254 research outputs found
Variation of the carrier-envelope phase of few-cycle laser pulses owing to the Gouy phase: a solid-state-based measurement
Stratification of sunspot umbral dots from inversion of Stokes profiles recorded by Hinode
This work aims to constrain the physical nature of umbral dots (UDs) using
high-resolution spectropolarimetry. Full Stokes spectra recorded by the
spectropolarimeter on Hinode of 51 UDs in a sunspot close to the disk center
are analyzed. The height dependence of the temperature, magnetic field vector,
and line-of-sight velocity across each UD is obtained from an inversion of the
Stokes vectors of the two FeI lines at 630 nm. No difference is found at higher
altitudes (-3 <= log(tau) <= -2) between the UDs and the diffuse umbral
background. Below that level the difference rapidly increases, so that at the
continuum formation level (log(tau) = 0) we find on average a temperature
enhancement of 570 K, a magnetic field weakening of 510 G, and upflows of 800
m/s for peripheral UDs, whereas central UDs display an excess temperature of on
average 550 K, a field weakening of 480 G, and no significant upflows. The
results for, in particular, the peripheral UDs, including cuts of magnetic
vector and velocity through them, look remarkably similar to the output of
recent radiation MHD simulations. They strongly suggest that UDs are produced
by convective upwellings
Twist, Writhe & Helicity in the inner penumbra of a sunspot
The aim of this work is the determination of the twist, writhe, and self
magnetic helicity of penumbral filaments located in an inner Sunspot penumbra.
To this extent, we inverted data taken with the spectropolarimeter (SP) aboard
Hinode with the SIR (Stokes Inversion based on Response function) code. For the
construction of a 3D geometrical model we applied a genetic algorithm
minimizing the divergence of the magnetic field vector and the net
magnetohydrodynamic force, consequently a force-free solution would be reached
if possible. We estimated two proxies to the magnetic helicity frequently used
in literature: the force-free parameter and the current helicity term. We show
that both proxies are only qualitative indicators of the local twist as the
magnetic field in the area under study significantly departures from a
force-free configuration. The local twist shows significant values only at the
borders of bright penumbral filaments with opposite signs on each side. These
locations are precisely correlated to large electric currents. The average
twist (and writhe) of penumbral structures is very small. The spines (dark
filaments in the background) show a nearly zero writhe. The writhe per unit
length of the intraspines diminishes with increasing length of the tube axes.
Thus, the axes of tubes related to intraspines are less wrung when the tubes
are more horizontal. As the writhe of the spines is very small, we can conclude
that the writhe reaches only significant values when the tube includes the
border of a intraspine.Comment: 7 pages, 4 figures; Astrophysical Journal, in pres
The Fine-Structure of the Net-Circular Polarization in a Sunspot Penumbra
We present novel evidence for a fine structure observed in the net-circular
polarization (NCP) of a sunspot penumbra based on spectropolarimetric
measurements utilizing the Zeeman sensitive FeI 630.2 nm line. For the first
time we detect a filamentary organized fine structure of the NCP on spatial
scales that are similar to the inhomogeneities found in the penumbral flow
field. We also observe an additional property of the visible NCP, a
zero-crossing of the NCP in the outer parts of the center-side penumbra, which
has not been recognized before. In order to interprete the observations we
solve the radiative transfer equations for polarized light in a model penumbra
with embedded magnetic flux tubes. We demonstrate that the observed
zero-crossing of the NCP can be explained by an increased magnetic field
strength inside magnetic flux tubes in the outer penumbra combined with a
decreased magnetic field strength in the background field. Our results strongly
support the concept of the uncombed penumbra
Flow Field Evolution of a Decaying Sunspot
We study the evolution of the flows and horizontal proper motions in and
around a decaying follower sunspot based on time sequences of two-dimensional
spectroscopic observations in the visible and white light imaging data obtained
over six days from June~7 to~12, 2005. During this time period the sunspot
decayed gradually to a pore. The spectroscopic observations were obtained with
the Fabry-P\'{e}rot based Visible-Light Imaging Magnetograph (VIM) in
conjunction with the high-order adaptive optics (AO) system operated at the 65
