Dali Georgobiani

Excitation of solar-like oscillations across the HR diagram

Aims. 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 Methods. 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. Results. The excitation rates obtained from the 3D simulations are systematically lower than those computed from the semi-analytical excitation model. We find that Pmax ,t heP 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 χk, the eddy time-correlation; using a Lorentzian, χ L , results in s = 2.6, whereas a Gaussian, χ G ,g ivess = 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 χk 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.

Validation of time-distance helioseismology by use of realistic simulations of solar convection

Recent progress in realistic simulations of solar convection have given us an unprecedented opportunity to evaluate the robustness of solar interior structures and dynamics obtained by methods of local helioseismology. We present results of testing the time-distance method using realistic simulations. By computing acoustic wave propagation time and distance relations for different depths of the simulated data, we confirm that acoustic waves propagate into the interior and then turn back to the photosphere. This demonstrates that in numerical simulations properties of acoustic waves (p-modes) are similar to the solar conditions, and that these properties can be analyzed by the time-distance technique. For surface gravity waves (f-modes), we calculate perturbations of their travel times caused by localized downdrafts and demonstrate that the spatial pattern of these perturbations (representing so-called sensitivity kernels) is similar to the patterns obtained from the real Sun, displaying characteristic hyperbolic structures. We then test time-distance measurements and inversions by calculating acoustic travel times from a sequence of vertical velocities at the photosphere of the simulated data and inferring mean three-dimensional flow fields by performing inversion based on the ray approximation. The inverted horizontal flow fields agree very well with the simulated data in subsurface areas up to 3 Mm deep, but differ in deeper areas. Due to the cross talk effects between the horizontal divergence and downward flows, the inverted vertical velocities are significantly different from the mean convection velocities of the simulation data set. These initial tests provide important validation of time-distance helioseismology measurements of supergranular-scale convection, illustrate limitations of this technique, and provide guidance for future improvements.

Local helioseismology and correlation tracking analysis of surface structures in realistic simulations of solar convection

We apply time-distance helioseismology, local correlation tracking, and Fourier spatial-temporal filtering methods to realistic supergranule scale simulations of solar convection and compare the results with high-resolution observations from the Solar and Heliospheric Observatory (SOHO) Michelson Doppler Imager (MDI). Our objective is to investigate the surface and subsurface convective structures and test helioseismic measurements. The size and grid of the computational domain are sufficient to resolve various convective scales from granulation to supergranulation. The spatial velocity spectrum is approximately a power law for scales larger than granules, with a continuous decrease in velocity amplitude with increasing size. Aside from granulation no special scales exist, although a small enhancement in power at supergranulation scales can be seen. We calculate the time-distance diagram for f- and p-modes and show that it is consistent with the SOHO MDI observations. From the simulation data we calculate travel-time maps for surface gravity waves (f-mode). We also apply correlation tracking to the simulated vertical velocity in the photosphere to calculate the corresponding horizontal flows. We compare both of these to the actual large-scale (filtered) simulation velocities. All three methods reveal similar large-scale convective patterns and provide an initial test of time-distance methods.

Solar Convection: Comparison of Numerical Simulations and Mixing-Length Theory

We compare the results of realistic numerical simulations of convection in the superadiabatic layer near the solar surface with the predictions of mixing-length theory. We find that the peak values of such quantities as the temperature gradient, the temperature fluctuations, and the velocity fluctuations, as well as the entropy jump in the simulation, can be reproduced by mixing-length theory for a ratio of mixing length to pressure scale height α ≈ 1.5. However, local mixing-length theory neither reproduces the profiles of these variables with depth nor allows penetration of convective motions into the overlying stable photosphere.

