257 research outputs found
Atomic alignment and Diagnostics of Magnetic Fields in Diffuse Media
We continue our studies of atomic alignment in diffuse media, in
particularly, in interstellar and circumstellar media, with the goal of
developing new diagnostics of magnetic fields in these environments. We
understand atomic alignment as alignment of atoms or ions in their ground
state. Such atoms are sensitive to weak magnetic fields. In particular, we
provide predictions of the polarization that arises from astrophysically
important aligned atoms (ions) with fine structure of the ground level, namely,
OI and SII and Ti II. Unlike our earlier papers which dealt with weak fields
only, a substantial part of our current paper is devoted to the studies of
atomic alignment when magnetic fields get strong enough to affect the emission
from the excited level, i.e. with the regime when the magnetic splitting is
comparable to the line-width. This is a regime of Hanle effect modified by the
atomic alignment. Using an example of emission and absorption lines of SII ion
we demonstrate how polarimetric studies can probe magnetic fields in
circumstellar regions and accretion disks. In addition, we show that atomic
alignment induced by anisotropic radiation can induce substantial variations of
magnetic dipole transitions within the ground state, thus affecting abundance
studies based on this emission. Moreover, the radio emission is polarized,
provides a new way to study magnetic fields, e.g. at the epoch of Universe
reionization.Comment: Minor changes, accepted to Ap
Advanced Forward Modeling and Inversion of Stokes Profiles Resulting from the Joint Action of the Hanle and Zeeman Effects
A big challenge in solar and stellar physics in the coming years will be to
decipher the magnetism of the solar outer atmosphere (chromosphere and corona)
along with its dynamic coupling with the magnetic fields of the underlying
photosphere. To this end, it is important to develop rigorous diagnostic tools
for the physical interpretation of spectropolarimetric observations in suitably
chosen spectral lines. Here we present a computer program for the synthesis and
inversion of Stokes profiles caused by the joint action of atomic level
polarization and the Hanle and Zeeman effects in some spectral lines of
diagnostic interest, such as those of the He I 10830 A and D_3 multiplets. It
is based on the quantum theory of spectral line polarization, which takes into
account all the relevant physical mechanisms and ingredients (optical pumping,
atomic level polarization, Zeeman, Paschen-Back and Hanle effects). The
influence of radiative transfer on the emergent spectral line radiation is
taken into account through a suitable slab model. The user can either calculate
the emergent intensity and polarization for any given magnetic field vector or
infer the dynamical and magnetic properties from the observed Stokes profiles
via an efficient inversion algorithm based on global optimization methods. The
reliability of the forward modeling and inversion code presented here is
demonstrated through several applications, which range from the inference of
the magnetic field vector in solar active regions to determining whether or not
it is canopy-like in quiet chromospheric regions. This user-friendly diagnostic
tool called "HAZEL" (from HAnle and ZEeman Light) is offered to the
astrophysical community, with the hope that it will facilitate new advances in
solar and stellar physics.Comment: 62 pages, 19 figures, 3 tables. Accepted for publication in Ap
Pharmaceutical innovation, reference pricing and therapeutic classes
This paper is a first attempt to model the effects of reference pricing on the
innovation effort of pharmaceutical firms. The model is based on a dynamic
game involving three types of agents: pharmaceutical firms, consumers and
a regulatory entity. The games includes research stages where the innovation
efforts by the firms are determined and introductory stages where a price
for a new medicament is fixed. We model the negotiation between the drug
owner and the regulator to fix the price, first without legal constraint, second
under the regime of reference pricing in therapeutic classes. We then solve
the innovation game where the firms anticipate the results of the negotiation
round on prices. We thus consider the effect of the therapeutic class regulation on both prices and the innovation pace. The final stage consists in calibrating the model with a small data on anti-statine in France and
simulates the effect of the change in regulatory regime
Magnetic Field Structures in a Facular Region Observed by THEMIS and Hinode
The main objective of this paper is to build and compare vector magnetic maps
obtained by two spectral polarimeters, i.e. THEMIS/MTR and Hinode SOT/SP, using
two inversion codes (UNNOFIT and MELANIE) based on the Milne-Eddington solar
atmosphere model. To this end, we used observations of a facular region within
active region NOAA 10996 on 23 May 2008, and found consistent results
concerning the field strength, azimuth and inclination distributions. Because
SOT/SP is free from the seeing effect and has better spatial resolution, we
were able to resolve small magnetic polarities with sizes of 1" to 2", and we
could detect strong horizontal magnetic fields, which converge or diverge in
negative or positive facular polarities. These findings support models which
suggest the existence of small vertical flux tube bundles in faculae. A new
method is proposed to get the relative formation heights of the multi-lines
observed by MTR assuming the validity of a flux tube model for the faculae. We
found that the Fe 1 6302.5 \AA line forms at a greater atmospheric height than
the Fe 1 5250.2 \AA line.Comment: 20 pages, 9 figures, 3 tables, accepted for publication in Solar
Physic
Scattering polarization of hydrogen lines in the presence of turbulent electric fields
We study the broadband polarization of hydrogen lines produced by scattering
of radiation, in the presence of isotropic electric fields. In this paper, we
focus on two distinct problems: a) the possibility of detecting the presence of
turbulent electric fields by polarimetric methods, and b) the influence of such
fields on the polarization due to a macroscopic, deterministic magnetic field.
We found that isotropic electric fields decrease the degree of linear
polarization in the scattered radiation, with respect to the zero-field case.
