684 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
Penumbral thermal structure below the visible surface
. The thermal structure of the penumbra below its visible surface
(i.e., ) has important implications for our present understanding
of sunspots and their penumbrae: their brightness and energy transport, mode
conversion of magneto-acoustic waves, sunspot seismology, and so forth. .
We aim at determining the thermal stratification in the layers immediately
beneath the visible surface of the penumbra: ( km below the visible continuum-forming layer). . We analyzed
spectropolarimetric data (i.e., Stokes profiles) in three Fe \textsc{i} lines
located at 1565 nm observed with the GRIS instrument attached to the 1.5-meter
solar telescope GREGOR. The data are corrected for the smearing effects of
wide-angle scattered light and then subjected to an inversion code for the
radiative transfer equation in order to retrieve, among others, the temperature
as a function of optical depth . . We find that the
temperature gradient below the visible surface of the penumbra is smaller than
in the quiet Sun. This implies that in the region the penumbral
temperature diverges from that of the quiet Sun. The same result is obtained
when focusing only on the thermal structure below the surface of bright
penumbral filaments. We interpret these results as evidence of a thick
penumbra, whereby the magnetopause is not located near its visible surface. In
addition, we find that the temperature gradient in bright penumbral filaments
is lower than in granules. This can be explained in terms of the limited
expansion of a hot upflow inside a penumbral filament relative to a granular
upflow, as magnetic pressure and tension forces from the surrounding penumbral
magnetic field hinder an expansion like this.Comment: 5 pages; 2 figures; accepted for publication in Astronomy and
Astrophysics Letter
Ranking efficient DMUs using cooperative game theory
The problem of ranking Decision Making Units (DMUs) in Data Envelopment Analysis (DEA) has been widely studied in the literature. Some of the proposed approaches use cooperative game theory as a tool to perform the ranking. In this paper, we use the Shapley value of two different cooperative games in which the players are the efficient DMUs and the characteristic function represents the increase in the discriminant power of DEA contributed by each efficient DMU. The idea is that if the efficient DMUs are not included in the modified reference sample then the efficiency score of some inefficient DMUs would be higher. The characteristic function represents, therefore, the change in the efficiency scores of the inefficient DMUs that occurs when a given coalition of efficient units is dropped from the sample. Alternatively, the characteristic function of the cooperative game can be defined as the change in the efficiency scores of the inefficient DMUs that occurs when a given coalition of efficient DMUs are the only efficient DMUs that are included in the sample. Since the two cooperative games proposed are dual games, their corresponding Shapley value coincide and thus lead to the same ranking. The more an ef- ficient DMU impacts the shape of the efficient frontier, the higher the increase in the efficiency scores of the inefficient DMUs its removal brings about and, hence, the higher its contribution to the overall discriminant power of the method. The proposed approach is illustrated on a number of datasets from the literature and compared with existing methods
Observations of solar small-scale magnetic flux-sheet emergence
Aims. Moreno-Insertis et al. (2018) recently discovered two types of flux
emergence in their numerical simulations: magnetic loops and magnetic sheet
emergence. Whereas magnetic loop emergence has been documented well in the last
years, by utilising high-resolution full Stokes data from ground-based
telescopes as well as satellites, magnetic sheet emergence is still an
understudied process. We report here on the first clear observational evidence
of a magnetic sheet emergence and characterise its development.
Methods. Full Stokes spectra from the Hinode spectropolarimeter were inverted
with the SIR code to obtain solar atmospheric parameters such as temperature,
line-of-sight velocities and full magnetic field vector information.
Results. We analyse a magnetic flux emergence event observed in the quiet-sun
internetwork. After a large scale appearance of linear polarisation, a magnetic
sheet with horizontal magnetic flux density of up to 194 Mx/cm hovers in
the low photosphere spanning a region of 2 to 3 arcsec. The magnetic field
azimuth obtained through Stokes inversions clearly shows an organised structure
of transversal magnetic flux density emerging. The granule below the magnetic
flux-sheet tears the structure apart leaving the emerged flux to form several
magnetic loops at the edges of the granule.
