230 research outputs found
Structure and dynamics of penumbral filaments
High-resolution observations of sunspots have revealed the existence of dark
cores inside the bright filaments of the penumbra. Here we present the
stationary solution of the heat transfer equation in a stratified penumbra
consisting of nearly horizontal magnetic flux tubes embedded in a stronger and
more vertical field. The tubes and the external medium are in horizontal
mechanical equilibrium. This model produces bright filaments with dark cores as
a consequence of the higher density of the plasma inside the flux tube, which
shifts the surface of optical depth unity toward higher (cooler) layers. Our
results suggest that the surplus brightness of the penumbra is a natural
consequence of the Evershed flow, and that magnetic flux tubes about 250 km in
diameter can explain the morphology of sunspot penumbra.Comment: 6 pages, 4 figures, conference proceedings: SEA, 2008, Santander,
Spai
Fluxtube model atmospheres and Stokes V zero-crossing wavelengths
First results of the inversion of Stokes I and V profiles from plage regions
near disk center are presented. Both low and high spatial resolution spectra of
FeI 6301.5 and FeI 6302.5 A obtained with the Advanced Stokes Polarimeter (ASP)
have been considered for analysis. The thin flux tube approximation,
implemented in an LTE inversion code based on response functions, is used to
describe unresolved magnetic elements. The code allows the simultaneous and
consistent inference of all atmospheric quantities determining the radiative
transfer with the sole assumption of hydrostatic equilibrium. By considering
velocity gradients within the tubes we are able to match the full ASP Stokes
profiles. The magnetic atmospheres derived from the inversion are characterized
by the absence of significant motions in high layers and strong velocity
gradients in deeper layers. These are essential to reproduce the asymmetries of
the observed profiles. Our scenario predicts a shift of the Stokes V
zero-crossing wavelengths which is indeed present in observations made with the
Fourier Transform Spectrometer.Comment: To appear in ApJ Letters (1997) (in press
Photospheric structure of an extended penumbra
The photospheric structure of an extended penumbra belonging to a complex spot observed near disk center is investigated by means of the inversion of the full Stokes vector of two Fe I lines at 1.56 μm. An attempt is made to classify the observed Stokes V profiles in terms of their shapes. It turns out that about 28% of the profiles have abnormal shapes indicative of two different magnetic-field
components within the resolution element. The spatial distribution of abnormal Stokes V profiles is studied. It is found that such profiles are evenly distributed in the penumbra, without any particular tendency to concentrate near the so-called neutral line. Anomalous profiles are not only seen in the outer penumbra and beyond, but also in the middle part of it. A Milne-Eddington–like inversion is
carried out first, revealing a smooth picture of the spatial distribution of magneticfield vector and velocities along the line of sight. In particular, dark spines with
stronger and more vertical magnetic fields are seen to coexist with nearly horizontal magnetic fields throughout the penumbra. A full inversion allowing for gradients of
the atmospheric parameters along the line of sight indicates the existence of cool magnetic tubes returning back to the solar surface (inclination angles greater than
90◦) and carrying the largest material flows
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
Spectral Inversion of Multi-Line Full-Disk Observations of Quiet Sun Magnetic Fields
Spectral inversion codes are powerful tools to analyze spectropolarimetric
observations, and they provide important diagnostics of solar magnetic fields.
Inversion codes differ by numerical procedures, approximations of the
atmospheric model, and description of radiative transfer. Stokes Inversion
based on Response functions (SIR) is an implementation widely used by the solar
physics community. It allows to work with different atmospheric components,
where gradients of different physical parameters are possible, e.g., magnetic
field strength and velocities. The spectropolarimetric full-disk observations
were carried out with the Stokesmeter of the Solar Telescope for Operative
Predictions (STOP) at the Sayan Observatory on 3 February 2009, when neither an
active region nor any other extended flux concentration was present on the Sun.
