Dr. Peter Goldreich, at the California Institute of Technology, will direct theoretical investigations in several topics in astrophysics. Most of his effort will be in the area of interstellar scintillation and magneto-hydrodynamic (MHD) turbulence. Most of the baryonic matter in the universe has such high electrical conductivity that magnetic fields diffuse very slowly through it. Thus fluid motions and motions of magnetic field lines are closely coupled. Large scale motions are generally turbulent, and incompressible MHD is the simplest approximation under which these complex coupled motions can be investigated. Dr. Peter Goldreich will carry out a series of numerical simulations of incompressible MHD turbulence to test specific predictions of a theory set forth earlier by Goldreich & Sridhar. These predictions include: 1. A scale dependent anisotropy relationship between correlation lengths that are parallel and perpendicular to the local magnetic field. 2. That velocity and magnetic field differences in planes perpendicular to the local magnetic field obey the Kolmogorov scaling characteristic of hydrodynamic turbulence. 3. That shear Alfven waves control the cascade dynamics and slow waves play only a passive role.
Scintillations of small angular diameter radio sources reveal a spectrum of interstellar electron density fluctuations. To a good approximation, these conform to the Kolmogorov scaling, with a constant of proportionality that varies over a wide range in different regions. It is plausible to assume that this spectrum is a consequence of MHD turbulence. Elliptical, scatter-broadened images imply the density spectrum is anisotropic, presumably due to the presence of a large scale magnetic field. A quantitative connection will be made between velocity and magnetic field fluctuations predicted by the Goldreich & Sridhar theory of MHD turbulence and the fluctuations of interstellar electron density. The amount of ionized gas along lines of site to pulsars will be correlated with the pulsars' scattering measures to help identify the sources and sites of the turbulence.
Dr. Goldreich will also examine processes in planet formation in which he will concentrate on dynamical problems posed by the large orbital eccentricities of detected extra-solar planets. Finally, he will explore the phenomenon of amplitude saturation in multi-mode stellar pulsations. He will attempt to explain why Cepheid variables and RR Lyrae stars pulsate with large amplitudes while many other types of stars pulsate at much smaller amplitudes. ***
