arXiv subjects

A. Nordlund

Publications and source records attributed to A. Nordlund.

At least 37 records · Page 2Linked to original sources

arXiv2006

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.

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arXiv2005

Radiative transfer in decomposed domains

An efficient algorithm for calculating radiative transfer on massively parallel computers using domain decomposition is presented. The integral formulation of the transfer equation is used to divide the problem into a local but compute-intensive part for calculating the intensity and optical depth integrals, and a nonlocal part for communicating the intensity between adjacent processors. The waiting time of idle processors during the nonlocal communication part does not have a severe impact on the scaling. The wall clock time thus scales nearly linearly with the inverse number of processors.

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arXiv2005

Stable magnetic fields in stellar interiors

We investigate the 50-year old hypothesis that the magnetic fields of the Ap stars are stable equilibria that have survived in these stars since their formation. With numerical simulations we find that stable magnetic field configurations indeed appear to exist under the conditions in the radiative interior of a star. Confirming a hypothesis by Prendergast (1956), the configurations have roughly equal poloidal and toroidal field strengths. We find that tori of such twisted fields can form as remnants of the decay of an unstable random initial field. In agreement with observations, the appearance at the surface is an approximate dipole with smaller contributions from higher multipoles, and the surface field strength can increase with the age of the star. The results of this paper were summarised by Braithwaite & Spruit (2004).

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arXiv2005

Effect of the radiative background flux in convection

Numerical simulations of turbulent stratified convection are used to study models with approximately the same convective flux, but different radiative fluxes. As the radiative flux is decreased, for constant convective flux: the entropy jump at the top of the convection zone becomes steeper, the temperature fluctuations increase and the velocity fluctuations decrease in magnitude, and the distance that low entropy fluid from the surface can penetrate increases. Velocity and temperature fluctuations follow mixing length scaling laws.

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arXiv2005

Spectrum and amplitudes of internal gravity waves excited by penetrative convection in solar-type stars

The excitation of internal gravity waves by penetrative convective plumes is investigated using 2-D direct simulations of compressible convection. The wave generation is quantitatively studied from the linear response of the radiative zone to the plumes penetration, using projections onto the g-modes solutions of the associated linear eigenvalue problem for the perturbations. This allows an accurate determination of both the spectrum and amplitudes of the stochastically excited modes. Using time-frequency diagrams of the mode amplitudes, we then show that the lifetime of a mode is around twice its period and that during times of significant excitation up to 40% of the total kinetic energy may be contained into g-modes.

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arXiv2005

Collisionless Plasma Shocks. Field Generation and Particle Acceleration

Gamma ray bursts are among the most energetic events in the known universe. A highly relativistic fireball is ejected. In most cases the burst itself is followed by an afterglow, emitted under deceleration as the fireball plunges through the circum-stellar media. To interpret the observations of the afterglow emission, two physical aspects need to be understood: 1) The origin and nature of the magnetic field in the fireball and 2) the particle velocity distribution function behind the shock. Both are necessary in existing afterglow models to account for what is believed to be synchrotron radiation. To answer these questions, we need to understand the microphysics at play in collisionless shocks. Using 3D particle-in-cell simulations we can gain insight in the microphysical processes that take place in such shocks. We discuss the results of such computer experiments. It is shown how a Weibel-like two-stream plasma instability is able to create a strong transverse intermittent magnetic field and points to a connected mechanism for in situ particle acceleration in the shock region.

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arXiv2005

Forward modeling of stellar coronae: from a 3D MHD model to synthetic EUV spectra

A forward model is described in which we synthesize spectra from an ab-initio 3D MHD simulation of an outer stellar atmosphere, where the coronal heating is based on braiding of magnetic flux due to photospheric footpoint motions. We discuss the validity of assumptions such as ionization equilibrium and investigate the applicability of diagnostics like the differential emission measure inversion. We find that the general appearance of the synthesized corona is similar to the solar corona and that, on a statistical basis, integral quantities such as average Doppler shifts or differential emission measures are reproduced remarkably well. The persistent redshifts in the transition region, which have puzzled theorists since their discovery, are explained by this model as caused by the flows induced by the heating through braiding of magnetic flux. While the model corona is only slowly evolving in intensity, as is observed, the amount of structure and variability in Doppler shift is very large. This emphasizes the need for fast coronal spectroscopy, as the dynamical response of the corona to the heating process manifests itself in a comparably slow evolving coronal intensity but rapid changes in Doppler shift.

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arXiv2004

Excitation rates of p modes: mass luminosity relation across the HR diagram

We compute the rates P at which energy is injected into the p modes for a set of 3D simulations of outer layers of stars. We found that Pmax - the maximum in P - scales as (L/M)^s where s is the slope of the power law, L and M are the luminosity and the mass of the 1D stellar models associated with the simulations. The slope is found to depend significantly on the adopted representation for the turbulent eddy-time correlation function, chi_k. According to the expected performances of COROT, it will likely be possible to measure Pmax as a function of L/M and to constrain the properties of stellar turbulence as the turbulent eddy time-correlation.

