2,035 research outputs found
Hanle effect in the CN violet system with LTE modeling
Weak entangled magnetic fields with mixed polarity occupy the main part of
the quiet Sun. The Zeeman effect diagnostics fails to measure such fields
because of cancellation in circular polarization. However, the Hanle effect
diagnostics, accessible through the second solar spectrum, provides us with a
very sensitive tool for studying the distribution of weak magnetic fields on
the Sun. Molecular lines are very strong and even dominate in some regions of
the second solar spectrum. The CN system is
one of the richest and most promising systems for molecular diagnostics and
well suited for the application of the differential Hanle effect method. The
aim is to interpret observations of the CN
system using the Hanle effect and to obtain an estimation of the magnetic field
strength. We assume that the CN molecular layer is situated above the region
where the continuum radiation is formed and employ the single-scattering
approximation. Together with the Hanle effect theory this provides us with a
model that can diagnose turbulent magnetic fields. We have succeeded in fitting
modeled CN lines in several regions of the second solar spectrum to
observations and obtained a magnetic field strength in the range from 10--30 G
in the upper solar photosphere depending on the considered lines.Comment: Accepted for publication in Astronomy and Astrophysic
Rogue waves in the atmosphere
The appearance of rogue waves is well known in oceanographics, optics, and
cold matter systems. Here we show a possibility for the existence of
atmospheric rogue waves.Comment: 2 pages, 1 figur
Horizontal or vertical magnetic fields on the quiet Sun: Angular distributions and their height variations
Different analyses of identical Hinode SOT/SP data of quiet-sun magnetic
fields have in the past led to contradictory answers to the question of whether
the angular distribution of field vectors is preferentially horizontal or
vertical. These answers have been obtained by combining the measured circular
and linear polarizations in different ways to derive the field inclinations. A
problem with these combinations is that the circular and linear polarizations
scale with field strength in profoundly different ways. Here, we avoid these
problems by using an entirely different approach that is based exclusively on
the fundamental symmetry properties of the transverse Zeeman effect for
observations away from the disk center without any dependence on the circular
polarization. Systematic errors are suppressed by the application of a doubly
differential technique with the 5247-5250 \AA\ line pair for observations with
the ZIMPOL-2 imaging polarimeter on the French THEMIS telescope on Tenerife.
This allows us to determine in a model- and resolution-independent way how the
angular distribution of the intranetwork magnetic fields changes from being
preferentially vertical in the lower and middle photosphere to become
preferentially horizontal in the upper photosphere
On the physical origin of the second solar spectrum of the Sc II line at 4247 A
The peculiar three-peak structure of the linear polarization profile shown in
the second solar spectrum by the Ba II line at 4554 A has been interpreted as
the result of the different contributions coming from the barium isotopes with
and without hyperfine structure (HFS). In the same spectrum, a triple peak
polarization signal is also observed in the Sc II line at 4247 A. Scandium has
a single stable isotope (^{45}Sc), which shows HFS due to a nuclear spin I=7/2.
We investigate the possibility of interpreting the linear polarization profile
shown in the second solar spectrum by this Sc II line in terms of HFS. A
two-level model atom with HFS is assumed. Adopting an optically thin slab
model, the role of atomic polarization and of HFS is investigated, avoiding the
complications caused by radiative transfer effects. The slab is assumed to be
illuminated from below by the photospheric continuum, and the polarization of
the radiation scattered at 90 degrees is investigated. The three-peak structure
of the scattering polarization profile observed in this Sc II line cannot be
fully explained in terms of HFS. Given the similarities between the Sc II line
at 4247 A and the Ba II line at 4554 A, it is not clear why, within the same
modeling assumptions, only the three-peak Q/I profile of the barium line can be
fully interpreted in terms of HFS. The failure to interpret this Sc II
polarization signal raises important questions, whose resolution might lead to
significant improvements in our understanding of the second solar spectrum. In
particular, if the three-peak structure of the Sc II signal is actually
produced by a physical mechanism neglected within the approach considered here,
it will be extremely interesting not only to identify this mechanism, but also
to understand why it seems to be less important in the case of the barium line.Comment: 8 pages, 8 figures, and 1 table. Accepted for publication in
Astronomy and Astrophysic
Scaling laws for magnetic fields on the quiet Sun
The Sun's magnetic field is structured over a range of scales that span
approximately seven orders of magnitudes, four of which lie beyond the
resolving power of current telescopes. Here we have used a Hinode SOT/SP deep
mode data set for the quiet-sun disk center in combination with constraints
from the Hanle effect to derive scaling laws that describe how the magnetic
structuring varies from the resolved scales down to the magnetic diffusion
limit, where the field ceases to be frozen-in. The focus of the analysis is a
derivation of the magnetic energy spectrum, but we also discuss the scale
dependence of the probability density function (PDF) for the flux densities and
the role of the cancellation function for the average unsigned flux density.