In this study of quiet Sun magnetic fields, we apply the SIR code
simultaneously to 15 spectral lines. A tendency is found that weaker magnetic
field strengths occur closer to the limb. We explain this finding by the fact
that close to the limb, we are more sensitive to higher altitudes in an
expanding flux tube, where the field strength should be smaller since the
magnetic flux is conserved with height. Typically, the inversions deliver two
populations of magnetic elements: (1) high magnetic field strengths (1500-2000
G) and high temperatures (5500-6500 K) and (2) weak magnetic fields (50-150 G)
and low temperatures (5000-5300 K).Comment: 10 pages, 6 figures, accepted for Solar Physic
Wave propagation and shock formation in different magnetic structures
Velocity oscillations "measured" simultaneously at the photosphere and the
chromosphere -from time series of spectropolarimetric data in the 10830 A
region- of different solar magnetic features allow us to study the properties
of wave propagation as a function of the magnetic flux of the structure (i.e.
two different-sized sunspots, a tiny pore and a facular region). While
photospheric oscillations have similar characteristics everywhere, oscillations
measured at chromospheric heights show different amplitudes, frequencies and
stages of shock development depending on the observed magnetic feature. The
analysis of the power and the phase spectra, together with simple theoretical
modeling, lead to a series of results concerning wave propagation within the
range of heights of this study. We find that, while the atmospheric cut-off
frequency and the propagation properties of the different oscillating modes
depend on the magnetic feature, in all the cases the power that reaches the
high chromosphere above the atmospheric cut-off comes directly from the
photosphere by means of linear vertical wave propagation rather than from
non-linear interaction of modes.Comment: Accepted for publication in The Astrophysical Journal. 29 pages, 9
figures, 12pt, preprin
A study of the capabilities for inferring atmospheric information from high-spatial-resolution simulations
In this work, we study the accuracy that can be achieved when inferring the
atmospheric information from realistic numerical magneto-hydrodynamic
simulations that reproduce the spatial resolution we will obtain with future
observations made by the 4m class telescopes DKIST and EST. We first study
multiple inversion configurations using the SIR code and the Fe I transitions
at 630 nm until we obtain minor differences between the input and the inferred
atmosphere in a wide range of heights. Also, we examine how the inversion
accuracy depends on the noise level of the Stokes profiles. The results
indicate that when the majority of the inverted pixels come from strongly
magnetised areas, there are almost no restrictions in terms of the noise,
obtaining good results for noise amplitudes up to 1 of . At
the same time, the situation is different for observations where the dominant
magnetic structures are weak, and noise restraints are more demanding.
Moreover, we find that the accuracy of the fits is almost the same as that
obtained without noise when the noise levels are on the order of
1of . We, therefore, advise aiming for noise values on the
order of or lower than 5 of if observers seek reliable
interpretations of the results for the magnetic field vector reliably. We
expect those noise levels to be achievable by next-generation 4m class
telescopes thanks to an optimised polarisation calibration and the large
collecting area of the primary mirror.Comment: 14 pages, 13 figure
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
VFISV: Very Fast Inversion of the Stokes Vector for the Helioseismic and Magnetic Imager
In this paper we describe in detail the implementation and main properties of
a new inversion code for the polarized radiative transfer equation (VFISV: Very
Fast inversion of the Stokes vector). VFISV will routinely analyze pipeline
data from the Helioseismic and Magnetic Imager (HMI) on-board of the Solar
Dynamics Observatory (SDO). It will provide full-disk maps (40964096
pixels) of the magnetic field vector on the Solar Photosphere every 10 minutes.
For this reason VFISV is optimized to achieve an inversion speed that will
allow it to invert 16 million pixels every 10 minutes with a modest number
(approx. 50) of CPUs. Here we focus on describing a number of important
details, simplifications and tweaks that have allowed us to significantly speed
up the inversion process. We also give details on tests performed with data
from the spectropolarimeter on-board of the Hinode spacecraft.Comment: 23 pages, 9 figures (2 color). Submitted for publication to Solar
Physic
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