118 research outputs found
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
High-pressure effects on the optical-absorption edge of CdIn2S4, MgIn2S4, and MnIn2S4 thiospinels
The effect of pressure on the optical-absorption edge of CdIn2S4, MgIn2S4,
and MnIn2S4 thiospinels at room temperature is investigated up to 20 GPa. The
pressure dependence of their band-gaps has been analyzed using the Urbach rule.
We have found that, within the pressure-range of stability of the low-pressure
spinel phase, the band-gap of CdIn2S4 and MgIn2S4 exhibits a linear blue-shift
with pressure, whereas the band-gap of MnIn2S4 exhibits a pronounced non-linear
shift. In addition, an abrupt decrease of the band-gap energies occurs in the
three compounds at pressures of 10 GPa, 8.5 GPa, and 7.2 GPa, respectively.
Beyond these pressures, the optical-absorption edge red-shifts upon compression
for the three studied thiospinels. All these results are discussed in terms of
the electronic structure of each compound and their reported structural
changes.Comment: 18 pages, 3 figure
Excitation of solar-like oscillations across the HR diagram
We extend semi-analytical computations of excitation rates for solar
oscillation modes to those of other solar-like oscillating stars to compare
them with recent observations. Numerical 3D simulations of surface convective
zones of several solar-type oscillating stars are used to characterize the
turbulent spectra as well as to constrain the convective velocities and
turbulent entropy fluctuations in the uppermost part of the convective zone of
such stars. These constraints, coupled with a theoretical model for stochastic
excitation, provide the rate 'P' at which energy is injected into the p-modes
by turbulent convection. These energy rates are compared with those derived
directly from the 3D simulations. The excitation rates obtained from the 3D
simulations are systematically lower than those computed from the
semi-analytical excitation model. We find that Pmax, the excitation rate
maximum, scales as (L/M)^s where s is the slope of the power law and L and M
are the mass and luminosity of the 1D stellar model built consistently with the
associated 3D simulation. The slope is found to depend significantly on the
adopted form of the eddy time-correlation ; using a Lorentzian form results in
s=2.6, whereas a Gaussian one gives s=3.1. Finally, values of Vmax, the maximum
in the mode velocity, are estimated from the computed power laws for Pmax and
we find that Vmax increases as (L/M)^sv. Comparisons with the currently
available ground-based observations show that the computations assuming a
Lorentzian eddy time-correlation yield a slope, sv, closer to the observed one
than the slope obtained when assuming a Gaussian. We show that the spatial
resolution of the 3D simulations must be high enough to obtain accurate
computed energy rates.Comment: 14 pages ; 7 figures ; accepted for publication in Astrophysics &
Astronom
Solar Flux Emergence Simulations
We simulate the rise through the upper convection zone and emergence through
the solar surface of initially uniform, untwisted, horizontal magnetic flux
with the same entropy as the non-magnetic plasma that is advected into a domain
48 Mm wide from from 20 Mm deep. The magnetic field is advected upward by the
diverging upflows and pulled down in the downdrafts, which produces a hierarchy
of loop like structures of increasingly smaller scale as the surface is
approached. There are significant differences between the behavior of fields of
10 kG and 20 or 40 kG strength at 20 Mm depth. The 10 kG fields have little
effect on the convective flows and show little magnetic buoyancy effects,
reaching the surface in the typical fluid rise time from 20 Mm depth of 32
hours. 20 and 40 kG fields significantly modify the convective flows, leading
to long thin cells of ascending fluid aligned with the magnetic field and their
magnetic buoyancy makes them rise to the surface faster than the fluid rise
time. The 20 kG field produces a large scale magnetic loop that as it emerges
through the surface leads to the formation of a bipolar pore-like structure.Comment: Solar Physics (in press), 12 pages, 13 figur
- …