91 research outputs found
A study on disability glare vision in young adult subjects
The full assessment of the visual system must include the evaluation of the optical quality of the eye and neural visual functions. The objective evaluation of the retinal image quality is often carried out by computing the point spread function (PSF) of the eye. The central part of the PSF is associated with optical aberrations and the peripheral areas with scattering contributions. In that sense, visual acuity and contrast sensitivity function tests can be considered the perceptual neural response to those contributions characterizing the eyeâs PSF. However, in natural viewing conditions, visual acuity tests may provide good vision while contrast sensitivity tests can reveal visual impairment in glare vision conditions, such as exposure to bright light sources or night driving conditions. Here we present an optical instrument for the study of disability glare vision under extended Maxwellian illumination to assess the contrast sensitivity function under glare conditions. The limit of the Total Disability Glare threshold, tolerance, and glare adaptation will be investigated as a function of the angular size of the glare source (GA) and the contrast sensitivity function in young adult subjects
A near-IR line of Mn I as a diagnostic tool of the average magnetic energy in the solar photosphere
We report on spectropolarimetric observations of a near-IR line of Mn I
located at 15262.702 A whose intensity and polarization profiles are very
sensitive to the presence of hyperfine structure. A theoretical investigation
of the magnetic sensitivity of this line to the magnetic field uncovers several
interesting properties. The most important one is that the presence of strong
Paschen-Back perturbations due to the hyperfine structure produces an intensity
line profile whose shape changes according to the absolute value of the
magnetic field strength. A line ratio technique is developed from the intrinsic
variations of the line profile. This line ratio technique is applied to
spectropolarimetric observations of the quiet solar photosphere in order to
explore the probability distribution function of the magnetic field strength.
Particular attention is given to the quietest area of the observed field of
view, which was encircled by an enhanced network region. A detailed theoretical
investigation shows that the inferred distribution yields information on the
average magnetic field strength and the spatial scale at which the magnetic
field is organized. A first estimation gives ~250 G for the mean field strength
and a tentative value of ~0.45" for the spatial scale at which the observed
magnetic field is horizontally organized.Comment: 42 pages, 17 figures, accepted for publication in the Astrophysical
Journal. Figures 1 and 9 are in JPG forma
Observation and modeling of anomalous CN polarization profiles produced by the molecular Paschen-Back effect in sunspots
We report novel spectropolarimetric observations of sunspots carried out with
the Tenerife Infrared Polarimeter (TIP) in a near-IR spectral region around
15410 A, which is known to contain two groups of prominent OH lines that show
circular polarization signals of opposite polarity. Surrounding these
well-known OH lines, we have discovered the presence of CN lines of the band which show anomalous polarization profiles. Although the Stokes V
signals of the OH lines are antisymmetric and with a sizable amplitude, the CN
lines show almost negligible circular polarization. On the contrary, the linear
polarization signals turn out to be much stronger in the CN lines than in the
OH lines. Interestingly, these CN lines present striking antisymmetric linear
polarization profiles, which we are able to explain and model via the
Paschen-Back effect theory for diatomic molecules. The presence of such
peculiar CN lines in the same spectral region of the OH lines may be useful to
improve our empirical knowledge of solar magnetic fields via the simultaneous
observation and modeling of the transverse and longitudinal Zeeman effects in
two different molecular species.Comment: 11 pages, 3 figures, published in ApJ 623, L57 (2005
Theoretical modeling of propagation of magneto-acoustic waves in magnetic regions below sunspots
We use 2D numerical simulations and eikonal approximation, to study
properties of MHD waves traveling below the solar surface through the magnetic
structure of sunspots. We consider a series of magnetostatic models of sunspots
of different magnetic field strengths, from 10 Mm below the photosphere to the
low chromosphere. The purpose of these studies is to quantify the effect of the
magnetic field on local helioseismology measurements by modeling waves excited
by sub-photospheric sources. Time-distance propagation diagrams and wave travel
times are calculated for models of various field strength and compared to the
non-magnetic case. The results clearly indicate that the observed time-distance
helioseismology signals in sunspot regions correspond to fast MHD waves. The
slow MHD waves form a distinctly different pattern in the time-distance
diagram, which has not been detected in observations. The numerical results are
