87 research outputs found
Effect of suppressed excitation on the amplitude distribution of 5-min oscillations in sunspots
Five-minute oscillations on the Sun (acoustic and surface gravity waves) are
excited by subsurface turbulent convection. However, in sunspots the excitation
is suppressed because strong magnetic field inhibits convection. We use 3D
simulations to investigate how the suppression of excitation sources affects
the distribution of the oscillation power in sunspot regions. The amplitude of
random acoustic sources was reduced in circular-shaped regions to simulate the
suppression in sunspots. The simulation results show that the amplitude of the
oscillations can be approximately 2-4 times lower in the sunspot regions in
comparison to the quiet Sun, just because of the suppressed sources. Using
SOHO/MDI data we measured the amplitude ratio for the same frequency bands
outside and inside sunspots, and found that this ratio is approximately 3-4.
Hence, the absence of excitation sources inside sunspots makes a significant
contribution (about 50% or higher) to the observed amplitude ratio and must be
taken into account in sunspot seismology.Comment: 12 pages, 5 figures, accepted to ApJ
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
Influence of Non-Uniform Distribution of Acoustic Wavefield Strength on Time-Distance Helioseismology Measurements
By analyzing numerically simulated solar oscillation data, we study the
influence of non-uniform distribution of acoustic wave amplitude, acoustic
source strength, and perturbations of the sound speed on the shifts of acoustic
travel times measured by the time-distance helioseismology method. It is found
that for short distances, the contribution to the mean travel time shift caused
by non-uniform distribution of acoustic sources in sunspots may be comparable
to (but smaller than) the contribution from the sound speed perturbation in
sunspots, and that it has the opposite sign to the sound-speed effect. This
effect may cause some underestimation of the negative sound-speed perturbations
in sunspots just below the surface, that was found in previous time-distance
helioseismology inferences. This effect cannot be corrected by artificially
increasing the amplitude of oscillations in sunspots. For large time-distance
annuli, the non-uniform distribution of wavefields does not have significant
effects on the mean travel times, and thus the sound-speed inversion results.
The measured travel time differences, which are used to determine the mass
flows beneath sunspots, can also be systematically shifted by this effect, but
only in an insignificant magnitude.Comment: 16 pages, 6 figures, accepted for publication in Ap
Numerical simulation of excitation and propagation of helioseismic MHD waves: Effects of inclined magnetic field
Investigation of propagation, conversion, and scattering of MHD waves in the
Sun is very important for understanding the mechanisms of observed oscillations
and waves in sunspots and active regions. We have developed 3D linear MHD
numerical model to investigate influence of the magnetic field on excitation
and properties of the MHD waves. The results show that the magnetic field can
substantially change the properties of the surface gravity waves (f-mode), but
their influence on the acoustic-type waves (p-modes) is rather moderate.
Comparison our simulations with the time-distance helioseismology results from
SOHO/MDI shows that the travel time variations caused by the inclined magnetic
field do not exceed 25% of the observed amplitude even for strong fields of
1400-1900 G. This can be an indication that other effects (e.g. background
flows and non-uniform distribution of magnetic field) can contribute to the
observed travel time variations. The travel time variations caused by the wave
interaction with magnetic field are in phase with the observations for strong
fields of 1400-1900 G if Doppler velocities are taken at the height of 300 km
above the photosphere where plasma parameter beta<<1. The simulations show that
the travel times only weakly depend on the height of velocity observation. For
the photospheric level the travel times are systematically smaller on
approximately 0.12 min then for the hight of 300 km above the photosphere for
all studied ranges of the magnetic field strength and inclination angles. The
numerical MHD wave modeling and new data from the HMI instrument of the Solar
Dynamics Observatory will substantially advance our knowledge of the wave
interaction with strong magnetic fields on the Sun and improve the local
helioseismology diagnostics.Comment: 24 pages, 10 figures, submitted to Ap
Travel Time Shifts due to Amplitude Modulation in Time-Distance Helioseismology
Correct interpretation of acoustic travel times measured by time-distance
helioseismology is essential to get an accurate understanding of the solar
properties that are inferred from them. It has long been observed that sunspots
suppress p-mode amplitude, but its implications on travel times has not been
fully investigated so far. It has been found in test measurements using a
'masking' procedure, in which the solar Doppler signal in a localized quiet
region of the Sun is artificially suppressed by a spatial function, and using
numerical simulations that the amplitude modulations in combination with the
phase-speed filtering may cause systematic shifts of acoustic travel times. To
understand the properties of this procedure, we derive an analytical expression
for the cross-covariance of a signal that has been modulated locally by a
spatial function that has azimuthal symmetry, and then filtered by a phase
speed filter typically used in time-distance helioseismology. Comparing this
expression to the Gabor wavelet fitting formula without this effect, we find
that there is a shift in the travel times, that is introduced by the amplitude
modulation. The analytical model presented in this paper can be useful also for
interpretation of travel time measurements for non-uniform distribution of
oscillation amplitude due to observational effects.Comment: 17 pages, 1 figure, accepted for publication in Ap
Excitation of acoustic waves by vortices in the quiet Sun
Five-minutes oscillations is one of the basic properties of solar convection.
Observations show mixture of a large number of acoustic wave fronts propagating
from their sources. We investigate the process of acoustic waves excitation
from the point of view of individual events, by using realistic 3D radiative
hydrodynamic simulation of the quiet Sun. The results show that the excitation
events are related to dynamics vortex tubes (or swirls) in the intergranular
lanes. These whirlpool-like flows are characterized by very strong horizontal
velocities (7 - 11 km/s) and downflows (~ 7 km/s), and are accompanied by
strong decreases of the temperature, density and pressure at the surface and in
a ~ 0.5-1 Mm deep layer below the surface. High-speed whirlpool flows can
attract and capture other vortices. According to our simulation results, the
processes of the vortex interaction, such as vortex annihilation, can cause the
excitation of acoustic waves.Comment: 10 pages, 5 figure, submitted to ApJ
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
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