699 research outputs found
Distribution of Magnetic Shear Angle in an Emerging Flux Region
We study the distribution of magnetic shear in an emerging flux region using
the high-resolution Hinode/SOT SP observations. The distribution of mean
magnetic shear angle across the active region shows large values near region of
flux emergence i.e., in the middle of existing bipolar region and decreases
while approaching the periphery of the active region.Comment: 3 pages, Proceedings of IAU Symposium 273 "Physics of Sun and
Starspots" Eds. D.P. Choudhary and K.G. Strassmeie
Statistical analysis of the very quiet Sun magnetism
The behavior of the observed polarization amplitudes with spatial resolution
is a strong constraint on the nature and organization of solar magnetic fields
below the resolution limit. We study the polarization of the very quiet Sun at
different spatial resolutions using ground- and space-based observations. It is
shown that 80% of the observed polarization signals do not change with spatial
resolution, suggesting that, observationally, the very quiet Sun magnetism
remains the same despite the high spatial resolution of space-based
observations. Our analysis also reveals a cascade of spatial scales for the
magnetic field within the resolution element. It is manifest that the Zeeman
effect is sensitive to the microturbulent field usually associated to Hanle
diagnostics. This demonstrates that Zeeman and Hanle studies show complementary
perspectives of the same magnetism.Comment: Accepted for publication in Ap
Polarimetric Calibration of Large-Aperture Telescopes II: The sub-aperture method
A new method for absolute polarimetric calibration of large telescopes is
presented. The proposed method is highly accurate and is based on the
calibration of a small sub-aperture, which is then extended to the full system
by means of actual observations of an astronomical source. The calibration
procedure is described in detail along with numerical simulations that explore
its robustness and accuracy. The advantages and disadvantages of this technique
with respect to other possible alternatives are discussed.Comment: Journal of the Optical Society of America-A, submitte
Interaction between Granulation and Small-Scale Magnetic Flux Observed by Hinode
We study the relationship between granular development and magnetic field
evolution in the quiet Sun. 6 typical cases are displayed to exhibit
interaction between granules and magnetic elements, and we have obtained the
following results. (1) A granule develops centrosymmetrically when no magnetic
flux emerges within the granular cell. (2) A granule develops and splits
noncentrosymmetrically while flux emerges at an outer part of the granular
cell. (3) Magnetic flux emergence as a cluster of mixed polarities is detected
at the position of a granule as soon as the granule breaks up. (4) A dipole
emerges accompanying with the development of a granule, and the two elements of
the dipole root in the adjacent intergranular lanes and face each other across
the granule. Advected by the horizontal granular motion, the positive element
of the dipole then cancels with pre-existing negative flux. (5) Flux
cancellation also takes place between a positive element, which is advected by
granular flow, and its surrounding negative flux. (6) While magnetic flux
cancellation takes place at a granular cell, the granule shrinks and then
disappears. (7) Horizontal magnetic fields enhance at the places where dipoles
emerge and where opposite polarities cancel with each other, but only the
horizontal fields between the dipolar elements point orderly from the positive
element to the negative one. Our results reveal that granules and small-scale
magnetic flux influence each other. Granular flow advects magnetic flux, and
magnetic flux evolution suppresses granular development. There exist extremely
large Doppler blue-shifts at the site of one cancelling magnetic element. This
phenomenon may be caused by the upward flow produced by magnetic reconnection
below the photosphere.Comment: 8 figures, 13 pages. RAA, in pres
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
On the magnetic structure of the solar transition region
We examine the hypothesis that ``cool loops'' dominate emission from solar
transition region plasma below temperatures of K. We compare
published VAULT images of H L, a lower transition region line, with
near-contemporaneous magnetograms from Kitt Peak, obtained during the second
flight (VAULT-2) on 14 June 2002. The measured surface fields and potential
extrapolations suggest that there are too few short loops, and that L
emission is associated with the base regions of longer, coronal loops. VAULT-2
data of network boundaries have an asymmetry on scales larger than
supergranules, also indicating an association with long loops. We complement
the Kitt Peak data with very sensitive vector polarimetric data from the
Spectro-Polarimeter on board Hinode, to determine the influence of very small
magnetic concentrations on our analysis. From these data two classes of
behavior are found: within the cores of strong magnetic flux concentrations ( Mx) associated with active network and plage, small-scale mixed
fields are absent and any short loops can connect just the peripheries of the
flux to cell interiors. Core fields return to the surface via longer, most
likely coronal, loops. In weaker concentrations, short loops can connect
between concentrations and produce mixed fields within network boundaries as
suggested by Dowdy and colleagues. The VAULT-2 data which we examined are
associated with strong concentrations. We conclude that the cool loop model
applies only to a small fraction of the VAULT-2 emission, but we cannot
discount a significant role for cool loops in quieter regions. We suggest a
physical picture for how network L emission may occur through the
cross-field diffusion of neutral atoms from chromospheric into coronal plasma.Comment: Accepted by ApJ, 9 May 200
The properties of horizontal magnetic elements in quiet solar intranetwork
