55 research outputs found
The power spectrum of solar convection flows from high-resolution observations and 3D simulations
We compare Fourier spectra of photospheric velocity fields from very high
resolution IMaX observations to those from recent 3D numerical
magnetoconvection models. We carry out a proper comparison by synthesizing
spectral lines from the numerical models and then applying to them the adequate
residual instrumental degradation that affects the observational data. Also,
the validity of the usual observational proxies is tested by obtaining
synthetic observations from the numerical boxes and comparing the velocity
proxies to the actual velocity values from the numerical grid.
For the observations, data from the SUNRISE/IMaX instrument with about 120 km
spatial resolution are used, thus allowing the calculation of observational
Fourier spectra well into the subgranular range. For the simulations, we use
four series of runs obtained with the STAGGER code and synthesize the IMaX
spectral line (FeI 5250.2 A) from them. Proxies for the velocity field are
obtained via Dopplergrams (vertical component) and local correlation tracking
(horizontal component).
A very good match between observational and simulated Fourier power spectra
is obtained for the vertical velocity data for scales between 200 km and 6 Mm.
Instead, a clear vertical shift is obtained when the synthetic observations are
not degraded. The match for the horizontal velocity data is much less
impressive because of the inaccuracies of the LCT procedure. Concerning the
internal comparison of the direct velocity values of the numerical boxes with
those from the synthetic observations, a high correlation (0.96) is obtained
for the vertical component when using the velocity values on the
log() = -1 surface in the box. The corresponding Fourier spectra are
near each other. A lower maximum correlation (0.5) is reached (at =
1) for the horizontal velocities as a result of the coarseness of the LCT
procedure.Comment: 12 pages, 9 figures, accepted in A&
Analysis of altimetric movements of the seismic region of Chlef by Monte Carlo method
Comunicación presntada a la 3ª Asamblea Hispano-Portuguesa de Geodesia y Geofísica = 3ª Assembleia Luso-Espanhola de Geodesia e Geofisica, celebrada en Valencia entre el 4 y el 8 de febrero de 2002.1n the region o/ Ch/ef (ex El AsnC/l17,Northwest ofAlgeriat numerous geodesi campaigns have been mude for the
study ofterrestrial crustal movements associated 10 the S ismicity of the region.
The results of the works of different timesof leveling measure.ss..¿ J of j 976, /986, /987 are comparad with the
results obtained in 198/, /988 and /98, rhey confirm the tendency lo ,VW-SE overthrust and they indicate the
persistence ofre/ative 1110Vel17el1i/ns the same sense.
The evaluation and the simultaneous representa/ion of these deformations and o/ their errors are made by the
method of Monte Carlo that allows /0 simulate a great number ofseries ofmeasurec_c' e' This method has pul in
evidence the existence of significant movements in 90% of the simulations of Monte Carlo about {he speed of the
altimetric displacements of different sections of leveling net in the region.
In this work the results obtained in the different catnpaign are presentedPeer reviewe
Nonlinear force-free modelling: influence of inaccuracies in the measured magnetic vector
Context: Solar magnetic fields are regularly extrapolated into the corona
starting from photospheric magnetic measurements that can suffer from
significant uncertainties. Aims: Here we study how inaccuracies introduced into
the maps of the photospheric magnetic vector from the inversion of ideal and
noisy Stokes parameters influence the extrapolation of nonlinear force-free
magnetic fields. Methods: We compute nonlinear force-free magnetic fields based
on simulated vector magnetograms, which have been produced by the inversion of
Stokes profiles, computed froma 3-D radiation MHD simulation snapshot. These
extrapolations are compared with extrapolations starting directly from the
field in the MHD simulations, which is our reference. We investigate how line
formation and instrumental effects such as noise, limited spatial resolution
and the effect of employing a filter instrument influence the resulting
magnetic field structure. The comparison is done qualitatively by visual
inspection of the magnetic field distribution and quantitatively by different
metrics. Results: The reconstructed field is most accurate if ideal Stokes data
are inverted and becomes less accurate if instrumental effects and noise are
