37 research outputs found
Center to lim variation of solar photospheric structures
El estudio de propiedades de la granulación y las fáculas del centro al borde del disco es un tema clásico en Física Solar. Aunque se ha dedicado mucho trabajo observacional, semi-empírico y teórico a este asunto, todavía no existe acuerdo generalizado sobre el escenario físico adecuado para reproducir los numerosos observables medidos en diversas condiciones (alta y baja resolución espacial, son su evolución temporal, en 1 ó 2 dimensiones espaciales, en distintas longitudes de onda, etc.) y utilizando diferentes técnicas (fotometría, espectroscopía ó espectropolarmetría). Un estudio observacional cuidadoso y su comparación con modelos semi-empíricos y sofisticados simulaciones numéricos como el abordado en esta tesis, contribuye a -- luz sobre esta cuestión y a proporcionar las prescindibles restricciones observacionales a la teoría. Este trabajo parte de observaciones fotométricas con alta resolución espacio obtenidos en la SVST@ORM (La Palma) en varias longitudes de -- en el visible e IR, estas últimas de especial interés por no ser tan abundantes las observaciones de la fotosfera en dicho rango del espectro solar. Los datos corresponden a granulación en calma y varias regiones activas en distintas posiciones del centro al borde del disco solar. Algunas observaciones se obtuvieron durante un eclipse parcial del sol, por lo que han podido ser satisfactoriamente restauradas de la degradación atmosférica e instrumental gracias al desarrollo de un novedoso filtro ad hoc drivado de los propios datos. Las observaciones han sido comparadas con -- numéricas y datos sintetizadas aplicando un código 3-D de transporte radiactivo a modelos semi-empíricos. La tesis aborda un problema refrescante y el trabajo ha sido realizado con gran rigor, de modo que ha producido ya como artículos en revistas internacionales -- y varias comunicaciones a congresos, lo que avala y confirma su calidad
Detection of Small-Scale Granular Structures in the Quiet Sun with the New Solar Telescope
Results of a statistical analysis of solar granulation are presented. A data
set of 36 images of a quiet Sun area on the solar disk center was used. The
data were obtained with the 1.6 m clear aperture New Solar Telescope (NST) at
Big Bear Solar Observatory (BBSO) and with a broad-band filter centered at the
TiO (705.7 nm) spectral line. The very high spatial resolution of the data
(diffraction limit of 77 km and pixel scale of 0.0375) augmented by the very
high image contrast (15.50.6%) allowed us to detect for the first time a
distinct subpopulation of mini-granular structures. These structures are
dominant on spatial scales below 600 km. Their size is distributed as a power
law with an index of -1.8 (which is close to the Kolmogorov's -5/3 law) and no
predominant scale. The regular granules display a Gaussian (normal) size
distribution with a mean diameter of 1050 km. Mini-granular structures
contribute significantly to the total granular area. They are predominantly
confined to the wide dark lanes between regular granules and often form chains
and clusters, but different from magnetic bright points. A multi-fractality
test reveals that the structures smaller than 600 km represent a multi-fractal,
whereas on larger scales the granulation pattern shows no multi-fractality and
can be considered as a Gaussian random field. The origin, properties and role
of the newly discovered population of mini-granular structures in the solar
magneto-convection are yet to be explored.Comment: 13 pages, 5 figure
The intensity contrast of solar granulation: comparing Hinode SP results with MHD simulations
The contrast of granulation is an important quantity characterizing solar
surface convection. We compare the intensity contrast at 630 nm, observed using
the Spectro-Polarimeter (SP) aboard the Hinode satellite, with the 3D radiative
MHD simulations of V{\"o}gler & Sch{\"u}ssler (2007). A synthetic image from
the simulation is degraded using a theoretical point-spread function of the
optical system, and by considering other important effects. The telescope
aperture and the obscuration by the secondary mirror and its attachment spider,
reduce the simulated contrast from 14.4 % to 8.5 %. A slight effective defocus
of the instrument brings the simulated contrast down to 7.5 %, close to the
observed value of 7.0 %. A proper consideration of the effects of the optical
system and a slight defocus, lead to sufficient degradation of the synthetic
image from the MHD simulation, such that the contrast reaches almost the
observed value. The remaining small discrepancy can be ascribed to straylight
and slight imperfections of the instrument, which are difficult to model.
