6,129 research outputs found
Three-dimensional simulations of solar magneto-convection including effects of partial ionization
Over the last decades, realistic 3D radiative-MHD simulations have become the
dominant theoretical tool for understanding the complex interactions between
the plasma and the magnetic field on the Sun. Most of such simulations are
based on approximations of magnetohydrodynamics, without directly considering
the consequences of the very low degree of ionization of the solar plasma in
the photosphere and bottom chromosphere. The presence of large amount of
neutrals leads to a partial decoupling of the plasma and the magnetic field. As
a consequence of that, a series of non-ideal effects (ambipolar diffusion, Hall
effect and battery effect) arises. The ambipolar effect is the dominant one in
the solar chromosphere. Here we report on the first three-dimensional realistic
simulations of magneto-convection including ambipolar diffusion and battery
effects. The simulations are done using the newly developed Mancha3D code. Our
results reveal that ambipolar diffusion causes measurable effects on the
amplitudes of waves excited by convection in the simulations, on the absorption
of Poynting flux and heating and on the formation of chromospheric structures.
We provide a low limit on the chromospheric temperature increase due to the
ambipolar effect using the simulations with battery-excited dynamo fields.Comment: To appear in Astronomy & Astrophysic
Numerical simulations of quiet Sun magnetic fields seeded by Biermann battery
The magnetic fields of the quiet Sun cover at any time more than 90\% of its
surface and their magnetic energy budget is crucial to explain the thermal
structure of the solar atmosphere. One of the possible origins of these fields
is due to the action of local dynamo in the upper convection zone of the Sun.
Existing simulations of the local solar dynamo require an initial seed field,
and sufficiently high spatial resolution, in order to achieve the amplification
of the seed field to the observed values in the quiet Sun. Here we report an
alternative model of seeding based on the action of the Bierman battery effect.
This effect generates a magnetic field due to the local imbalances in electron
pressure in the partially ionized solar plasma. We show that the battery effect
self-consistently creates from zero an initial seed field of a strength of the
order of micro G, and together with dynamo amplification, allows the generation
of quiet Sun magnetic fields of a similar strength to those from solar
observations.Comment: To appear in Astronomy & Astrophysic
High frequency waves in the corona due to null points
This work aims to understand the behavior of non-linear waves in the vicinity
of a coronal null point. In previous works we have showed that high frequency
waves are generated in such magnetic configuration. This paper studies those
waves in detail in order to provide a plausible explanation of their
generation. We demonstrate that slow magneto-acoustic shock waves generated in
the chromosphere propagate through the null point and produce a train of
secondary shocks that escape along the field lines. A particular combination of
the shock wave speeds generates waves at a frequency of 80 mHz. We speculate
that this frequency may be sensitive to the atmospheric parameters in the
corona and therefore can be used to probe the structure of this solar layer
High-resolution radio imaging of two luminous quasars beyond redshift 4.5
Context. Radio-loud active galactic nuclei in the early Universe are rare.
The quasars J0906+6930 at redshift z=5.47 and J2102+6015 at z=4.57 stand out
from the known sample with their compact emission on milliarcsecond (mas)
angular scale with high (0.1-Jy level) flux densities measured at GHz radio
frequencies. This makes them ideal targets for very long baseline
interferometry (VLBI) observations. Aims. By means of VLBI imaging we can
reveal the inner radio structure of quasars and model their brightness
distribution to better understand the geometry of the jet and the physics of
the sources. Methods. We present sensitive high-resolution VLBI images of
J0906+6930 and J2102+6015 at two observing frequencies, 2.3 and 8.6 GHz. The
data were taken in an astrometric observing programme involving a global
five-element radio telescope array. We combined the data from five different
epochs from 2017 February to August. Results. For one of the highest redshift
blazars known, J0906+6930, we present the first-ever VLBI image obtained at a
frequency below 8 GHz. Based on our images at 2.3 and 8.6 GHz, we confirm that
this source has a sharply bent helical inner jet structure within ~3 mas from
the core. The quasar J2102+6015 shows an elongated radio structure in the
east-west direction within the innermost ~2 mas that can be described with a
symmetric three-component brightness distribution model at 8.6 GHz. Because of
their non-pointlike mas-scale structure, these sources are not ideal as
astrometric reference objects. Our results demonstrate that VLBI observing
programmes conducted primarily with astrometric or geodetic goals can be
utilized for astrophysical purposes as well.Comment: 8 pages, 3 figures, accepted for publication in Astronomy &
Astrophysic
Years of sunlight exposure and cataract: a case-control study in a Mediterranean population.
BACKGROUND: We aimed to investigate the relation between sunlight exposure and risk of cataract. METHODS: We carried out a frequency-matched case-control study of 343 cases and 334 controls attending an ophthalmology outpatient clinic at a primary health-care center in a small town near Valencia, Spain. All cases were diagnosed as having a cataract in at least one eye based on the Lens Opacification Classification system (LOCS II). Controls had no opacities in either eye. All cases and controls were interviewed for information on outdoor exposure, "usual" diet, history of severe episodes of diarrhea illness, life-style factors and medical and socio-demographic variables. Blood antioxidant vitamin levels were also analyzed. We used logistic regression models to estimate sex and age-adjusted odds ratios (ORs) by quintiles of years of occupational outdoor exposure, adjusting for potential confounders such as smoking, alcohol consumption, serum antioxidants and education. RESULTS: No association was found between years of outdoor exposure and risk of cataract. However, exploratory analyses suggested a positive association between years of outdoor exposure at younger ages and risk of nuclear cataract later in life. CONCLUSION: Our study does not support an association with cataract and sunlight exposure over adult life
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