7,210 research outputs found
Scattering Polarization in the Presence of Magnetic and Electric Fields
The polarization of radiation by scattering on an atom embedded in combined
external quadrupole electric and uniform magnetic fields is studied
theoretically. Limiting cases of scattering under Zeeman effect and Hanle
effect in weak magnetic fields are discussed. The theory is general enough to
handle scattering in intermediate magnetic fields (Hanle-Zeeman effect) and for
arbitrary orientation of magnetic field. The quadrupolar electric field
produces asymmetric line shifts and causes interesting level-crossing phenomena
either in the absence of an ambient magnetic field or in its presence. It is
shown that the quadrupolar electric field produces an additional depolarization
in the profiles and rotation of the plane of polarization in the
profile over and above that arising from magnetic field itself. This
characteristic may have a diagnostic potential to detect steady state and time
varying electric fields that surround radiating atoms in Solar atmospheric
layers.Comment: 41 pages, 6 figure
Quiet Sun Magnetic Field Measurements Based on Lines with Hyperfine Structure
The Zeeman pattern of MnI lines is sensitive to hyperfine structure (HFS)
and, they respond to hG magnetic field strengths differently from the lines
used in solar magnetometry. This peculiarity has been employed to measure
magnetic field strengths in quiet Sun regions. However, the methods applied so
far assume the magnetic field to be constant in the resolution element. The
assumption is clearly insufficient to describe the complex quiet Sun magnetic
fields, biasing the results of the measurements. We present the first syntheses
of MnI lines in realistic quiet Sun model atmospheres. The syntheses show how
the MnI lines weaken with increasing field strength. In particular, kG magnetic
concentrations produce NnI 5538 circular polarization signals (Stokes V) which
can be up to two orders of magnitude smaller than the weak magnetic field
approximation prediction. Consequently, (1) the polarization emerging from an
atmosphere having weak and strong fields is biased towards the weak fields, and
(2) HFS features characteristic of weak fields show up even when the magnetic
flux and energy are dominated by kG fields. For the HFS feature of MnI 5538 to
disappear the filling factor of kG fields has to be larger than the filling
factor of sub-kG fields. Stokes V depends on magnetic field inclination
according to the simple consine law. Atmospheres with unresolved velocities
produce asymmetric line profiles, which cannot be reproduced by simple
one-component model atmospheres. The uncertainty of the HFS constants do not
limit the use of MnI lines for magnetometry.Comment: Accepted for publication in ApJ. 10 pages, 14 figure
Multimodal Differential Emission Measure in the Solar Corona
The Atmospheric Imaging Assembly (AIA) telescope on board the Solar Dynamics
Observatory (SDO) provides coronal EUV imaging over a broader temperature
sensitivity range than the previous generations of instruments (EUVI, EIT, and
TRACE). Differential emission measure tomography (DEMT) of the solar corona
based on AIA data is presented here for the first time. The main product of
DEMT is the three-dimensional (3D) distribution of the local differential
emission measure (LDEM). While in previous studies, based on EIT or EUVI data,
there were 3 available EUV bands, with a sensitivity range
MK, the present study is based on the 4 cooler AIA bands (aimed at studying the
quiet sun), sensitive to the range MK. The AIA filters allow
exploration of new parametric LDEM models. Since DEMT is better suited for
lower activity periods, we use data from Carrington Rotation 2099, when the Sun
was in its most quiescent state during the AIA mission. Also, we validate the
parametric LDEM inversion technique by applying it to standard bi-dimensional
(2D) differential emission measure (DEM) analysis on sets of simultaneous AIA
images, and comparing the results with DEM curves obtained using other methods.
Our study reveals a ubiquitous bimodal LDEM distribution in the quiet diffuse
corona, which is stronger for denser regions. We argue that the nanoflare
heating scenario is less likely to explain these results, and that alternative
mechanisms, such as wave dissipation appear better supported by our results.Comment: 52 pages, 18 figure
Using HINODE/Extreme-Ultraviolet Imaging Spectrometer to confirm a seismologically inferred coronal temperature
The Extreme-Ultraviolet Imaging Spectrometer on board the HINODE satellite is used to examine the loop system described in Marsh et al. (2009) by applying spectroscopic diagnostic methods. A simple isothermal mapping algorithm is applied to determine where the assumption of isothermal plasma may be valid, and the emission measure locii technique is used to determine the temperature profile along the base of the loop system. It is found that, along the base, the loop has a uniform temperature profile with a mean temperature of 0.89 +- 0.09 MK which is in agreement with the temperature determined seismologically in Marsh et al. (2009), using observations interpreted as the slow magnetoacoustic mode. The results further strengthen the slow mode interpretation, propagation at a uniform sound speed, and the analysis method applied in Marsh et al. (2009). It is found that it is not possible to discriminate between the slow mode phase speed and the sound speed within the precision of the present observations
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