Hence, Hinode SP data are consistent with a granulation contrast which is
predicted by 3D radiation MHD simulations.Comment: 5 pages, 4 figures, to be published in A&
Photometric properties of resolved and unresolved magnetic elements
We investigate the photometric signature of magnetic flux tubes in the solar
photosphere. We developed two dimensional, static numerical models of isolated
and clustered magnetic flux tubes. We investigated the emergent intensity
profiles at different lines-of-sight for various spatial resolutions and
opacity models. We found that both geometric and photometric properties of
bright magnetic features are determined not only by the physical properties of
the tube and its surroundings, but also by the particularities of the
observations, including the line/continuum formation height, the spatial
resolution and the image analyses techniques applied. We show that some
observational results presented in the literature can be interpreted by
considering bright magnetic features to be clusters of smaller elements, rather
than a monolithic flux tube.Comment: 12 page
Polarimetric Diagnostics of Unresolved Chromospheric Magnetic Fields
For about a decade, spectro-polarimetry of HeI 10830 has been applied to the
magnetic diagnostics of the solar chromosphere. This resonance line is very
versatile, as it is visible both on disk and in off-limb structures, and it has
a good sensitivity to both the weak-field Hanle effect and the strong-field
Zeeman effect. Recent observations of an active-region filament showed that the
linear polarization was dominated by the transverse Zeeman effect, with very
little or no hint of scattering polarization. This is surprising, since the HeI
levels should be significantly polarized in a conventional scattering scenario.
To explain the observed level of atomic depolarization by collisional or
radiative processes, one must invoke plasma densities larger by several orders
of magnitude than currently known values for prominences. We show that such
depolarization can be explained quite naturally by the presence of an
unresolved, highly entangled magnetic field, which averages to give the ordered
field inferred from spectro-polarimetric data, over the typical temporal and
spatial scales of the observations. We present a modeling of the polarized HeI
10830 in this scenario, and discuss its implications for the magnetic
diagnostics of prominences and spicules, and for the general study of
unresolved magnetic field distributions in the solar atmosphere
On the continuum intensity distribution of the solar photosphere
We present a detailed comparison between simulations and seeing-free
observations that takes into account the crucial influence of instrumental
image degradation. We use images of quiet Sun granulation taken in the blue,
green and red continuum bands of the Broadband Filter Imager of the Solar
Optical Telescope (SOT) onboard Hinode. The images are deconvolved with Point
Spread Functions (PSF) that account for non-ideal contributions due to
instrumental stray-light and imperfections. In addition, synthetic intensity
images are degraded with the corresponding PSFs. ... Removing the influence of
the PSF unveils much broader intensity distributions with a secondary component
that is otherwise only visible as an asymmetry between the darker and brighter
than average part of the distribution. The contrast values increase to (26.7
+/- 1.3) %, (19.4 +/- 1.4) %, and (16.6 +/- 0.7) % for blue, green, and red
continuum, respectively. The power spectral density of the images exhibits a
pronounced peak at spatial scales characteristic for the granulation pattern
and a steep decrease towards smaller scales. The observational findings like
the absolute values and centre-to-limb variation of the intensity contrast,
intensity histograms, and power spectral density are well matched with
corresponding synthetic observables from three-dimensional radiation
(magneto-)hydrodynamic simulations. We conclude that the intensity contrast of
the solar continuum intensity is higher than usually derived from ground-based
observations and is well reproduced by modern radiation (magneto-)hydrodynamic
models. Properly accounting for image degradation effects is of crucial
importance for comparisons between observations and numerical models.Comment: 15 pages, 10 figures; A&A, in pres
Is it possible to detect planets around young active G and K dwarfs?
Theoretical predictions suggest that the distribution of planets in very young stars could be very different to that typically observed in Gyr old systems that are the current focus of radial velocity surveys. However, the detection of planets around young stars is hampered by the increased stellar activity associated with young stars, the signatures of which can bias the detection of planets. In this paper, we place realistic limitations on the possibilities for detecting planets around young active G and K dwarfs. The models of stellar activity based on tomographic imaging of the G dwarf HD 141943 and the K1 dwarf AB Dor also include contributions from plage and many small random starspots. Our results show that the increased stellar activity levels present on young solar-type stars strongly impacts the detection of Earth-mass and Jupiter-mass planets and that the degree of activity jitter is directly correlated with stellar v sin i. We also show that for G and K dwarfs, the distribution of activity in individual stars is more important than the differences in induced radial velocities as a function of spectral type. We conclude that Jupiter-mass planets can be detected close-in around fast-rotating young active stars, Neptune-mass planets around moderate rotators and that Super-Earths are only detectable around very slowly rotating stars. The effects of an increase in stellar activity jitter by observing younger stars can be compensated for by extending the observational base-line to at least 100 epochs.Peer reviewe
The formation of sunspot penumbra. I. Magnetic field properties
We study the formation of a sunspot penumbra in the active region NOAA11024.
