345 research outputs found
Temporal power spectra of the horizontal velocity of the solar photosphere
We have derived the temporal power spectra of the horizontal velocity of the
solar photosphere. The data sets for 14 quiet regions observed with the Gband
filter of Hinode/SOT are analyzed to measure the temporal fluctuation of the
horizontal velocity by using the local correlation tracking (LCT) method. Among
the high resolution (~0.2") and seeing-free data sets of Hinode/SOT, we
selected the observations whose duration is longer than 70 minutes and cadence
is about 30 s. The so-called k-{\omega} diagrams of the photospheric horizontal
velocity are derived for the first time to investigate the temporal evolution
of convection. The power spectra derived from k-omega diagrams typically have a
double power law shape bent over at a frequency of 4.7 mHz. The power law index
in the high frequency range is -2.4 while the power law index in the low
frequency range is -0.6. The root mean square of the horizontal speed is about
1.1 km/s when we use a tracer size of 0.4" in LCT method. Autocorrelation
functions of intensity fluctuation, horizontal velocity, and its spatial
derivatives are also derived in order to measure the correlation time of the
stochastic photospheric motion. Since one of possible energy sources of the
coronal heating is the photospheric convection, the power spectra derived in
the present study will be of high value to quantitatively justify various
coronal heating models.Comment: 17 pages, 5 figures, accepted for publication in Astrophysical
Journa
Simulations of Oscillation Modes of the Solar Convection Zone
We use the three-dimensional hydrodynamic code of Stein and Nordlund to
realistically simulate the upper layers of the solar convection zone in order
to study physical characteristics of solar oscillations. Our first result is
that the properties of oscillation modes in the simulation closely match the
observed properties. Recent observations from SOHO/MDI and GONG have confirmed
the asymmetry of solar oscillation line profiles, initially discovered by
Duvall et al. In this paper we compare the line profiles in the power spectra
of the Doppler velocity and continuum intensity oscillations from the SOHO/MDI
observations with the simulation. We also compare the phase differences between
the velocity and intensity data. We have found that the simulated line profiles
are asymmetric and have the same asymmetry reversal between velocity and
intensity as observed. The phase difference between the velocity and intensity
signals is negative at low frequencies and jumps in the vicinity of modes as is
also observed. Thus, our numerical model reproduces the basic observed
properties of solar oscillations, and allows us to study the physical
properties which are not observed.Comment: Accepted for publication in ApJ Letter
Numerical MHD Simulations of Solar Magnetoconvection and Oscillations in Inclined Magnetic Field Regions
The sunspot penumbra is a transition zone between the strong vertical
magnetic field area (sunspot umbra) and the quiet Sun. The penumbra has a fine
filamentary structure that is characterized by magnetic field lines inclined
toward the surface. Numerical simulations of solar convection in inclined
magnetic field regions have provided an explanation of the filamentary
structure and the Evershed outflow in the penumbra. In this paper, we use
radiative MHD simulations to investigate the influence of the magnetic field
inclination on the power spectrum of vertical velocity oscillations. The
results reveal a strong shift of the resonance mode peaks to higher frequencies
in the case of a highly inclined magnetic field. The frequency shift for the
inclined field is significantly greater than that in vertical field regions of
similar strength. This is consistent with the behavior of fast MHD waves.Comment: 9 pages, 6 figures, Solar Physics (in press
What Causes P-mode Asymmetry Reversal?
The solar acoustic p-mode line profiles are asymmetric. Velocity spectra have
more power on the low-frequency sides, whereas intensity profiles show the
opposite sense of asymmetry. Numerical simulations of the upper convection zone
have resonant p-modes with the same asymmetries and asymmetry reversal as the
observed modes. The temperature and velocity power spectra at optical depth
have the opposite asymmetry as is observed for the
intensity and velocity spectra. At a fixed geometrical depth, corresponding to
, however, the temperature and velocity spectra have the
same asymmetry. This indicates that the asymmetry reversal is produced by
radiative transfer effects and not by correlated noise.Comment: 16 pages, 10 figures, submitted to Astrophysical Journa
Comparison of High-degree Solar Acoustic Frequencies and Asymmetry between Velocity and Intensity Data
Using the local helioseismic technique of ring diagram we analyze the
frequencies of high--degree f- and p-modes derived from both velocity and
continuum intensity data observed by MDI. Fitting the spectra with asymmetric
peak profiles, we find that the asymmetry associated with velocity line
profiles is negative for all frequency ranges agreeing with previous
observations while the asymmetry of the intensity profiles shows a complex and
frequency dependent behavior. We also observe systematic frequency differences
between intensity and velocity spectra at the high end of the frequency range,
mostly above 4 mHz. We infer that this difference arises from the fitting of
the intensity rather than the velocity spectra. We also show that the frequency
differences between intensity and velocity do not vary significantly from the
disk center to the limb when the spectra are fitted with the asymmetric profile
and conclude that only a part of the background is correlated with the
intensity oscillations.Comment: Accepted for publication in Astrophysical Journa
Imaging the Solar Tachocline by Time-Distance Helioseismology
The solar tachocline at the bottom of the convection zone is an important
region for the dynamics of the Sun and the solar dynamo. In this region, the
sound speed inferred by global helioseismology exhibits a bump of approximately
0.4% relative to the standard solar model. Global helioseismology does not
provide any information on possible latitudinal variations or asymmetries
between the Northern and Southern hemisphere. Here, we develop a time-distance
helioseismology technique, including surface- and deep-focusing measurement
schemes and a combination of both, for two-dimensional tomographic imaging of
the solar tachocline that infers radial and latitudinal variations in the sound
speed. We test the technique using artificial solar oscillation data obtained
from numerical simulations. The technique successfully recovers major features
of the simplified tachocline models. The technique is then applied to SOHO/MDI
medium-l data and provides for the first time a full two-dimensional
sound-speed perturbation image of the solar tachocline. The one-dimensional
radial profile obtained by latitudinal averaging of the image is in good
agreement with the previous global helioseismology result. It is found that the
amplitude of the sound-speed perturbation at the tachocline varies with
latitude, but it is not clear whether this is in part or fully an effect of
instrumental distortion. Our initial results demonstrate that time-distance
helioseismology can be used to probe the deep interior structure of the Sun,
including the solar tachocline.Comment: accepted for publication by Ap
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