755 research outputs found
Modified p-modes in penumbral filaments?
Aims: The primary objective of this study is to search for and identify wave
modes within a sunspot penumbra.
Methods: Infrared spectropolarimetric time series data are inverted using a
model comprising two atmospheric components in each spatial pixel. Fourier
phase difference analysis is performed on the line-of-sight velocities
retrieved from both components to determine time delays between the velocity
signals. In addition, the vertical separation between the signals in the two
components is calculated from the Stokes velocity response functions.
Results: The inversion yields two atmospheric components, one permeated by a
nearly horizontal magnetic field, the other with a less-inclined magnetic
field. Time delays between the oscillations in the two components in the
frequency range 2.5-4.5 mHz are combined with speeds of atmospheric wave modes
to determine wave travel distances. These are compared to expected path lengths
obtained from response functions of the observed spectral lines in the
different atmospheric components. Fast-mode (i.e., modified p-mode) waves
exhibit the best agreement with the observations when propagating toward the
sunspot at an angle ~50 degrees to the vertical.Comment: 8 pages, 12 figures, accepted for publication in Astronomy &
Astrophysic
The uncombed penumbra
The uncombed penumbral model explains the structure of the sunspot penumbra
in terms of thick magnetic fibrils embedded in a magnetic surrounding
atmosphere. This model has been successfully applied to explain the
polarization signals emerging from the sunspot penumbra. Thick penumbral
fibrils face some physical problems, however. In this contribution we will
offer possible solutions to these shortcomings.Comment: 6 pages, 2 figures. to appear in the proceedings of the Solar
Polarization Workshop I
Convective motions and net circular polarization in sunspot penumbrae
We have employed a penumbral model, that includes the Evershed flow and
convective motions inside penumbral filaments, to reproduce the azimuthal
variation of the net circular polarization (NCP) in sunspot penumbrae at
different heliocentric angles for two different spectral lines. The theoretical
net circular polarization fits the observations as satisfactorily as penumbral
models based on flux-tubes. The reason for this is that the effect of
convective motions on the NCP is very small compared to the effect of the
Evershed flow. In addition, the NCP generated by convective upflows cancels out
the NCP generated by the downflows. We have also found that, in order to fit
the observed NCP, the strength of the magnetic field inside penumbral filaments
must be very close to 1000 G. In particular, field-free or weak-field filaments
fail to reproduce both the correct sign of the net circular polarization, as
well as its dependence on the azimuthal and heliocentric angles.Comment: Accepted for publication in the Astrophysical Journal. 10 pages, 7
figures (3 in color). Uses emulatedap
Temporal relation between quiet-Sun transverse fields and the strong flows detected by IMaX/SUNRISE
Localized strongly Doppler-shifted Stokes V signals were detected by
IMaX/SUNRISE. These signals are related to newly emerged magnetic loops that
are observed as linear polarization features. We aim to set constraints on the
physical nature and causes of these highly Doppler-shifted signals. In
particular, the temporal relation between the appearance of transverse fields
and the strong Doppler shifts is analyzed in some detail. We calculated the
time difference between the appearance of the strong flows and the linear
polarization. We also obtained the distances from the center of various
features to the nearest neutral lines and whether they overlap or not. These
distances were compared with those obtained from randomly distributed points on
observed magnetograms. Various cases of strong flows are described in some
detail. The linear polarization signals precede the appearance of the strong
flows by on average 84+-11 seconds. The strongly Doppler-shifted signals are
closer (0.19") to magnetic neutral lines than randomly distributed points
(0.5"). Eighty percent of the strongly Doppler-shifted signals are close to a
neutral line that is located between the emerging field and pre-existing
fields. That the remaining 20% do not show a close-by pre-existing field could
be explained by a lack of sensitivity or an unfavorable geometry of the
pre-existing field, for instance, a canopy-like structure. Transverse fields
occurred before the observation of the strong Doppler shifts. The process is
most naturally explained as the emergence of a granular-scale loop that first
gives rise to the linear polarization signals, interacts with pre-existing
fields (generating new neutral line configurations), and produces the observed