cm vacuum reflector of the Big Bear Solar Observatory (BBSO). We apply local
correlation tracking (LCT) to the speckle reconstructed time sequences of
white-light images around 600 nm to infer horizontal proper motions while the
Doppler shifts of the scanned \FeI line at 630.15 nm are used to calculate
line-of-sight (LOS) velocities with sub-arcsecond resolution. We find that the
dividing line between radial inward and outward proper motions in the inner and
outer penumbra, respectively, survives the decay phase. In particular the moat
flow is still detectable after the penumbra disappeared. Based on our
observations three major processes removed flux from the sunspot: (a)
fragmentation of the umbra, (b) flux cancelation of moving magnetic features
(MMFs; of the same polarity as the sunspot) that encounter the leading opposite
polarity network and plages areas, and (c) flux transport by MMFs (of the same
polarity as the sunspot) to the surrounding network and plage regions that have
the same polarity as the sunspot.Comment: 9 pages, 7 figures, The Astrophysical Journal, accepted September,
200
Brightness, distribution, and evolution of sunspot umbral dots
We present a 106-minute TiO (705.7nm) time series of high spatial and
temporal resolution that contains thousands of umbral dots (UDs) in a mature
sunspot in the active region NOAA 10667 at =0.95. The data were acquired
with the 1-m Swedish Solar Telescope on La Palma. With the help of a multilevel
tracking (MLT) algorithm the sizes, brightnesses, and trajectories of 12836
umbral dots were found and analyzed. The MLT allows UDs with very low contrast
to be reliably identified. Inside the umbra we determine a UD filling factor of
11%. The histogram of UD lifetimes is monotonic, i.e. a UD does not have a
typical lifetime. Three quarters of the UDs lived for less than 150s and showed
no or little motion. The histogram of the UD diameters exhibits a maximum at
225km, i.e. most of the UDs are spatially resolved. UDs display a typical
horizontal velocity of 420m/s and a typical peak intensity of 51% of the mean
intensity of the quiet photosphere, making them on average 20% brighter than
the local umbral background. Almost all mobile UDs (large birth-death distance)
were born close to the umbra-penumbra boundary, move towards the umbral center,
and are brighter than average. Notably bright and mobile UDs were also observed
along a prominent UD chain, both ends of which are located at the
umbra-penumbra boundary. Their motion started primarily at either of the ends
of the chain, continued along the chain, and ended near the chain's center. We
observed the splitting and merging of UDs and the temporal succession of both.
For the first time the evolution of brightness, size, and horizontal speed of a
typical UD could be determined in a statistically significant way. Considerable
differences between the evolution of central and peripheral UDs are found,
which point to a difference in origin
Magnetic fields in solar plage regions: insights from high-sensitivity spectropolarimetry
Plage regions are patches of concentrated magnetic field in the Sun's
atmosphere where hot coronal loops are rooted. While previous studies have shed
light on the properties of plage magnetic fields in the photosphere, there are
still challenges in measuring the overlying chromospheric magnetic fields,
which are crucial to understanding the overall heating and dynamics. Here, we
utilize high-sensitivity, spectropolarimetric data obtained by the four-meter
Daniel K. Inouye Solar Telescope (DKIST) to investigate the dynamic environment
and magnetic field stratification of an extended, decaying plage region. The
data show strong circular polarization signals in both plage cores and
surrounding fibrils. Notably, weak linear polarization signals clearly
differentiate between plage patches and the fibril canopy, where they are
relatively stronger. Inversions of the Ca II 8542 spectra show
an imprint of the fibrils in the chromospheric magnetic field, with typical
field strength values ranging from 200-300 G in fibrils. We confirm the
weak correlation between field strength and cooling rates in the lower
chromosphere. Additionally, we observe supersonic downflows and strong velocity
gradients in the plage periphery, indicating dynamical processes occurring in
the chromosphere. These findings contribute to our understanding of the
magnetic field and dynamics within plages, emphasizing the need for further
research to explore the expansion of magnetic fields with height and the
three-dimensional distribution of heating rates in the lower chromosphere.Comment: 17 pages, 8 figures, accepted for publication in ApJ
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