Analysis of the Hinode data set with the line-ratio method reveals a collapsed
flux population in the form of flux tubes with a size distribution that is
peaked in the 10-100 km range. Magnetic energy is injected into this scale
range by the instability mechanism of flux tube collapse, which is driven by
the external gas pressure in the superadiabatic region at the top of the
convection zone. This elevates the magnetic energy spectrum just beyond the
telescope resolution limit. Flux tube decay feeds an inertial range that
cascades down the scale spectrum to the magnetic diffusion limit, and which
contains the tangled, "hidden" flux that is known to exist from observations of
the Hanle effect. The observational constraints demand that the total magnetic
energy in the hidden flux must be of the same order as the total energy in the
kG flux tubes. Both the flux tubes and the hidden flux are found to be
preferentially located in the intergranular lanes, which is to be expected
since they are physically related.Comment: accepted for publication in Astronomy & Astrophysic
Measuring the Hidden Aspects of Solar Magnetism
2008 marks the 100th anniversary of the discovery of astrophysical magnetic
fields, when George Ellery Hale recorded the Zeeman splitting of spectral lines
in sunspots. With the introduction of Babcock's photoelectric magnetograph it
soon became clear that the Sun's magnetic field outside sunspots is extremely
structured. The field strengths that were measured were found to get larger
when the spatial resolution was improved. It was therefore necessary to come up
with methods to go beyond the spatial resolution limit and diagnose the
intrinsic magnetic-field properties without dependence on the quality of the
telescope used. The line-ratio technique that was developed in the early 1970s
revealed a picture where most flux that we see in magnetograms originates in
highly bundled, kG fields with a tiny volume filling factor. This led to
interpretations in terms of discrete, strong-field magnetic flux tubes embedded
in a rather field-free medium, and a whole industry of flux tube models at
increasing levels of sophistication. This magnetic-field paradigm has now been
shattered with the advent of high-precision imaging polarimeters that allow us
to apply the so-called "Second Solar Spectrum" to diagnose aspects of solar
magnetism that have been hidden to Zeeman diagnostics. It is found that the
bulk of the photospheric volume is seething with intermediately strong, tangled
fields. In the new paradigm the field behaves like a fractal with a high degree
of self-similarity, spanning about 8 orders of magnitude in scale size, down to
scales of order 10 m.Comment: To appear in "Magnetic Coupling between the Interior and the
Atmosphere of the Sun", eds. S.S. Hasan and R.J. Rutten, Astrophysics and
Space Science Proceedings, Springer-Verlag, Heidelberg, Berlin, 200
The Scattering Polarization of the Sr I 4607 \AA Line at the Diffraction Limit Resolution of a 1-m Telescope
One of the greatest challenges in solar and stellar physics in coming years
will be to observe the Second Solar Spectrum with a spatial resolution
significantly better than 1 arcsec. This type of scattering polarization
observations would probably allow us to discover hitherto unknown aspects of
the Sun's hidden magnetism. Here we report on some theoretical predictions for
the photospheric line of Sr I at 4607 \AA, which we have obtained by solving
the three-dimensional (3D) radiative transfer problem of scattering line
polarization in a realistic hydrodynamical model of the solar photosphere. We
have taken into account not only the anisotropy of the radiation field in the
3D medium and the Hanle effect of a tangled magnetic field, but also the
symmetry breaking effects caused by the horizontal atmospheric inhomogeneities
produced by the solar surface convection. Interestingly, the Q/I and U/I linear
polarization signals of the emergent spectral line radiation have sizable
values and fluctuations, even at the very center of the solar disk where we
meet the forward scattering case. The ensuing small-scale patterns in Q/I and
U/I turn out to be sensitive to the assumed magnetic field model, and are of
great diagnostic value. We argue that it should be possible to observe them
with the help of a 1-m telescope equipped with adaptive optics and a suitable
polarimeter.Comment: Accepted for publication in The Astrophysical Journal Letters (12
pages and 2 color figures
Spectral Properties of Acoustic Gravity Wave Turbulence
The nonlinear turbulent interactions between acoustic gravity waves are
investigated using two dimensional nonlinear fluid simulations. The acoustic
gravity waves consist of velocity and density perturbations and propagate
across the density gradients in the vertical direction in the Earth's
atmosphere. We find that the coupled two component model exhibits generation of
large scale velocity potential flows along the vertical direction, while the
density perturbations relax towards an isotropic random distribution. The
characteristic turbulent spectrum associated with the system has a
Kolmogorov-like feature and tends to relax towards a spectrum, where
is a typical wavenumber. The cross field diffusion associated with the
velocity potential grows linearly and saturates in the nonlinear phase
Iterated function systems with a given continuous stationary distribution
For any continuous probability measure on we construct an
IFS with probabilities having as its unique measure-attractor.Comment: 7 pages, 3 figure
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