261 research outputs found
Hanle effect in the CN violet system with LTE modeling
Weak entangled magnetic fields with mixed polarity occupy the main part of
the quiet Sun. The Zeeman effect diagnostics fails to measure such fields
because of cancellation in circular polarization. However, the Hanle effect
diagnostics, accessible through the second solar spectrum, provides us with a
very sensitive tool for studying the distribution of weak magnetic fields on
the Sun. Molecular lines are very strong and even dominate in some regions of
the second solar spectrum. The CN system is
one of the richest and most promising systems for molecular diagnostics and
well suited for the application of the differential Hanle effect method. The
aim is to interpret observations of the CN
system using the Hanle effect and to obtain an estimation of the magnetic field
strength. We assume that the CN molecular layer is situated above the region
where the continuum radiation is formed and employ the single-scattering
approximation. Together with the Hanle effect theory this provides us with a
model that can diagnose turbulent magnetic fields. We have succeeded in fitting
modeled CN lines in several regions of the second solar spectrum to
observations and obtained a magnetic field strength in the range from 10--30 G
in the upper solar photosphere depending on the considered lines.Comment: Accepted for publication in Astronomy and Astrophysic
NLTE modeling of Stokes vector center-to-limb variations in the CN violet system
The solar surface magnetic field is connected with and even controls most of
the solar activity phenomena. Zeeman effect diagnostics allow for measuring
only a small fraction of the fractal-like structured magnetic field. The
remaining hidden magnetic fields can only be accessed with the Hanle effect.
Molecular lines are very convenient for applying the Hanle effect diagnostics
thanks to the broad range of magnetic sensitivities in a narrow spectral
region. With the UV version of the Zurich Imaging Polarimeter ZIMPOL II
installed at the 45 cm telescope of the Istituto Ricerche Solari Locarno
(IRSOL), we simultaneously observed intensity and linear polarization
center-to-limb variations in two spectral regions containing the (0,0) and
(1,1) bandheads of the CN B 2 {\Sigma} - X 2 {\Sigma} system. Here we present
an analysis of these observations. We have implemented coherent scattering in
molecular lines into a NLTE radiative transfer code. A two-step approach was
used. First, we separately solved the statistical equilibrium equations and
compute opacities and intensity while neglecting polariza- tion. Then we used
these quantities as input for calculating scattering polarization and the Hanle
effect. We have found that it is impossible to fit the intensity and
polarization simultaneously at different limb angles in the frame- work of
standard 1D modeling. The atmosphere models that provide correct intensity
center-to-limb variations fail to fit linear polar- ization center-to-limb
variations due to lacking radiation field anisotropy. We had to increase the
anisotropy by means of a specially introduced free parameter. This allows us to
successfully interpret our observations. We discuss possible reasons for
underestimating the anisotropy in the 1D modeling.Comment: 15 pages, 10 figures, accepted for publication in
Astronomy&Astrophysic
Abnormal Visuo-vestibular Interactions in Vestibular Migraine: a Cross Sectional Study
Vestibular migraine is amongst the commonest causes of episodic vertigo. Chronically, patients with vestibular migraine develop abnormal responsiveness to both vestibular and visual stimuli characterised by heightened self-motion sensitivity and visually-induced dizziness. Yet, the neural mechanisms mediating such symptoms remain unknown. We postulate that such symptoms are attributable to impaired visuo-vestibular cortical interactions, which in-turn disrupts normal vestibular function. To assess this, we investigated whether prolonged, full-field visual motion exposure, which has previously been shown to modulate visual cortical excitability in both healthy individuals and avestibular patients, could disrupt vestibular ocular reflex (VOR) and vestibular-perceptual thresholds of self-motion during rotations. Our findings reveal that vestibular migraine patients exhibited abnormally elevated reflexive and perceptual vestibular thresholds at baseline. Following visual motion exposure, both reflex and perceptual thresholds were significantly further increased in vestibular migraine patients relative to healthy controls, migraineurs without vestibular symptoms and patients with episodic vertigo due to a peripheral inner-ear disorder. Our results provide support for the notion of altered visuo-vestibular cortical interactions in vestibular migraine, as evidenced by vestibular threshold elevation following visual motion exposure