We simultaneously observed the Stokes parameters of the photospheric iron lines
at 1089.6 nm with the TIP and 617.3 nm with the GFPI spectropolarimeters along
with broad-band images using G-band and CaIIK filters at the German VTT. The
formation of the penumbra is intimately related to the inclined magnetic field.
Within 4.5 h observing time, the magnetic flux of the penumbra increases from
9.7E+20 to 18.2E+20 Mx, while the magnetic flux of the umbra remains constant
at about 3.8E+20 Mx. Magnetic flux in the immediate surroundings is
incorporated into the spot, and new flux is supplied via small flux patches
(SFPs), which on average have a flux of 2-3E+18 Mx. The spot's flux increase
rate of 4.2E+16 Mx/s corresponds to the merging of one SFP per minute. We also
find that during the formation of the spot penumbra: a) the maximum magnetic
field strength of the umbra does not change, b) the magnetic neutral line keeps
the same position relative to the umbra, c) the new flux arrives on the
emergence side of the spot while the penumbra forms on the opposite side, d)
the average LRF inclination of the light bridges decreases from 50 to 37 deg,
and e) as the penumbra develops, the mean magnetic field strength at the spot
border decreases from 1.0 to 0.8 kG. The SFPs associated with elongated
granules are the building blocks of structure formation in active regions.
During the sunspot formation, their contribution is comparable to the
coalescence of pores. A quiet environment in the surroundings is important for
penumbral formation. As remnants of trapped granulation between merging pores,
the light bridges are found to play a crucial role in the formation process.
They seem to channel the magnetic flux through the spot during its formation.
Light bridges are also the locations where the first penumbral filaments form.Comment: 14 pages, 12 figures, accepted by A&
Two-dimensional spectroscopy of a sunspot. III Thermal and kinematic structure of the penumbra at 0.5"
We investigate the thermal and kinematic configuration of a sunspot penumbra
using very high spectral and spatial resolution intensity profiles of the
non-magnetic Fe I 557.6 nm line. The dataset was acquired with the 2D solar
spectrometer TESOS. The profiles are inverted using a one-component model
atmosphere with gradients of the physical quantities. From this inversion we
obtain the stratification with depth of temperature, line-of-sight velocity,
and microturbulence across the penumbra. Our results suggest that the physical
mechanism(s) responsible for the penumbral filaments operate preferentially in
the lower photosphere. We confirm the existence of a thermal asymmetry between
the center and limb-side penumbra, the former being hotter by 100-150 K on
average. We also investigate the nature of the bright ring that appears in the
inner penumbra when sunspots are observed in the wing of spectral lines. The
line-of-sight velocities retrieved from the inversion are used to determine the
flow speed and flow angle at different heights in the photosphere. Both the
flow speed and flow angle increase with optical depth and radial distance.
Downflows are detected in the mid and outer penumbra, but only in deep layers
(log tau_{500} < -1.4). We demonstrate that the velocity stratifications
retrieved from the inversion are consistent with the idea of penumbral flux
tubes channeling the Evershed flow. Finally, we show that larger Evershed flows
are associated with brighter continuum intensities in the inner center-side
penumbra. Dark structures, however, are also associated with significant
Evershed flows. This leads us to suggest that the bright and dark filaments
seen at 0.5" resolution are not individual flow channels, but a collection of
them.Comment: 15 pages with 18 Postscript figures. Submitted to Astronomy and
Astrophysics (December 2005
A 3-D sunspot model derived from an inversion of spectropolarimetric observations and its implications for the penumbral heating
I deduced a 3-D sunspot model that is in agreement with spectropolarimetric
observations, to address the question of penumbral heating by the repetitive
rise of flow channels. I performed inversions of data taken simultaneously in
infrared and visible spectral lines. I used two independent magnetic components
to reproduce the irregular Stokes profiles in the penumbra. I studied the
averaged and individual properties of the two components. By integrating the
field inclination to the surface, I developed a 3-D model of the spot from
inversion results without intrinsic height information. I find that the
Evershed flow is harbored by the weaker of the two field components. This
component forms flow channels that show upstreams in the inner and mid
penumbra, continue horizontally as slightly elevated loops throughout the
penumbra, and finally bend down in the outer penumbra. I find several examples,
where two or more flow channels are found along a radial cut from the umbra to
the outer boundary of the spot. I find that a model of horizontal flow channels
in a static background field is in good agreement with the observed spectra.
The properties of the flow channels correspond very well to the simulations of
Schlichenmaier et al. (1998). From the temporal evolution in intensity images
and the properties of the flow channels in the inversion, I conclude that
interchange convection of rising hot flux tubes in a thick penumbra still seems
a possible mechanism for maintaining the penumbral energy balance.Comment: 17 pages, 21 figures, accepted by A&