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arXiv2004

Coronal heating through braiding of magnetic field lines

Cool stars like our Sun are surrounded by a million degree hot outer atmosphere, the corona. Since more than 60 years the physical nature of the processes heating the corona to temperatures well in excess of those on the stellar surface remain puzzling. Recent progress in observational techniques and numerical modeling now opens a new window to approach this problem. We present the first coronal emission line spectra synthesized from three-dimensional numerical models describing the evolution of the dynamics and energetics as well as of the magnetic field in the corona. In these models the corona is heated through motions on the stellar surface that lead to a braiding of magnetic field lines inducing currents which are finally dissipated. These forward models enable us to synthesize observed properties like (average) emission line Doppler shifts or emission measures in the outer atmosphere, which until now have not been understood theoretically, even though many suggestions have been made in the past. As our model passes these observational tests, we conclude that the flux braiding mechanism is a prime candidate for being the dominant heating process of the magnetically closed corona of the Sun and solar-like stars.

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arXiv2004

Oscillation power spectra of the Sun and of Alpha Cen A: observations versus models

Hydrodynamical, 3D simulations of the outer layers of the Sun and Alpha Cen A are used to obtain constraints on the properties of turbulent convection in such stars. These constraints enable us to compute - on the base of a theoretical model of stochastic excitation - the rate P at which p modes are excited by turbulent convection in those two stars. Results are then compared with solar seismic observations and recent observations of Alpha Cen A. For the Sun, a good agreement between observations and computed P is obtained. For Alpha Cen A a large discrepancy is obtained which origin cannot be yet identified: it can either be caused by the present data quality which is not sufficient for our purpose or by the way the intrinsic amplitudes and the life-times of the modes are determined or finally attributed to our present modelling. Nevertheless, data with higher quality or/and more adapted data reductions will likely provide constraints on the p-mode excitation mechanism in Alpha Cen A.

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arXiv2004

Magnetic Field Generation in Collisionless Shocks; Pattern Growth and Transport

We present results from three-dimensional particle simulations of collisionless shocks with relativistic counter-streaming ion-electron plasmas. Particles are followed over many skin depths downstream of the shock. Open boundaries allow the experiments to be continued for several particle crossing times. The experiments confirm the generation of strong magnetic and electric fields by a Weibel-like kinetic streaming instability, and demonstrate that the electromagnetic fields propagate far downstream of the shock. The magnetic fields are predominantly transversal, and are associated with merging ion current channels. The total magnetic energy grows as the ion channels merge, and as the magnetic field patterns propagate down stream. The electron populations are quickly thermalized, while the ion populations retain distinct bulk speeds in shielded ion channels and thermalize much more slowly. These results may help explain the origin of the magnetic fields responsible for afterglow synchrotron/jitter radiation from Gamma-Ray Bursts.

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arXiv2003

Numerical 3D constraints on convective eddy time-correlations : consequences for stochastic excitation of solar p modes

A 3D simulation of the upper part of the solar convective zone is used to obtain information on the frequency component, chi_k, of the correlation product of the turbulent velocity field. This component plays an important role in the stochastic excitation of acoustic oscillations. A time analysis of the solar simulation shows that a gaussian function does not correctly reproduce the nu-dependency of chi_k inferred from the 3D simuation in the frequency range where the acoustic energy injected into the solar p modes is important (nu ~ 2 - 4 mHz). The nu-dependency of chi_k is fitted with different analytical functions which can then conveniently be used to compute the acoustic energy supply rate P injected into the solar radial oscillations. With constraints from a 3D simulation, adjustment of free parameters to solar data is no longer necessary and is not performed here. The result is compared with solar seismic data. Computed values of P obtained with the analytical function which fits best chi_k are found ~ 2.7 times larger than those obtained with the gaussian model and reproduce better the solar seismic observations. This non-gaussian description also leads to a Reynolds stress contribution of the same order as the one arising from the advection of the turbulent fluctuations of entropy by the turbulent motions. Some discrepancy between observed and computed P values still exist at high frequency and possible causes for this discrepancy are discussed.

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arXiv2003

Collisionless Shocks -- Magnetic Field Generation and Particle Acceleration

We present numerical results from plasma particle simulations of collisionless shocks and ultra-relativistic counter-streaming plasmas. We demonstrate how the field-particle interactions lead to particle acceleration behind the shock-front. Further, we demonstrate how ultra relativistic counter-streaming plasmas create large scale patchy magnetic field structures and that these field structures propagate down-stream of the shock front. These results may help explain the origin of the magnetic fields and accelerated electrons responsible for afterglow synchrotron radiation from gamma ray bursts.

