1,601 research outputs found
Magneto-acoustic waves in sunspots from observations and numerical simulations
We study the propagation of waves from the photosphere to the chromosphere of
sunspots. From time series of cospatial Ca II H (including its line blends)
intensity spectra and polarimetric spectra of Si I 1082.7 nm and He I 1083.0 nm
we retrieve the line-of-sight velocity at several heights. The analysis of the
phase difference and amplification spectra shows standing waves for frequencies
below 4 mHz and propagating waves for higher frequencies, and allows us to
infer the temperature and height where the lines are formed. Using these
observational data, we have constructed a model of sunspot, and we have
introduced the velocity measured with the photospheric Si I 1082.7 nm line as a
driver. The numerically propagated wave pattern fits reasonably well with the
observed using the lines formed at higher layers, and the simulations reproduce
many of the observed features. The observed waves are slow MHD waves
propagating longitudinally along field lines.Comment: proceedings of GONG 2010/SOHO 24 meeting, June 27 - July 2, 2010,
Aix-en-Provence, Franc
Numerical simulations of conversion to Alfven waves in sunspots
We study the conversion of fast magneto-acoustic waves to Alfven waves by
means of 2.5D numerical simulations in a sunspot-like magnetic configuration. A
fast, essentially acoustic, wave of a given frequency and wave number is
generated below the surface and propagates upward though the Alfven/acoustic
equipartition layer where it splits into upgoing slow (acoustic) and fast
(magnetic) waves. The fast wave quickly reflects off the steep Alfven speed
gradient, but around and above this reflection height it partially converts to
Alfven waves, depending on the local relative inclinations of the background
magnetic field and the wavevector. To measure the efficiency of this conversion
to Alfven waves we calculate acoustic and magnetic energy fluxes. The
particular amplitude and phase relations between the magnetic field and
velocity oscillations help us to demonstrate that the waves produced are indeed
Alfven waves. We find that the conversion to Alfven waves is particularly
important for strongly inclined fields like those existing in sunspot
penumbrae. Equally important is the magnetic field orientation with respect to
the vertical plane of wave propagation, which we refer to as "field azimuth".
For field azimuth less than 90 degrees the generated Alfven waves continue
upwards, but above 90 degrees downgoing Alfven waves are preferentially
produced. This yields negative Alfven energy flux for azimuths between 90 and
180 degrees. Alfven energy fluxes may be comparable to or exceed acoustic
fluxes, depending upon geometry, though computational exigencies limit their
magnitude in our simulations.Comment: Accepted for publication in Ap
Magneto-acoustic waves in sunspots: first results from a new 3D nonlinear magnetohydrodynamic code
Waves observed in the photosphere and chromosphere of sunspots show complex
dynamics and spatial patterns. The interpretation of high-resolution sunspot
wave observations requires modeling of three-dimensional non-linear wave
propagation and mode transformation in the sunspot upper layers in realistic
spot model atmospheres. Here we present the first results of such modeling. We
have developed a 3D non-linear numerical code specially designed to calculate
the response of magnetic structures in equilibrium to an arbitrary
perturbation. The code solves the 3D nonlinear MHD equations for perturbations;
it is stabilized by hyper-diffusivity terms and is fully parallelized. The
robustness of the code is demonstrated by a number of standard tests. We
analyze several simulations of a sunspot perturbed by pulses of different
periods at subphotospheric level, from short periods, introduced for academic
purposes, to longer and realistic periods of three and five minutes. We present
a detailed description of the three-dimensional mode transformation in a
non-trivial sunspot-like magnetic field configuration, including the conversion
between fast and slow magneto-acoustic waves and the Alfv\'en wave, by
calculation of the wave energy fluxes. Our main findings are the following: (1)
the conversion from acoustic to the Alfv\'en mode is only observed if the the
driving pulse is located out of the sunspot axis, but this conversion is
energetically inefficient; (2) as a consequence of the cut-off effects and
refraction of the fast magneto-acoustic mode, the energy of the evanescent
waves with periods around 5 minutes remains almost completely below the level
beta=1; (3) waves with frequencies above the cut-off propagate field-aligned to
the chromosphere and their power becomes dominating over that of evanescent
5-minute oscillations, in agreement with observations
Rayleigh-Taylor instability in partially ionized compressible plasmas: one fluid approach
We study the modification of the classical criterion for the linear onset and
