Author name code: skartlien ADS astronomy entries on 2022-09-14 author:"Skartlien, Roar" ------------------------------------------------------------------------ Title: Numerical simulations of stochastically excited sound waves in a random medium Authors: Selwa, M.; Skartlien, R.; Murawski, K. Bibcode: 2004A&A...420.1123S Altcode: In turbulent acoustic media such as the solar envelope, both wave sources and the propagation characteristics (background density, refractive index, dissipation, etc.) are stochastic quantities. By means of numerical simulation of the Euler equations, we study two cases in a homogeneous stochastic medium in which the background density fluctuations and wave sources are 1) correlated and 2) uncorrelated. We find that in the uncorrelated case, the coherent (or mean) acoustic field is zero, leaving only an incoherent field. In the correlated case, the coherent field is nonzero, yielding both coherent and incoherent fields. We question the use of mean-field dispersion relations to determine frequency shifts in p-mode and f-mode spectra, since the coherent field can be non-existent or weak relative to the incoherent field. We demonstrate the importance of accounting for a stochastic wave source by showing that the two cases give very different frequency shifts. Title: Effects in the Solar p-Mode Power Spectrum from Scattering on a Turbulent Background Flow with Stochastic Wave Sources Authors: Skartlien, R. Bibcode: 2002ApJ...578..621S Altcode: This work demonstrates how the scattered wave field, in combination with stochastic wave sources, influences the p-mode power spectrum. I adopt a turbulent zone with random fluctuations in sound speed and velocity (characterized by the respective correlation functions). I present a general formalism (for plane-parallel conditions) to calculate the expectation value of the p-mode power spectrum. The power due to the direct field from the sources dominates, and depends only on the source correlation function. Smaller, but significant, ``corrections'' are due to scattered wave field components. These corrections depend in general on the correlation functions of the turbulent fluctuations in the medium. I adopt a simple waveguide for the purpose of clear demonstration, and show that the corrections generate three important effects: (1) the line profiles are shifted along the wavenumber (frequency) axis, with varying effects depending on which ``turbulent'' physical quantity we consider; (2) the shapes and widths of the line profiles are altered; and (3) the ``troughs'' between the line profiles are to some extent filled in, contributing to the ``solar background power.'' It is demonstrated that turbulent boundary zones can generate important contributions to the power spectrum. Title: Local Helioseismology as an Inverse Source-Inverse Scattering Problem Authors: Skartlien, R. Bibcode: 2002ApJ...565.1348S Altcode: The inverse source and inverse scattering problems for a general inhomogeneous medium is investigated within the framework of helioseismic holography. Holographic images, defined by the method of Lindsey and Braun, or via the Porter-Bojarski equation, define a Fredholm integral equation of the first kind in terms of acoustic sources, scatterers, and absorbers. This integral equation is well posed in the sense that it can be inverted by standard constrained inversion methods. The inversion produces an image that does not distinguish between sources, scatterers, and absorbers. The inversion is necessarily approximate because of the null space of the kernel that defines the Fredholm equation. Physically, the null space corresponds to the nonradiating source contribution, as previously shown by Devaney and Porter. Numerical experiments based on a solar model show that the sidelobes in the ``imaging point-spread function'' are greatly reduced after inversion, such that the image is sharper than the corresponding holographic image. Title: Imaging of Acoustic Wave Sources inside the Sun Authors: Skartlien, R. Bibcode: 2001ApJ...554..488S Altcode: A holographic imaging technique, previously used in underwater acoustics, is applied to the solar inverse source problem. This problem consists of forming acoustic images so as to localize impulsive sources in space and time in the solar interior. The difficulty with such an imaging method is that the image of a single point source is spatiotemporally extended and will produce significant overlap between images of separated sources if the source density is large. I propose a modification of the existing method in order to make reliable source images also in the case for large source density. It is suggested that the method can be used on solar observations to detect relatively strong and impulsive wave sources inside the sun and to perhaps clarify unanswered problems regarding excitation of p modes. Title: Excitation of Chromospheric Wave Transients by Collapsing Granules Authors: Skartlien, R.; Stein, R. F.; Nordlund, Å. Bibcode: 2000ApJ...541..468S Altcode: The excitation of acoustic waves is studied using three-dimensional numerical simulations of the nonmagnetic solar atmosphere and the upper convection zone. Transient acoustic waves in the atmosphere are excited at the top of the convective zone (the cooling layer) and immediately above in the convective overshoot zone, by small granules that undergo a rapid collapse, in the sense that upflow reverses to downflow, on a timescale shorter than the atmospheric acoustic cutoff period (3 minutes). These collapsing granules tend to be located above downflows at the boundaries of mesogranules where the upward enthalpy flux is smaller than average. An extended downdraft between larger cells is formed at the site of the collapse. The waves produced are long wavelength, gravity modified acoustic waves with periods close to the 3 minute cutoff period of the solar atmosphere. The oscillation is initially horizontally localized with a size of about 1 Mm. The wave amplitude decays in time as energy is transported horizontally and vertically away from the site of the event. Observed ``acoustic events'' and darkening of intergranular lanes could be explained by this purely hydrodynamical process. Furthermore, the observed ``internetwork bright grains'' in the Ca II H and K line cores and associated shock waves in the chromosphere may also be linked to such wave transients. Title: A Multigroup Method for Radiation with Scattering in Three-Dimensional Hydrodynamic Simulations Authors: Skartlien, R. Bibcode: 2000ApJ...536..465S Altcode: Substantial approximations in the treatment of radiation are still necessary in three-dimensional simulations in order to avoid extremely large computational costs. Solar radiation hydrodynamic simulations in