197 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
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
Milne-Eddington inversion of the Fe I line pair at 630~nm
The iron lines at 630.15 and 630.25 nm are often used to determine the
physical conditions of the solar photosphere. A common approach is to invert
them simultaneously under the Milne-Eddington approximation. The same
thermodynamic parameters are employed for the two lines, except for their
opacities, which are assumed to have a constant ratio. We aim at investigating
the validity of this assumption, since the two lines are not exactly the same.
We use magnetohydrodynamic simulations of the quiet Sun to examine the behavior
of the ME thermodynamic parameters and their influence on the retrieval of
vector magnetic fields and flow velocities. Our analysis shows that the two
lines can be coupled and inverted simultaneously using the same thermodynamic
parameters and a constant opacity ratio. The inversion of two lines is
significantly more accurate than single-line inversions because of the larger
number of observables.Comment: Accepted for publication in Astronomy and Astrophysics (Research
Note
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
Two-dimensional solar spectropolarimetry with the KIS/IAA Visible Imaging Polarimeter
Spectropolarimetry at high spatial and spectral resolution is a basic tool to
characterize the magnetic properties of the solar atmosphere. We introduce the
KIS/IAA Visible Imaging Polarimeter (VIP), a new post-focus instrument that
upgrades the TESOS spectrometer at the German VTT into a full vector
polarimeter. VIP is a collaboration between the KIS and the IAA. We describe
the optical setup of VIP, the data acquisition procedure, and the calibration
of the spectropolarimetric measurements. We show examples of data taken between
2005 and 2008 to illustrate the potential of the instrument. VIP is capable of
measuring the four Stokes profiles of spectral lines in the range from 420 to
700 nm with a spatial resolution better than 0.5". Lines can be sampled at 40
wavelength positions in 60 s, achieving a noise level of about 2 x 10E-3 with
exposure times of 300 ms and pixel sizes of 0.17" x 0.17" (2 x 2 binning). The
polarization modulation is stable over periods of a few days, ensuring high
polarimetric accuracy. The excellent spectral resolution of TESOS allows the
use of sophisticated data analysis techniques such as Stokes inversions. One of
the first scientific results of VIP presented here is that the ribbon-like
magnetic structures of the network are associated with a distinct pattern of
net circular polarization away from disk center. VIP performs
spectropolarimetric measurements of solar magnetic fields at a spatial
resolution that is only slightly worse than that of the Hinode
spectropolarimeter, while providing a 2D field field of view and the
possibility to observe up to four spectral regions sequentially with high
cadence. VIP can be used as a stand-alone instrument or in combination with
other spectropolarimeters and imaging systems of the VTT for extended
wavelength coverage.Comment: 10 pages, 8 figures, accepted by Astronomy and Astrophysics v2:
figures updated with improved qualit
Classical Evolution of Quantum Elliptic States
The hydrogen atom in weak external fields is a very accurate model for the
multiphoton excitation of ultrastable high angular momentum Rydberg states, a
process which classical mechanics describes with astonishing precision. In this
paper we show that the simplest treatment of the intramanifold dynamics of a
hydrogenic electron in external fields is based on the elliptic states of the
hydrogen atom, i.e., the coherent states of SO(4), which is the dynamical
symmetry group of the Kepler problem. Moreover, we also show that classical
perturbation theory yields the {\it exact} evolution in time of these quantum
states, and so we explain the surprising match between purely classical
perturbative calculations and experiments. Finally, as a first application, we
propose a fast method for the excitation of circular states; these are
ultrastable hydrogenic eigenstates which have maximum total angular momentum
and also maximum projection of the angular momentum along a fixed direction. %Comment: 8 Pages, 2 Figures. Accepted for publication in Phys. Rev.
Estimation of solar prominence magnetic fields based on the reconstructed 3D trajectories of prominence knots
We present an estimation of the lower limits of local magnetic fields in
quiescent, activated, and active (surges) promineces, based on reconstructed
3-dimensional (3D) trajectories of individual prominence knots. The 3D
trajectories, velocities, tangential and centripetal accelerations of the knots
were reconstructed using observational data collected with a single
ground-based telescope equipped with a Multi-channel Subtractive Double Pass
imaging spectrograph. Lower limits of magnetic fields channeling observed
plasma flows were estimated under assumption of the equipartition principle.
Assuming approximate electron densities of the plasma n_e = 5*10^{11} cm^{-3}
in surges and n_e = 5*10^{10} cm^{-3} in quiescent/activated prominences, we
found that the magnetic fields channeling two observed surges range from 16 to
40 Gauss, while in quiescent and activated prominences they were less than 10
Gauss. Our results are consistent with previous detections of weak local
magnetic fields in the solar prominences.Comment: 14 pages, 12 figures, 1 tabl
An active region filament studied simultaneously in the chromosphere and photosphere: I - Magnetic structure
A thorough multiwavelength, multiheight study of the vector magnetic field in
a compact active region (AR) filament (NOAA10781) is presented. We suggest an
evolutionary scenario for this filament. Full Stokes vectors were acquired with
TIP-II in a spectral range which comprises the chromospheric He I 10830 A
multiplet and the photospheric Si I 10827 A line. An AR filament (that was
formed before our observing run) was detected in the He I absorption images on
2005 July 3rd. The chromospheric vector magnetic field in this portion of the
filament was strongly sheared whereas the photospheric field lines underneath
had an inverse polarity configuration. From July 3rd to July 5th, an opening
and closing of the polarities at either side of the polarity inversion line
(PIL) was recorded, resembling the recently discovered process of the sliding
door effect seen by Hinode. During this time, a newly created region that
contained pores and orphan penumbrae at the PIL was observed.On July 5th, a
normal polarity configuration was inferred from the chromospheric spectra,
while strongly sheared field lines aligned with the PIL were found in the
photosphere. In this same data set, the spine of the filament is also observed
in a different portion of the FOV and is clearly mapped by the Silicon line
core. The inferred vector magnetic fields of the filament suggest a flux rope
topology. Furthermore, the observations indicate that the filament is divided
in two parts, one which lies in the chromosphere and another one that stays
trapped in the photosphere. Therefore, only the top of the helical structure is
seen by the Helium lines. The pores and orphan penumbrae at the PIL appear to
be the photospheric counterpart of the extremely low-lying filament. We suggest
that orphan penumbrae are formed in very narrow PILs of compact ARs and are the
photospheric manifestation of flux ropes in the photosphere.Comment: Accepted for publication in Astronomy & Astrophysics, 16 pages, 13
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