included. The results demonstrate that the non-linear force-free field
extrapolation method tested here is relatively insensitive to the effects of
noise in measured polarization spectra at levels consistent with present-day
instruments. Conclusions heading: Our results show that we can reconstruct the
coronal magnetic field as a nonlinear force-free field from realistic
photospheric measurements with an accuracy of a few percent, at least in the
absence of sunspots.Comment: A&A, accepted, 9 Pages, 4 Figure
Expansion of magnetic flux concentrations: a comparison of Hinode SOT d ata and models
Context: The expansion of network magnetic fields with height is a
fundamental property of flux tube models. A rapid expansion is required to form
a magnetic canopy. Aims: We characterize the observed expansion properties of
magnetic network elements and compare them with the thin flux tube and sheet
approximations, as well as with magnetoconvection simulations. Methods: We used
data from the Hinode SOT NFI NaD1 channel and spectropolarimeter to study the
appearance of magnetic flux concentrations seen in circular polarization as a
function of position on the solar disk. We compared the observations with
synthetic observables from models based on the thin flux tube approximation and
magnetoconvection simulations with two different upper boundary conditions for
the magnetic field (potential and vertical). Results: The observed circular
polarization signal of magnetic flux concentrations changes from unipolar at
disk center to bipolar near the limb, which implies an expanding magnetic
field. The observed expansion agrees with expansion properties derived from the
thin flux sheet and tube approximations. Magnetoconvection simulations with a
potential field as the upper boundary condition for the magnetic field also
produce bipolar features near the limb while a simulation with a vertical field
boundary condition does not. Conclusions: The near-limb apparent bipolar
magnetic features seen in high-resolution Hinode observations can be
interpreted using a simple flux sheet or tube model. This lends further support
to the idea that magnetic features with vastly varying sizes have similar
relative expansion rates. The numerical simulations presented here are less
useful in interpreting the expansion since the diagnostics we are interested in
are strongly influenced by the choice of the upper boundary condition for the
magnetic field in the purely photospheric simulations.Comment: accepted for publication in A&
Simulation of a flux emergence event and comparison with observations by Hinode
We study the observational signature of flux emergence in the photosphere
using synthetic data from a 3D MHD simulation of the emergence of a twisted
flux tube. Several stages in the emergence process are considered. At every
stage we compute synthetic Stokes spectra of the two iron lines Fe I 6301.5
{\AA} and Fe I 6302.5 {\AA} and degrade the data to the spatial and spectral
resolution of Hinode's SOT/SP. Then, following observational practice, we apply
Milne-Eddington-type inversions to the synthetic spectra in order to retrieve
various atmospheric parameters and compare the results with recent Hinode
observations. During the emergence sequence, the spectral lines sample
different parts of the rising flux tube, revealing its twisted structure. The
horizontal component of the magnetic field retrieved from the simulations is
close to the observed values. The flattening of the flux tube in the
photosphere is caused by radiative cooling, which slows down the ascent of the
tube to the upper solar atmosphere. Consistent with the observations, the
rising magnetized plasma produces a blue shift of the spectral lines during a
large part of the emergence sequence.Comment: A&A Letter, 3 figure
Decay of a simulated mixed-polarity magnetic field in the solar surface layers
Magnetic flux is continuously being removed and replenished on the solar
surface. To understand the removal process we carried out 3D radiative MHD
simulations of the evolution of patches of photospheric magnetic field with
equal amounts of positive and negative flux. We find that the flux is removed
at a rate corresponding to an effective turbulent diffusivity, of 100--340
km^2/s, depending on the boundary conditions. For average unsigned flux
densities above about 70 Gauss, the percentage of surface magnetic energy
coming from different field strengths is almost invariant. The overall process
is then one where magnetic elements are advected by the horizontal granular
motions and occasionally come into contact with opposite-polarity elements.
