The objectives of this project, led by Dr. Barney Rickett, are: (1) to determine the angular structure of some very compact radio sources; (2) to measure the small-scale turbulence in the interstellar plasma; and (3) to study the propagation of waves through turbulent media. These objectives are quite disparate areas of science, related by the radio scattering methods that will be employed in the work. As the radio signals from quasars and pulsars travel toward the Earth, they suffer scattering analogous to the twinkling of starlight in the Earth's atmosphere, except that the cause is fluctuations in the density of the interstellar plasma. Though this scattering hampers the direct study of the quasar or pulsar, it provides a probe of the interstellar plasma and a method of measuring very small structure in the source. In its simplest form it is based on the familiar observation that stars twinkle and planets don't - because planets have a larger angular size than stars and this smoothes out the twinkling. The sources that will be studied with these techniques are particularly interesting for different reasons. One is the recently discovered double pulsar binary system J0737-3039. This system is a remarkable laboratory for studies of gravitational physics and pulsar magneto-spheres. Dr. Rickett's studies will help realize the potential for gravitational and magnetospheric studies. Another class of source is the so-called "intra-day variable." These are active galactic nuclei which are so compact that they must be incredibly bright, in fact so bright that they come very close to the "inverse Compton limit." Dr. Rickett's studies will help determine if indeed the inverse Compton limit is violated. Some of the scattering techniques are mature and need only refinement, whereas others, such as the "parabolic arcs" recently discovered in pulsar spectra, have great promise but are still in their infancy. The development of these techniques is an important part of this work. This includes the development of digital simulation tools to study cases which are analytically intractable. The microstructure of the interstellar plasma is not well understood and measurements have been for the most part restricted to one dimension, but new techniques have already started to provide a two dimensional view and this will be pursued vigorously.
The work impacts several areas of astronomy: pulsars; gravitation; extra-galactic sources; and interstellar medium. The development of techniques, particularly digital simulations, will also have a broad impact outside astronomy, including the solar wind plasma, and wave propagation in the atmosphere and in the ocean. This award is primarily for the support of graduate and undergraduate students. The training they will receive has broad applicability in many branches of science and engineering.