First polarimetric measurements and modeling of the Paschen-Back effect in CaH transitions
We report the first spectropolarimetric observations and modeling of CaH
transitions in sunspots. We have detected strong polarization signals in many
CaH lines from the A-X system, and we provide the first successful fit to the
observed Stokes profiles using the previously developed theory of the
Paschen-Back effect in arbitrary electronic states of diatomic molecules and
polarized radiative transfer in molecular lines in stellar atmospheres. We
analyze the CaH Stokes profiles together with quasi-simultaneous observations
in TiO bands and conclude that CaH provides a valuable diagnostic of magnetic
fields in sunspots, starspots, cool stars, and brown dwarfs
Measurement of the proton and deuteron structure functions, F2p and F2d, and of the ratio sigma(L)/sigma(T)
The muon-proton and muon-deuteron inclusive deep inelastic scattering cross
sections were measured in the kinematic range 0.002 < x < 0.60 and 0.5 < Q2 <
75 GeV2 at incident muon energies of 90, 120, 200 and 280 GeV. These results
are based on the full data set collected by the New Muon Collaboration,
including the data taken with a small angle trigger. The extracted values of
the structure functions F2p and F2d are in good agreement with those from other
experiments. The data cover a sufficient range of y to allow the determination
of the ratio of the longitudinally to transversely polarised virtual photon
absorption cross sections, R= sigma(L)/sigma(T), for 0.002 < x < 0.12 . The
values of R are compatible with a perturbative QCD prediction; they agree with
earlier measurements and extend to smaller x.Comment: In this replacement the erroneously quoted R values in tables 3-6 for
x>0.12, and R1990 values in tables 5-6 for all x, have been corrected, and
the cross sections in tables 3-4 have been adapted. Everything else,
including the structure functions F2, remained unchanged. 22 pages, LateX,
including figures, with two .sty files, and three separate f2tab.tex files
for the F2-tables. Accepted for publication in Nucl.Phys.B 199
Debye-Hueckel solution for steady electro-osmotic flow of a micropolar fluid in a cylindrical microcapillary
Analytic expressions for the speed, flux, microrotation, stress, and couple
stress in a micropolar fluid exhibiting steady, symmetric and one-dimensional
electro-osmotic flow in a uniform cylindrical microcapillary were derived under
the constraint of the Debye-Hueckel approximation, which is applicable when the
cross-sectional radius of the microcapillary exceeds the Debye length, provided
that the zeta potential is sufficiently small in magnitude. As the aciculate
particles in a micropolar fluid can rotate without translation, micropolarity
influences fluid speed, fluid flux, and one of the two non-zero components of
the stress tensor. The axial speed in a micropolar fluid intensifies as the
radius increases. The stress tensor is confined to the region near the wall of
the microcapillary but the couple stress tensor is uniform across the
cross-section.Comment: 19 page
Recurrent stroke risk and cerebral microbleed burden in ischemic stroke and TIA A meta-analysis
OBJECTIVE: To determine associations between cerebral microbleed (CMB) burden with recurrent
ischemic stroke (IS) and intracerebral hemorrhage (ICH) risk after IS or TIA.
METHODS: We identified prospective studies of patients with IS or TIA that investigated CMBs and
stroke (ICH and IS) risk during 5 CMBs)
and distribution. We calculated absolute event rates and pooled risk ratios (RR) using randomeffects
meta-analysis.
RESULTS: We included 5,068 patients from 15 studies. There were 115/1,284 (9.6%) recurrent IS
events in patients with CMBs vs 212/3,781 (5.6%) in patients without CMBs (pooled RR 1.8 for
CMBs vs no CMBs; 95% confidence interval [CI] 1.4–2.5). There were 49/1,142 (4.3%) ICH
events in those with CMBs vs 17/2,912 (0.58%) in those without CMBs (pooled RR 6.3 for CMBs
vs no CMBs; 95% CI 3.5–11.4). Increasing CMB burden increased the risk of IS (pooled RR [95%
CI] 1.8 [1.0–3.1], 2.4 [1.3–4.4], and 2.7 [1.5–4.9] for 1 CMB, 2–4 CMBs, and 5 CMBs, respectively).
CONCLUSIONS: CMBs are associated with increased stroke risk after IS or TIA. With increasing CMB
burden (compared to no CMBs), the risk of ICH increases more steeply than that of IS. However, IS
absolute event rates remain higher than ICH absolute event rates in all CMB burden categories
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