Conclusions. A large amount of flux with strong horizontal magnetic fields
surfaces through the interplay of buried magnetic flux and convective motions.
The magnetic flux emerges within 10 minutes and we find a longitudinal magnetic
flux at the foot points of the order of Mx. This is one to two
orders of magnitude larger than what has been reported for small-scale magnetic
loops. The convective flows feed the newly emerged flux into the pre-existing
magnetic population on a granular scale.Comment: 6 pages, 5 figures, accepted as a letter in A&
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
Combining magneto-hydrostatic constraints with Stokes profiles inversions
Inversion codes for the polarized radiative transfer equation can be used to
infer the temperature , line-of-sight velocity , and magnetic
field as a function of the continuum optical-depth .
However, they do not directly provide the gas pressure or density
. In order to obtain these latter parameters, inversion codes rely
instead on the assumption of hydrostatic equilibrium (HE) in addition to the
equation of state (EOS). Unfortunately, the assumption of HE is rather
unrealistic across magnetic field lines. This is because the role of the
Lorentz force, among other factors, is neglected. This translates into an
inaccurate conversion from optical depth to geometrical height
. We aim at improving this conversion via the application of
magneto-hydrostatic (MHS) equilibrium instead of HE. We develop a method to
solve the momentum equation under MHS equilibrium (i.e., taking the Lorentz
force into account) in three dimensions. The method is based on the solution of
a Poisson-like equation. Considering the gas pressure and density
from three-dimensional magneto-hydrodynamic (MHD) simulations of
sunspots as a benchmark, we compare the results from the application of HE and
MHS equilibrium. We find that HE retrieves the gas pressure and density within
an order of magnitude of the MHD values in only about 47 \% of the domain. This
translates into an error of about km in the determination of the
conversion. On the other hand, the application of MHS
equilibrium allows determination of and within an order of
magnitude in 84 \% of the domain. In this latter case, the
conversion is obtained with an accuracy of km.Comment: Accepted for publication in Astronomy \& Astrophysics. 12 pages, 10
figure
Ocupação tardia e o desenvolvimento da agropecuária no Estado de Rondônia Uma história da bovinocultura no desenvolvimento regional.
Neste artigo se discute a ocupação tardia e sem planejamento de Rondônia e sua influência na bovinocultura. É uma pesquisa bibliográfica e qualitativa na qual foram levantadas informações sobre penetração do gado bovino no Estado do século XVIII ao XXI cujo objetivo é mostrar a configuração da atividade no desenvolvimento regional. Extraiu-se que somente a partir da década de setenta a criação de gado tornou-se importante para a economia regional, mas faltaram políticas públicas de ordenação e planejamento do território ocupado, tornando-a uma atividade competitiva. Aqui as baixas tecnologias provocaram a substituição da floresta pela criação extensiva, e sabe-se que na exploração bovina feita de forma correta poucos danos se verificam em sua área de atuação
Electronic depth profiles with atomic layer resolution from resonant soft x-ray reflectivity
The analysis of x-ray reflectivity data from artificial heterostructures
usually relies on the homogeneity of optical properties of the constituent
materials. However, when the x-ray energy is tuned to an absorption edge, this
homogeneity no longer exists. Within the same material, spatial regions
containing elements at resonance will have optical properties very different
from regions without resonating sites. In this situation, models assuming
homogeneous optical properties throughout the material can fail to describe the
reflectivity adequately. As we show here, resonant soft x-ray reflectivity is
sensitive to these variations, even though the wavelength is typically large as
compared to the atomic distances over which the optical properties vary. We
have therefore developed a scheme for analyzing resonant soft x-ray
reflectivity data, which takes the atomic structure of a material into account
by "slicing" it into atomic planes with characteristic optical properties.
Using LaSrMnO4 as an example, we discuss both the theoretical and experimental
implications of this approach. Our analysis not only allows to determine
important structural information such as interface terminations and stacking of
atomic layers, but also enables to extract depth-resolved spectroscopic
information with atomic resolution, thus enhancing the capability of the
technique to study emergent phenomena at surfaces and interfaces.Comment: Completely overhauled with respect to the previous version due to
peer revie
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