On the other hand, a distribution of isotropic electric fields superimposed
onto a deterministic magnetic field can generate a significant increase of the
degree of magnetic-induced, net circular polarization. This phenomenon has
important implications for the diagnostics of magnetic fields in plasmas using
hydrogen lines, because of the ubiquitous presence of the Holtsmark,
microscopic electric field from neighbouring ions. In particular, previous
solar magnetographic studies of the Balmer lines of hydrogen may need to be
revised because they neglected the effect of turbulent electric fields on the
polarization signals. In this work, we give explicit results for the
Lyman-alpha and Balmer-alpha lines.Comment: 15 pages, 6 figure
3D evolution of a filament disappearance event observed by STEREO
A filament disappearance event was observed on 22 May 2008 during our recent
campaign JOP 178. The filament, situated in the southern hemisphere, showed
sinistral chirality consistent with the hemispheric rule. The event was well
observed by several observatories in particular by THEMIS. One day before the
disappearance, H observations showed up and down flows in adjacent
locations along the filament, which suggest plasma motions along twisted flux
rope. THEMIS and GONG observations show shearing photospheric motions leading
to magnetic flux canceling around barbs. STEREO A, B spacecraft with separation
angle 52.4 degrees, showed quite different views of this untwisting flux rope
in He II 304 \AA\ images. Here, we reconstruct the 3D geometry of the filament
during its eruption phase using STEREO EUV He II 304 \AA\ images and find that
the filament was highly inclined to the solar normal. The He II 304 \AA\ movies
show individual threads, which oscillate and rise to an altitude of about 120
Mm with apparent velocities of about 100 km s, during the rapid
evolution phase. Finally, as the flux rope expands into the corona, the
filament disappears by becoming optically thin to undetectable levels. No CME
was detected by STEREO, only a faint CME was recorded by LASCO at the beginning
of the disappearance phase at 02:00 UT, which could be due to partial filament
eruption. Further, STEREO Fe XII 195 \AA\ images showed bright loops beneath
the filament prior to the disappearance phase, suggesting magnetic reconnection
below the flux rope
The Hanle Effect in 1D, 2D and 3D
This paper addresses the problem of scattering line polarization and the
Hanle effect in one-dimensional (1D), two-dimensional (2D) and
three-dimensional (3D) media for the case of a two-level model atom without
lower-level polarization and assuming complete frequency redistribution. The
theoretical framework chosen for its formulation is the QED theory of Landi
Degl'Innocenti (1983), which specifies the excitation state of the atoms in
terms of the irreducible tensor components of the atomic density matrix. The
self-consistent values of these density-matrix elements is to be determined by
solving jointly the kinetic and radiative transfer equations for the Stokes
parameters. We show how to achieve this by generalizing to Non-LTE polarization
transfer the Jacobi-based ALI method of Olson et al. (1986) and the iterative
schemes based on Gauss-Seidel iteration of Trujillo Bueno and Fabiani Bendicho
(1995). These methods essentially maintain the simplicity of the
Lambda-iteration method, but their convergence rate is extremely high. Finally,
some 1D and 2D model calculations are presented that illustrate the effect of
horizontal atmospheric inhomogeneities on magnetic and non-magnetic resonance
line polarization signals.Comment: 14 pages and 5 figure
Polarization from aligned atoms as a diagnostics of circumstellar, AGN and interstellar magnetic fields: II. Atoms with Hyperfine Structure
We show that atomic alignment presents a reliable way to study topology of
astrophysical magnetic fields. The effect of atomic alignment arises from
modulation of the relative population of the sublevels of atomic ground state
pumped by anisotropic radiation flux. As such aligned atoms precess in the
external magnetic field and this affects the properties of the polarized
radiation arising from both scattering and absorption by the atoms. As the
result the polarizations of emission and absorption lines depend on the 3D
geometry of the magnetic field as well as the direction and anisotropy of
incident radiation. We consider a subset of astrophysically important atoms
with hyperfine structure. For emission lines we obtain the dependencies of the
direction of linear polarization on the directions of magnetic field and the
incident pumping radiation. For absorption lines we establish when the
polarization is perpendicular and parallel to magnetic field. For both emission
and absorption lines we find the dependence on the degree of polarization on
the 3D geometry of magnetic field. We claim that atomic alignment provides a
unique tool to study magnetic fields in circumstellar regions, AGN,
interplanetary and interstellar medium. This tool allows studying of 3D
topology of magnetic fields and establish other important astrophysical
parameters. We consider polarization arising from both atoms in the steady
state and also as they undergo individual scattering of photons. We exemplify
the utility of atomic alignment for studies of astrophysical magnetic fields by
considering a case of Na alignment in a comet wake.Comment: 23 pages, 20 figures, ApJ, in press, minor change
Tangled Magnetic Fields in Solar Prominences
Solar prominences are an important tool for studying the structure and
evolution of the coronal magnetic field. Here we consider so-called "hedgerow"
prominences, which consist of thin vertical threads. We explore the possibility
that such prominences are supported by tangled magnetic fields. A variety of
different approaches are used. First, the dynamics of plasma within a tangled
field is considered. We find that the contorted shape of the flux tubes
significantly reduces the flow velocity compared to the supersonic free fall
that would occur in a straight vertical tube. Second, linear force-free models
of tangled fields are developed, and the elastic response of such fields to
gravitational forces is considered. We demonstrate that the prominence plasma
can be supported by the magnetic pressure of a tangled field that pervades not
only the observed dense threads but also their local surroundings. Tangled
fields with field strengths of about 10 G are able to support prominence
threads with observed hydrogen density of the order of 10^(11) cm^(-3).
Finally, we suggest that the observed vertical threads are the result of
Rayleigh-Taylor instability. Simulations of the density distribution within a
prominence thread indicate that the peak density is much larger than the
average density. We conclude that tangled fields provide a viable mechanism for
magnetic support of hedgerow prominences.Comment: 14 pages (emulateapj style), 10 figures, ApJ, in pres
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