about a near disc center sunspot region, NOAA AR 9661. We compare the
characteristics of Stokes profiles in terms of Doppler shifts and asymmetries
among the three spectral lines, which helps us to better understand the
chromospheric lines and the magnetic and flow fields in different magnetic
regions. The main results are: (1) For penumbral area observed by the
photospheric FeI lines, Doppler velocities derived from Stokes I (Vi) are very
close to those derived from linear polarization profiles (Vlp) but
significantly different from those derived from Stokes V profiles (Vzc), which
provides direct and strong evidence that the penumbral Evershed flows are
magnetized and mainly carried by the horizontal magnetic component. (2) The
rudimentary inverse Evershed effect observed by the MgI b2 line provides a
qualitative evidence on its formation height that is around or just above the
temperature minimum region. (3) Vzc and Vlp in penumbrae and Vzc in pores
generally approach their Vi observed by the chromospheric MgI line, which is
not the case for the photospheric FeI lines. (4) Outer penumbrae and pores show
similar behavior of the Stokes V asymmetries that tend to change from positive
values in the photosphere (FeI lines) to negative values in the low
chromosphere (MgI line). (5) The Stokes V profiles in plage regions are highly
asymmetric in the photosphere and more symmetric in the low chromosphere. (6)
Strong red shifts and large asymmetries are found around the magnetic polarity
inversion line within the common penumbra of the Delta spot. This study thus
emphasizes the importance of spectro-polarimetry using chromospheric lines.Comment: 10 pages, 7 figures, accepted to The Astrophysical Journa
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