500 research outputs found
Evershed clouds as precursors of moving magnetic features around sunspots
The relation between the Evershed flow and moving magnetic features (MMFs) is
studied using high-cadence, simultaneous spectropolarimetric measurements of a
sunspot in visible (630.2 nm) and near-infrared (1565 nm) lines. Doppler
velocities, magnetograms, and total linear polarization maps are calculated
from the observed Stokes profiles. We follow the temporal evolution of two
Evershed clouds that move radially outward along the same penumbral filament.
Eventually, the clouds cross the visible border of the spot and enter the moat
region, where they become MMFs. The flux patch farther from the sunspot has the
same polarity of the spot, while the MMF closer to it has opposite polarity and
exhibits abnormal circular polarization profiles. Our results provide strong
evidence that at least some MMFs are the continuation of the penumbral Evershed
flow into the moat. This, in turn, suggests that MMFs are magnetically
connected to sunspots.Comment: To appear in ApJ Letters, Vol 649, 2006 September 20 issu
Quiet Sun magnetic fields from space-borne observations: simulating Hinode's case
We examine whether or not it is possible to derive the field strength
distribution of quiet Sun internetwork regions from very high spatial
resolution polarimetric observations in the visible. In particular, we consider
the case of the spectropolarimeter attached to the Solar Optical Telescope
aboard Hinode. Radiative magneto-convection simulations are used to synthesize
the four Stokes profiles of the \ion{Fe}{1} 630.2 nm lines. Once the profiles
are degraded to a spatial resolution of 0\farcs32 and added noise, we infer the
atmospheric parameters by means of Milne-Eddington inversions. The comparison
of the derived values with the real ones indicates that the visible lines yield
correct internetwork field strengths and magnetic fluxes, with uncertainties
smaller than 150 G, when a stray light contamination factor is included
in the inversion. Contrary to the results of ground-based observations at
1\arcsec, weak fields are retrieved wherever the field is weak in the
simulation.Comment: Accepted for publication in ApJ Letter
The formation and disintegration of magnetic bright points observed by Sunrise/IMaX
The evolution of the physical parameters of magnetic bright points (MBPs)
located in the quiet Sun (mainly in the interwork) during their lifetime is
studied. First we concentrate on the detailed description of the magnetic field
evolution of three MBPs. This reveals that individual features follow
different, generally complex, and rather dynamic scenarios of evolution. Next
we apply statistical methods on roughly 200 observed MBP evolutionary tracks.
MBPs are found to be formed by the strengthening of an equipartition field
patch, which initially exhibits a moderate downflow. During the evolution,
strong downdrafts with an average velocity of 2.4 km/s set in. These flows,
taken together with the concurrent strengthening of the field, suggest that we
are witnessing the occurrence of convective collapses in these features,
although only 30% of them reach kG field strengths. This fraction might turn
out to be larger when the new 4 m class solar telescopes are operational as
observations of MBPs with current state of the art instrumentation could still
be suffering from resolution limitations. Finally, when the bright point
disappears (although the magnetic field often continues to exist) the magnetic
field strength has dropped to the equipartition level and is generally somewhat
weaker than at the beginning of the MBP's evolution. Noteworthy is that in
about 10% of the cases we observe in the vicinity of the downflows small-scale
strong (exceeding 2 km/s) intergranular upflows related spatially and
temporally to these downflows.Comment: 19 pages, 13 figures; final version published in "The Astrophysical
Journal
First high-resolution images of the Sun in the 2796 \AA{} Mg II k line
We present the first high-resolution solar images in the Mg II k 2796 \AA{}
line. The images, taken through a 4.8 \AA{} broad interference filter, were
obtained during the second science flight of SUNRISE in June 2013 by the SuFI
instrument. The Mg II k images display structures that look qualitatively very
similar to images taken in the core of Ca II H. The Mg II images exhibit
reversed granulation (or shock waves) in the internetwork regions of the quiet
Sun, at intensity contrasts that are similar to those found in Ca II H. Very
prominent in Mg II are bright points, both in the quiet Sun and in plage
regions, particularly near disk center. These are much brighter than at other
wavelengths sampled at similar resolution. Furthermore, Mg II k images also
show fibril structures associated with plage regions. Again, the fibrils are
similar to those seen in Ca II H images, but tend to be more pronounced,
particularly in weak plage.Comment: Accepted for publication in The Astrophysical Journal Letter
Applicability of Milne-Eddington inversions to high spatial resolution observations of the quiet Sun
The physical conditions of the solar photosphere change on very small spatial
scales both horizontally and vertically. Such a complexity may pose a serious
obstacle to the accurate determination of solar magnetic fields. We examine the
applicability of Milne-Eddington (ME) inversions to high spatial resolution
observations of the quiet Sun. Our aim is to understand the connection between
the ME inferences and the actual stratifications of the atmospheric parameters.