three dimensions have previously assumed local thermodynamic equilibrium (LTE) an assumption that works well in the deep photosphere. This work aims at bringing these simulations a step further by including scattered radiation, with the goal of modeling chromospheres in three dimensions. We allow for coherent isotropic scattering, which alters the thermal structure and wave amplitudes in the chromosphere. Group mean opacity coefficients are used in group mean source functions that contain approximate scattering terms and exact contributions from thermal emissivity. The resulting three-dimensional scattering problem allows for a computationally efficient solution by a new iteration method. We have compared exact wavelength-integrated monochromatic solutions with the corresponding approximate solutions for solar conditions. We find that the total flux divergence obtained from the groups deviates less than 10% from the exact solution. When using these groups rather than the full monochromatic solution, the CPU time is reduced by a factor of about 100 in a test case for solar conditions. Title: p-Mode Intensity-Velocity Phase Differences and Convective Sources Authors: Skartlien, R.; Rast, M. P. Bibcode: 2000ApJ...535..464S Altcode: We study the origin of the solar p-mode intensity-velocity phase differences at high degree (l>100). Observations show phase differences that are very different from those derived from linear theory alone. The theory predicts a smooth variation with frequency, dependent only on atmospheric parameters, while observations show large fluctuations across modal frequencies. We support previous suggestions that fluctuations in the intensity-velocity phase differences and line asymmetries in the intensity and velocity power spectra are produced by ``contamination'' of the p-mode signal with noise correlated with the excitation sources. It is demonstrated that the qualitative shapes of the observed phase-difference and power spectra can be realized only if both temperature (intensity) and velocity (Doppler shift) observations contain correlated noise. Moreover, the details of the observed spectra allow only a limited choice of noise parameters and constrain well the convective process responsible for p-mode excitation. The inferred correlated noise signals are consistent with the (visible) formation of convective downflows accompanied by darkening (lowered emergent intensity) and subsequent acoustic excitation. An upward velocity pulse follows after the wave excitation, which suggests overshoot of inflowing material that fills in the evacuated volume in the wake of the new downflow. Title: Three-Dimensional Modeling of Solar Convection and Atmosphere Dynamics Authors: Skartlien, R. Bibcode: 1998PhDT........34S Altcode: Results from non-magnetic numerical 3D simulations of the solar atmosphere and the convection zone below are presented. I find that transient acoustic wave trains in the atmosphere are excited by smaller granular cells that undergo a rapid collapse in the sense that upflow is reversed to downflow on a timescale shorter than the atmospheric acoustic cutoff period. An extended downdraft between larger cells is formed at the site of the collapse. The main wave excitation sources are found at the top of the convective layer (the cooling layer) and immediately above in the convective overshoot layer. The excitation lasts for 2-3 minutes as the upflow in the granule reverses to downflow. The following vertical wave components are long wavelength, gravity modified acoustic waves with periods close to the 3 minute eigenoscillation of the solar atmosphere. These waves shock at chromospheric layers. The oscillation is initially horizontally localized with a size of about 1 Mm. The wave amplitude decay in time as energy is transported horizontally and vertically away from the site of the event. This is the only convectively generated wave source I find that shows a clear correlation to large scale atmospheric wave motions. Observed darkening of intergranular lanes and the associated photospheric wave motions, the so called ``acoustic events'' could be explained by this purely hydrodynamical process. Furthermore, the observed ``internetwork bright grains'' in the CaII H and K line cores and associated shock wave trains in the chromosphere can also be linked to this wave train. The simulation is an extended Nordlund and Stein model of solar convection, so as to include the chromosphere. A large part of the work has been spent on developing a new method for calculating radiative flux divergence in the simulation, in which I treat photon scattering in atmospheric layers. The previous NS-model used the Planck function as source function, while the improved method includes a scattering term, which introduces a global radiation problem, requiring an iterative solution at each timestep in the simulation. By assuming opacity in LTE and coherent isotropic scattering, I calculate group mean opacity coefficients to be used in a group mean source function. This source function contains an approximate scattering term and an exact contribution from thermal emissivity. The resulting three dimensional scattering problems are solved by iteration using a new method based on a one-ray approximation in the angle integral for the mean intensity. The equations to be iterated are tri-diagonal matrix equations which require a minimum of computer time. I have compared exact wavelength integrated monochromatic solutions with the corresponding approximate group mean solutions for solar conditions. I find that the total flux divergence obtained from groups deviates with less than 10 % from the exact solution. Flux divergence in individual groups can deviate with typically 30 % in atmospheric layers. When using these groups, the CPU time is reduced by a factor of about 100 in a test case for solar conditions. The main difference for the atmospheric dynamics between LTE and scattering solutions, is that the wave amplitudes in the chromosphere are larger with scattering. This is because the radiative damping is smaller than in LTE for a given wave amplitude. The convection dynamics and therefore the convective wave sources are almost unaltered, since the radiative cooling that drives the convection is mainly unchanged. Collapsing granules is therefore also expected in LTE, but the chromospheric response would be of smaller amplitude, and the shock train would form at higher altitudes. Title: 3D modeling of solar convection and atmosphere dynamics Authors: Skartlien, Roar Bibcode: 1998PhDT.......590S Altcode: No abstract at ADS Title: Calcium II phase relations and chromospheric dynamics Authors: Skartlien, R.; Carlsson, M.; Stein, R. F. Bibcode: 1994chdy.conf...79S Altcode: No abstract at ADS