strong flows. This explanation is indicative of frequent small-scale
reconnection events in the quiet Sun.Comment: 11 pages, 8 figure
Models and Observations of Sunspot Penumbrae
The mysteries of sunspot penumbrae have been under an intense scrutiny for
the past 10 years. During this time, some models have been proposed and
refuted, while the surviving ones had to be modified, adapted and evolved to
explain the ever-increasing array of observational constraints. In this
contribution I will review two of the present models, emphasizing their
contributions to this field, but also pinpointing some of their inadequacies to
explain a number of recent observations at very high spatial resolution. To
help explaining these new observations I propose some modifications to each of
them. These modifications bring those two seemingly opposite models closer
together into a general picture that agrees well with recent 3D
magneto-hydrodynamic simulations.Comment: 9 pages, 1 color figure. Review talk to appear in the proceedings of
the International Workshop of 2008 Solar Total Eclipse: Solar Magnetism,
Corona and Space Weather--Chinese Space Solar Telescope Scienc
Kinematics and Magnetic Properties of a Light Bridge in a Decaying Sunspot
We present the results obtained by analyzing high spatial and spectral
resolution data of the solar photosphere acquired by the CRisp Imaging
SpectroPolarimeter at the Swedish Solar Telescope on 6 August 2011, relevant to
a large sunspot with a light bridge (LB) observed in NOAA AR 11263. These data
are complemented by simultaneous Hinode Spectropolarimeter (SP) observation in
the Fe I 630.15 nm and 630.25 nm lines. The continuum intensity map shows a
discontinuity of the radial distribution of the penumbral filaments in
correspondence with the LB, which shows a dark lane (about 0.3" wide and about
8.0" long) along its main axis. The available data were inverted with the
Stokes Inversion based on Response functions (SIR) code and physical parameters
maps were obtained. The line-of-sight (LOS) velocity of the plasma along the LB
derived from the Doppler effect shows motions towards and away from the
observer up to 0.6 km/s, which are lower in value than the LOS velocities
observed in the neighbouring penumbral filaments. The noteworthy result is that
we find motions toward the observer up to 0.6 km/s in the dark lane where the
LB is located between two umbral cores, while the LOS velocity motion toward
the observer is strongly reduced where the LB is located between an umbral core
at one side and penumbral filaments on the other side. Statistically, the LOS
velocities correspond to upflows/downflows andcomparing these results with
Hinode/SP data, we conclude that the surrounding magnetic field configuration
(whether more or less inclined) could have a role in maintaining the conditions
for the process of plasma piling up along the dark lane. The results obtained
from our study support and confirm outcomes of recent magnetohydro-dynamic
simulations showing upflows along the main axis of a LBs
Two-stage Robust Optimization Approach for Enhanced Community Resilience Under Tornado Hazards
Catastrophic tornadoes cause severe damage and are a threat to human
wellbeing, making it critical to determine mitigation strategies to reduce
their impact. One such strategy, following recent research, is to retrofit
existing structures. To this end, in this article we propose a model that
considers a decision-maker (a government agency or a public-private consortium)
who seeks to allocate resources to retrofit and recover wood-frame residential
structures, to minimize the population dislocation due to an uncertain tornado.
In the first stage the decision-maker selects the retrofitting strategies, and
in the second stage the recovery decisions are made after observing the
tornado. As tornado paths cannot be forecast reliably, we take a worst-case
approach to uncertainty where paths are modeled as arbitrary line segments on
the plane. Under the assumption that an area is affected if it is sufficiently
close to the tornado path, the problem is framed as a two-stage robust
optimization problem with a mixed-integer non-linear uncertainty set. We solve
this problem by using a decomposition column-and-constraint generation
algorithm that solves a two-level integer problem at each iteration. This
problem, in turn, is solved by a decomposition branch-and-cut method that
exploits the geometry of the uncertainty set. To illustrate the model's
applicability, we present a case study based on Joplin, Missouri. Our results
show that there can be up to 20 percent reductions in worst-case population
dislocation by investing 15 million dollars in retrofitting and recovery; that
our approach outperforms other retrofitting policies, and that the model is not
over-conservative
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