Using the data observed by the Solar Optical Telescope/Spectro-Polarimeter
aboard the Hinode satellite, the horizontal and vertical fields are derived
from the wavelength-integrated measures of Zeeman-induced linear and circular
polarizations. The quiet intranetwork regions are pervaded by horizontal
magnetic elements. We categorize the horizontal intranetwork magnetic elements
into two types: one is the non-isolated element which is accompanied by the
vertical magnetic elements during its evolution; another is the isolated
element which is not accompanied by the vertical magnetic elements. We identify
446 horizontal intranetwork magnetic elements, among them 87 elements are
isolated and 359 are non-isolated. Quantitative measurements reveal that the
isolated elements have relatively weaker horizontal magnetic fields, almost
equal size, and shorter lifetime comparing with the non-isolated elements. Most
non-isolated horizontal intranetwork magnetic elements are identified to
associate with the emergence of Omega-shaped flux loops. A few non-isolated
elements seem to indicate scenarios of submergence of Omega loops or emergence
of U-like loops. There is a positive correlation between the lifetime and the
size for both the isolated and non-isolated HIFs. It is also found that there
is also positive correlation between the lifetime and the magnetic flux density
for non-isolated HIFs, but no correlation for isolated HIFs. Even though the
horizontal elements show lower magnetic flux density, they could carry the
total magnetic flux in the order of magnitude close to 10^25 Mx to the solar
surface each day.Comment: 10 figures, 25 pages. ApJ, in pres
Analysis of Quiet-Sun Internetwork Magnetic Fields Based on Linear Polarization Signals
We present results from the analysis of Fe I 630 nm measurements of the quiet
Sun taken with the spectropolarimeter of the Hinode satellite. Two data sets
with noise levels of 1.2{\times}10-3 and 3{\times}10-4 are employed. We
determine the distribution of field strengths and inclinations by inverting the
two observations with a Milne-Eddington model atmosphere. The inversions show a
predominance of weak, highly inclined fields. By means of several tests we
conclude that these properties cannot be attributed to photon noise effects. To
obtain the most accurate results, we focus on the 27.4% of the pixels in the
second data set that have linear polarization amplitudes larger than 4.5 times
the noise level. The vector magnetic field derived for these pixels is very
precise because both circular and linear polarization signals are used
simultaneously. The inferred field strength, inclination, and filling factor
distributions agree with previous results, supporting the idea that
internetwork fields are weak and very inclined, at least in about one quarter
of the area occupied by the internetwork. These properties differ from those of
network fields. The average magnetic flux density and the mean field strength
derived from the 27.4% of the field of view with clear linear polarization
signals are 16.3 Mx cm-2 and 220 G, respectively. The ratio between the average
horizontal and vertical components of the field is approximately 3.1. The
internetwork fields do not follow an isotropic distribution of orientations.Comment: To appear in APJ, Vol 749, 201
On the inversion of Stokes profiles with local stray-light contamination
Obtaining the magnetic properties of non-resolved structures in the solar
photosphere is always challenging and problems arise because the inversion is
carried out through the numerical minimization of a merit function that depends
on the proposed model. We investigate the reliability of inversions in which
the stray-light contamination is obtained from the same observations as a local
average. In this case, we show that it is fundamental to include the covariance
between the observed Stokes profiles and the stray-light contamination. The
ensuing modified merit function of the inversion process penalizes large
stray-light contaminations simply because of the presence of positive
correlations between the observables and the stray-light, fundamentally
produced by spatially variable systematics. We caution that using the wrong
merit function, artificially large stray-light contaminations might be
inferred. Since this effect disappears if the stray-light contamination is
obtained as an average over the full field-of-view, we recommend to take into
account stray-light contamination using a global approach.Comment: 5 pages, 3 figures, accepted for publication in Ap
Polar Field Reversal Observations with Hinode
We have been monitoring yearly variation in the Sun's polar magnetic fields
with the Solar Optical Telescope aboard {\it Hinode} to record their evolution
and expected reversal near the solar maximum. All magnetic patches in the
magnetic flux maps are automatically identified to obtain the number density
and magnetic flux density as a function of th total magnetic flux per patch.
The detected magnetic flux per patch ranges over four orders of magnitude
( -- Mx). The higher end of the magnetic flux in the polar
regions is about one order of magnitude larger than that of the quiet Sun, and
nearly that of pores. Almost all large patches ( Mx) have the
same polarity, while smaller patches have a fair balance of both polarities.
The polarity of the polar region as a whole is consequently determined only by
the large magnetic concentrations. A clear decrease in the net flux of the
polar region is detected in the slow rising phase of the current solar cycle.
The decrease is more rapid in the north polar region than in the south. The
decrease in the net flux is caused by a decrease in the number and size of the
large flux concentrations as well as the appearance of patches with opposite
polarity at lower latitudes. In contrast, we do not see temporal change in the
magnetic flux associated with the smaller patches ( Mx) and that of
the horizontal magnetic fields during the years 2008--2012.Comment: 21 pages, 7 figures. Accepted for publication in Ap
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