We use magnetoconvection simulations of the solar surface to synthesize
asymmetric Stokes profiles such as those observed in the quiet Sun. We then
invert the profiles with the ME approximation. We perform an empirical analysis
of the heights of formation of ME measurements and analyze the uncertainties
brought about by the ME approximation. We also investigate the quality of the
fits and their relationship with the model stratifications. The atmospheric
parameters derived from ME inversions of high-spatial resolution profiles are
reasonably accurate and can be used for statistical analyses of solar magnetic
fields, even if the fit is not always good. We also show that the ME inferences
cannot be assigned to a specific atmospheric layer: different parameters sample
different ranges of optical depths, and even the same parameter may trace
different layers depending on the physical conditions of the atmosphere.
Despite this variability, ME inversions tend to probe deeper layers in granules
as compared with intergranular lanes.Comment: Accepted for publication in Astronomy and Astrophysic
Temporal evolution of the Evershed flow in sunspots. II. Physical properties and nature of Evershed clouds
Context: Evershed clouds (ECs) represent the most conspicuous variation of
the Evershed flow in sunspot penumbrae. Aims: We determine the physical
properties of ECs from high spatial and temporal resolution spectropolarimetric
measurements. Methods: The Stokes profiles of four visible and three infrared
spectral lines are subject to inversions based on simple one-component models
as well as more sophisticated realizations of penumbral flux tubes embedded in
a static ambient field (uncombed models). Results: According to the
one-component inversions, the EC phenomenon can be understood as a perturbation
of the magnetic and dynamic configuration of the penumbral filaments along
which these structures move. The uncombed inversions, on the other hand,
suggest that ECs are the result of enhancements in the visibility of penumbral
flux tubes. We conjecture that the enhancements are caused by a perturbation of
the thermodynamic properties of the tubes, rather than by changes in the vector
magnetic field. The feasibility of this mechanism is investigated performing
numerical experiments of thick penumbral tubes in mechanical equilibrium with a
background field. Conclusions: While the one-component inversions confirm many
of the properties indicated by a simple line parameter analysis (Paper I of
this series), we tend to give more credit to the results of the uncombed
inversions because they take into account, at least in an approximate manner,
the fine structure of the penumbra.Comment: Accepted for publication in A&
Solar Coronal Loops Associated with Small-scale Mixed Polarity Surface Magnetic Fields
How and where are coronal loops rooted in the solar lower atmosphere? The
details of the magnetic environment and its evolution at the footpoints of
coronal loops are crucial to understanding the processes of mass and energy
supply to the solar corona. To address the above question, we use
high-resolution line-of-sight magnetic field data from the Imaging Magnetograph
eXperiment instrument on the SUNRISE balloon-borne observatory and coronal
observations from the Atmospheric Imaging Assembly onboard the Solar Dynamics
Observatory of an emerging active region. We find that the coronal loops are
often rooted at the locations with minor small-scale but persistent
opposite-polarity magnetic elements very close to the larger dominant polarity.
These opposite-polarity small-scale elements continually interact with the
dominant polarity underlying the coronal loop through flux cancellation. At
these locations we detect small inverse Y-shaped jets in chromospheric Ca II H
images obtained from the SUNRISE Filter Imager during the flux cancellation.
Our results indicate that magnetic flux cancellation and reconnection at the
base of coronal loops due to mixed polarity fields might be a crucial feature
for the supply of mass and energy into the corona.Comment: Published in the Astrophysical Journal Supplement Serie
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