in good agreement with the solution in the short-wavelength (eikonal)
approximation, providing its validation. The frequency dependence of the travel
times is in a good qualitative agreement with observations.Comment: accepted by Ap
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,
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The GREGOR Fabry-P\'erot Interferometer
The GREGOR Fabry-P\'erot Interferometer (GFPI) is one of three first-light
instruments of the German 1.5-meter GREGOR solar telescope at the Observatorio
del Teide, Tenerife, Spain. The GFPI uses two tunable etalons in collimated
mounting. Thanks to its large-format, high-cadence CCD detectors with
sophisticated computer hard- and software it is capable of scanning spectral
lines with a cadence that is sufficient to capture the dynamic evolution of the
solar atmosphere. The field-of-view (FOV) of 50" x 38" is well suited for quiet
Sun and sunspot observations. However, in the vector spectropolarimetric mode
the FOV reduces to 25" x 38". The spectral coverage in the spectroscopic mode
extends from 530-860 nm with a theoretical spectral resolution R of about
250,000, whereas in the vector spectropolarimetric mode the wavelength range is
at present limited to 580-660 nm. The combination of fast narrow-band imaging
and post-factum image restoration has the potential for discovery science
concerning the dynamic Sun and its magnetic field at spatial scales down to
about 50 km on the solar surface.Comment: 14 pages, 17 figures, 4 tables; pre-print of AN 333, p.880-893, 2012
(AN special issue to GREGOR
The Imaging Magnetograph eXperiment (IMaX) for the Sunrise balloon-borne solar observatory
The Imaging Magnetograph eXperiment (IMaX) is a spectropolarimeter built by
four institutions in Spain that flew on board the Sunrise balloon-borne
telesocope in June 2009 for almost six days over the Arctic Circle. As a
polarimeter IMaX uses fast polarization modulation (based on the use of two
liquid crystal retarders), real-time image accumulation, and dual beam
polarimetry to reach polarization sensitivities of 0.1%. As a spectrograph, the
instrument uses a LiNbO3 etalon in double pass and a narrow band pre-filter to
achieve a spectral resolution of 85 mAA. IMaX uses the high Zeeman sensitive
line of Fe I at 5250.2 AA and observes all four Stokes parameters at various
points inside the spectral line. This allows vector magnetograms, Dopplergrams,
and intensity frames to be produced that, after reconstruction, reach spatial
resolutions in the 0.15-0.18 arcsec range over a 50x50 arcsec FOV. Time
cadences vary between ten and 33 seconds, although the shortest one only
includes longitudinal polarimetry. The spectral line is sampled in various ways
depending on the applied observing mode, from just two points inside the line
to 11 of them. All observing modes include one extra wavelength point in the
nearby continuum. Gauss equivalent sensitivities are four Gauss for
longitudinal fields and 80 Gauss for transverse fields per wavelength sample.
The LOS velocities are estimated with statistical errors of the order of 5-40
m/s. The design, calibration and integration phases of the instrument, together
with the implemented data reduction scheme are described in some detail.Comment: 17 figure
Mancha3D code: Multi-purpose Advanced Non-ideal MHD Code for High resolution simulations in Astrophysics
The Mancha3D code is a versatile tool for numerical simulations of
magnetohydrodynamic processes in solar/stellar atmospheres. The code includes
non-ideal physics derived from plasma partial ionization, a realistic equation
of state and radiative transfer, which allows performing high quality realistic
simulations of magneto-convection, as well as idealized simulations of
particular processes, such as wave propagation, instabilities or energetic
events. The paper summarizes the equations and methods used in the Mancha3D
code. It also describes its numerical stability and parallel performance and
efficiency. The code is based on a finite difference discretization and
memory-saving Runge-Kutta (RK) scheme. It handles non-ideal effects through
super-time stepping and Hall diffusion schemes, and takes into account thermal
conduction by solving an additional hyperbolic equation for the heat flux. The
code is easily configurable to perform different kinds of simulations. Several
examples of the code usage are given. It is demonstrated that splitting
variables into equilibrium and perturbation parts is essential for simulations
of wave propagation in a static background. A perfectly matched layer (PML)
boundary condition built into the code greatly facilitates a non-reflective
open boundary implementation. Spatial filtering is an important numerical
remedy to eliminate grid-size perturbations enhancing the code stability.
Parallel performance analysis reveals that the code is strongly memory bound,
which is a natural consequence of the numerical techniques used, such as split
variables and PML boundary conditions. Both strong and weak scalings show
adequate performance up till several thousands of CPUs
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