growth rate of the Rayleigh-Taylor instability (RTI) in a partially ionized
(PI) plasma in the one-fluid description, considering a generalized induction
equation. The governing linear equations and appropriate boundary conditions,
including gravitational terms, are derived and applied to the case of the RTI
in a single interface between two partially ionized plasmas. The boundary
conditions lead to an equation for the frequencies in which some of them have
positive complex parts, marking the appearance of the RTI. We study the
ambipolar term alone first, extending the result to the full induction equation
later. We find that the configuration is always unstable because of the
presence of a neutral species. In the classical stability regime the growth
rate is small, since the collisions prevent the neutral fluid to fully develop
the RTI. For parameters in the classical instability regime the growth rate is
lowered, but for the considered theoretical values of the collision frequencies
and diffusion coefficients for solar prominences the differences with the
compressible MHD case are small. We conclude that PI modifies some aspects of
the linear RTI instability, since it takes into account that neutrals do not
feel the stabilizing effect of the magnetic field. For the set of parameters
representative for solar prominences, our model gives the resulting timescale
comparable with observed lifetimes of RTI plumes.Comment: Accepted for publication in Astronomy & Astrophysic
Spiral-shaped wavefronts in a sunspot umbra
Solar active regions show a wide variety of oscillatory phenomena. The
presence of the magnetic field leads to the appearance of several wave modes,
whose behavior is determined by the sunspot thermal and magnetic structure. We
aim to study the relation between the umbral and penumbral waves observed at
the high photosphere and the magnetic field topology of the sunspot.
Observations of the sunspot in active region NOAA 12662 obtained with the
GREGOR telescope (Observatorio del Teide, Spain) were acquired on 2017 June 17.
The data set includes a temporal series in the Fe I 5435 \AA\ line obtained
with the imaging spectrograph GREGOR Fabry-P\'erot Interferometer (GFPI) and a
spectropolarimetric raster map acquired with the GREGOR Infrared Spectrograph
(GRIS) in the 10830 \AA\ spectral region. The Doppler velocity deduced from the
restored Fe I 5435 \AA\ line has been determined, and the magnetic field vector
of the sunspot has been inferred from spectropolarimetric inversions of the Ca
I 10839 \AA\ and the Si I 10827 \AA\ lines. A two-armed spiral wavefront has
been identified in the evolution of the two-dimensional velocity maps from the
Fe I 5435 \AA\ line. The wavefronts initially move counterclockwise in the
interior of the umbra, and develop into radially outward propagating running
penumbral waves when they reach the umbra-penumbra boundary. The horizontal
propagation of the wavefronts approximately follows the direction of the
magnetic field, which shows changes in the magnetic twist with height and
horizontal position. The spiral wavefronts are interpreted as the visual
pattern of slow magnetoacoustic waves which propagate upward along magnetic
field lines. Their apparent horizontal propagation is due to their sequential
arrival to different horizontal positions at the formation height of the Fe I
5435 \AA\ line, as given by the inclination and orientation of the magnetic
field.Comment: Accepted for publication in A&
Multi-layer study of wave propagation in sunspots
We analyze the propagation of waves in sunspots from the photosphere to the
chromosphere using time series of co-spatial Ca II H intensity spectra
(including its line blends) and polarimetric spectra of Si I 10827 and the He I
10830 multiplet. From the Doppler shifts of these lines we retrieve the
variation of the velocity along the line-of-sight at several heights. Phase
spectra are used to obtain the relation between the oscillatory signals. Our
analysis reveals standing waves at frequencies lower than 4 mHz and a
continuous propagation of waves at higher frequencies, which steepen into
shocks in the chromosphere when approaching the formation height of the Ca II H
core. The observed non-linearities are weaker in Ca II H than in He I lines.
Our analysis suggests that the Ca II H core forms at a lower height than the He
I 10830 line: a time delay of about 20 s is measured between the Doppler signal
detected at both wavelengths. We fit a model of linear slow magnetoacoustic
wave propagation in a stratified atmosphere with radiative losses according to
Newton's cooling law to the phase spectra and derive the difference in the
formation height of the spectral lines. We show that the linear model describes
well the wave propagation up to the formation height of Ca II H, where
non-linearities start to become very important.Comment: Accepted by The Astrophysical Journa
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