Excitation of Radial P-Modes in the Sun and Stars

P-mode oscillations in the Sun and stars are excited stochastically by Reynolds stress and entropy fluctuations produced by convection in their outer envelopes. The excitation rate of radial oscillations of stars near the main sequence from K to F and a subgiant K IV star have been calculated from numerical simulations of their surface convection zones. P-mode excitation increases with increasing effective temperature (until envelope convection ceases in the F stars) and also increases with decreasing gravity. The frequency of the maximum excitation decreases with decreasing surface gravity.

Excitation of solar-like oscillations: From PMS to MS stellar models

The amplitude of solar-like oscillations results from a balance between excitation and damping. As in the sun, the excitation is attributed to turbulent motions that stochastically excite thep modes in the uppermost part of the convective zone. We present here a model for the excitation mechanism. Comparisons between modeled amplitudes and helio and stellar seismic constraints are presented and the discrepancies discussed. Finally the possibility and the interest of detecting such stochastically excited modes in pre-main sequence stars are also discussed.

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 τcont = 1 have the opposite asymmetry, as is observed for the intensity and velocity spectra. At a fixed geometrical depth, corresponding to = 1, however, the temperature and velocity spectra have the same asymmetry. This indicates that the asymmetry reversal in the simulation is produced by radiative transfer effects and not by correlated noise. The cause of this reversal is the nonlinear amplitude of the displacements in the simulation and the nonlinear dependence of the H- opacity on temperature. Where the temperature is hotter the opacity is larger and photons escape from higher, cooler layers. This reduces the fluctuations in the radiation temperature compared to the gas temperature. The mode asymmetry reversal in the simulation is a small frequency-dependent differential effect within this overall reduction. Because individual solar modes have smaller amplitudes than the simulation modes, this effect will be smaller on the Sun.

Supergranulation Scale Convection Simulations

Results of realistic simulations of solar surface convection on the scale of supergranules (96 Mm wide by 20 Mm deep) are presented. The simulations cover only 10% of the geometric depth of the solar convection zone, but half its pressure scale heights. They include the hydrogen, first and most of the second helium ionization zones. The horizontal velocity spectrum is a power law and the horizontal size of the dominant convective cells increases with increasing depth. Convection is driven by buoyancy work which is largest close to the surface, but significant over the entire domain. Close to the surface buoyancy driving is balanced by the divergence of the kinetic energy flux, but deeper down it is balanced by dissipation. The damping length of the turbulent kinetic energy is 4 pressure scale heights. The mass mixing length is 1.8 scale heights. Two thirds of the area is upflowing fluid except very close to the surface. The internal (ionization) energy flux is the largest contributor to the convective flu...

Numerical Simulations of Oscillation Modes of the Solar Convection Zone.

We use the three-dimensional hydrodynamic code of Stein & 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 the Solar and Heliospheric Observatory (SOHO)/Michelson Doppler Imager (MDI) and Global Oscillations Network Group have confirmed the asymmetry of solar oscillation line profiles, initially discovered by Duvall et al. In this Letter, 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 phase jumps in the vicinity of modes are 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.

Design and Engineering of an HTS Dipole in the FRIB Fragment Separator

One of the challenges in the Facility for Rare Isotope Beams at Michigan State University is the 30 degree bending dipoles in the fragment separator operating in a high radiation environment. It is known that high temperature superconductors (HTS) have a much larger thermal margin due to high critical temperature > 90 K and high upper critical field > 100 T, which allows HTS magnets to operate stably so as to tolerate very high heat loads due to radiation. The HTS dipole magnets will utilize ReBCO conductor technology and operate at 38 K cooled by helium gas. High radiation deposits a large amount of heat into the iron yoke, cryostat, bobbin and HTS coil itself. For certain beams, over-bent particles will hit the cryostat with high intensity in the beam down-stream. Another difficulty is that the dipole coils generate significant Lorentz forces that need to be contained. All of these challenges have been analyzed separately and then integrated to find novel approaches. These approaches have been applied to optimize the magnet structure and enhance the 38 K helium gas cooling system. We present project status and progress of this HTS ReBCO dipole magnets and lay out a plan for magnet manufacturing.