These reconnect above the photosphere on a comparatively short time scale after
which the U loops produced rapidly escape through the upper surface while the
downward retraction of inverse-U loops is significantly slower, because of the
higher inertia and lower plasma beta in the deeper layers.Comment: 8 pages, 10 figures accepted in A&
Magneto-acoustic waves in a gravitationally stratified magnetized plasma: eigen-solutions and their applications to the solar atmosphere
Magneto-acoustic gravity (MAG) waves have been studied intensively in the context of astrophysical plasmas. There are three popular choices of analytic modeling using a Cartesian coordinate system: a magnetic field parallel, perpendicular, or at an angle to the gravitational field. Here, we study a gravitationally stratified plasma embedded in a parallel, so called vertical, magnetic field. We find a governing equation for the auxiliary quantity Θ = p 1/ρ 0, and find solutions in terms of hypergeometric functions. With the convenient relationship between Θ and the vertical velocity component, v z , we derive the solution for v z . We show that the four linearly independent functions for v z can also be cast as single hypergeometric functions, rather than the Frobenius series derived by Leroy & Schwartz. We are then able to analyze a case of approximation for a one-layer solution, taking the small wavelength limit. Motivated by solar atmospheric applications, we finally commence study of the eigenmodes of perturbations for a two-layer model using our solutions, solving the dispersion relation numerically. We show that, for a transition between a photospheric and chromospheric plasma embedded in a vertical magnetic field, modes exist that are between the observationally widely investigated three and five minute oscillation periods, interpreted as solar global oscillations in the lower solar atmosphere. It is also shown that, when the density contrast between the layers is large (e.g., applied to photosphere/chromosphere-corona), the global eigenmodes are practically a superposition of the same as in each of the separate one-layer systems
Power spectrum of turbulent convection in the solar photosphere
The solar photosphere provides us with a laboratory for understanding
turbulence in a layer where the fundamental processes of transport vary rapidly
and a strongly superadiabatic region lies very closely to a subadiabatic layer.
Our tools for probing the turbulence are high-resolution spectropolarimetric
observations such as have recently been obtained with the two sunrise missions,
and numerical simulations. Our aim is to study photospheric turbulence with the
help of Fourier power spectra that we compute from observations and
simulations. We also attempt to explain some properties of the photospheric
overshooting flow with the help of its governing equations and simulations. We
find that quiet-Sun observations and smeared simulations exhibit a power-law
behavior in the subgranular range of their Doppler velocity power spectra with
an index of. The unsmeared simulations exhibit a power-law index
of. The smearing considerably reduces the extent of the
power-law-like portion of the spectra. Therefore, the limited spatial
resolution in some observations might eventually result in larger uncertainties
in the estimation of the power-law indices.
The simulated vertical velocity power spectra as a function of height show a
rapid change in the power-law index from the solar surface to ~km above
it. A scale-dependent transport of the vertical momentum occurs. At smaller
scales, the vertical momentum is more efficiently transported sideways than at
larger scales. This results in less vertical velocity power transported upward
at small scales than at larger scales and produces a progressively steeper
vertical velocity power law below km. Above this height, the gravity work
progressively gains importance at all scales, making the atmosphere
progressively more hydrostatic and resulting in a gradually less steep power
law.Comment: 10 pages, 7 figures, Accepted in A and
Tsunami hazards in the Catalan Coast, a low-intensity seismic activity area
The final publication is available at Springer via http://dx.doi.org/10.1007/s11069-017-2918-zThe potential impacts of tsunamis along the Catalan Coast (NW Mediterranean) are analysed using numerical modelling. The region is characterized by moderate to low seismic activity and by moderate- to low-magnitude earthquakes. However, the occurrence of historical strong earthquakes and the location of several active offshore faults in front of the coast suggest that the possibility of an earthquake-triggered tsunami is not negligible although of low probability. Up to five faults have been identified to generate tsunamis, being the highest associated possible seismic magnitudes of up to 7.6. Coastal flooding and port agitation are characterized using the Worst-case Credible Tsunami Scenario Analysis approach. The results show a multiple fault source contribution to tsunami hazard. The shelf dimensions and the existence of submerged canyons control the tsunami propagation. In wide shelves, waves travelling offshore may become trapped by refraction causing the wave energy to reach the coastline at some distance from the origin. The free surface water elevation increases at the head of the canyons due to the sharp depth gradients. The effects of potential tsunamis would be very harmful in low-lying coastal stretches, such as deltas, with a high population concentration, assets and infrastructures. The Ebro delta appears to be the most exposed coast, and about the 20% of the delta surface is prone to flooding due to its extremely low-lying nature. The activity at Barcelona port will be severely affected by inflow backflow current at the entrance of up to 2 m/s.Peer ReviewedPostprint (author's final draft
The Algiers (north central Algeria) earthquake of August 1st, 2014 Mw 5.5 in the Algiers Bay tectonic context
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