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arXiv2003

Numerical constraints on the model of stochastic excitation of solar-type oscillations

Analyses of a 3D simulation of the upper layers of a solar convective envelope provide constraints on the physical quantities which enter the theoretical formulation of a stochastic excitation model of solar p modes, for instance the convective velocities and the turbulent kinetic energy spectrum. These constraints are then used to compute the acoustic excitation rate for solar p modes, P. The resulting values are found ~5 times larger than the values resulting from a computation in which convective velocities and entropy fluctuations are obtained with a 1D solar envelope model built with the time-dependent, nonlocal Gough (1977) extension of the mixing length formulation for convection (GMLT). This difference is mainly due to the assumed mean anisotropy properties of the velocity field in the excitation region. The 3D simulation suggests much larger horizontal velocities compared to vertical ones than in the 1D GMLT solar model. The values of P obtained with the 3D simulation constraints however are still too small compared with the values inferred from solar observations. Improvements in the description of the turbulent kinetic energy spectrum and its depth dependence yield further increased theoretical values of P which bring them closer to the observations. It is also found that the source of excitation arising from the advection of the turbulent fluctuations of entropy by the turbulent movements contributes ~ 65-75 % to the excitation and therefore remains dominant over the Reynolds stress contribution. The derived theoretical values of P obtained with the 3D simulation constraints remain smaller by a factor ~3 compared with the solar observations. This shows that the stochastic excitation model still needs to be improved.

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arXiv2002

A simulation of solar convection at supergranulation scale

We present here numerical simulations of surface solar convection which cover a box of 30$\times30\times$3.2 Mm$^3$ with a resolution of 315$\times315\times$82, which is used to investigate the dynamics of scales larger than granulation. No structure resembling supergranulation is present; possibly higher Reynolds numbers (i.e. higher numerical resolution), or magnetic fields, or greater depth are necessary. The results also show interesting aspects of granular dynamics which are briefly presented, like extensive p-mode ridges in the k-$ω$ diagram and a ringlike distribution of horizontal vorticity around granules. At large scales, the horizontal velocity is much larger than the vertical velocity and the vertical motion is dominated by p-mode oscillations.

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arXiv2002

Consequences of the non gaussian character of the stochastic excitation for solar-type oscillations

Our recent study, based on a 3D numerical simulation, suggested that the stochastic excitation of solar p-modes has a non gaussian nature and a non gaussian model was proposed which results in a good agreement between the computed rate $P$ at which p-modes are excited and the solar seismic observational constraints. In the present work, we study some consequences of this non-gaussian model on the rate P at which p-modes are excited in intermediate mass stars (1 Mo < M < 2 Mo). The non gaussian model changes substantially the spectrum of P at high frequency compared with the commonly assumed gaussian model. The largest effects on P are found for stars in the mass range ~ 1.5-1.6 Mo. They are found large enough that observatins in solar-like oscillating stars observed with space based seismology missions (e.g. COROT) will be able to confirm or reject the gaussian nature of stochastic excitation.

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arXiv2002

Seismic diagnostics on stellar convection treatment from oscillation amplitudes of p-modes

The excitation rate P of solar p-modes is computed with a model of stochatic excitation which involves constraints on the averaged properties of the solar turbulence. These constraints are obtained from a 3D simulation. Resulting values for P are found ~9 times larger than when the calculation assumes properties of turbulent convection which are derived from an 1D solar model based on Gough (1977)'s formulation of the mixing-length theory (GMLT). This difference is mainly due to the assumed values for the mean anisotropy of the velocity field in each case.Calculations based on 3D constraints bring the P maximum closer to the observational one.We also compute P for several models of intermediate mass stars (1Mo < M < 2Mo). Differences in the values of P_max between models computed with the classical mixing-length theory and GMLT models are found large enough for main sequence stars to suggest that measurements of P in this mass range will be able to discriminate between different models of turbulent convection.

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arXiv2002

Characterizing the dynamic properties of the solar turbulence with 3-D simulations: Consequences in terms of p-mode excitation

A 3D simulation of the upper part of the solar convective zone is used to derive constraints about the averaged and dynamic properties of solar turbulent convection. Theses constraints are then used to compute the acoustic energy supply rate P(nu) injected into the solar radial oscillations according to the theoretical expression in Samadi & Goupil (2001). The result is compared with solar seismic data. Assuming, as it is usually done, a gaussian model for the frequency (nu) component chi_k(nu) of the model of turbulence, it is found that the computed P(nu) is underestimated compared with the solar seismic data by a factor ~ 2.5. A frequency analysis of the solar simulation shows that the gaussian model indeed does not correctly model chi_k(nu) in the frequency range where the acoustic energy injected into the solar p-modes is important (nu ~ 2 - 4 mHz). One must consider an additional non-gaussian component for chi_k(nu) to reproduce its behavior. Computed values of P obtained with this non-gaussian component reproduce better the solar seismic observations. This non-gaussian component leads to a Reynolds stress contribution of the same order than the one arising from the advection of the turbulent fluctuations of entropy by the turbulent motions.

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