Low-frequency angular broadening and diffuse interstellar plasma turbulence

Abstract

A low-frequency radio interferometer is potentially a powerful tool for studies of diffuse plasma turbulence. The scattering angular size of a source due to interstellar scattering is proportional to the square of the observing wavelength. Synchrotron self-absorption, an intrinsic mechanism for making a source larger at lower frequencies, produces an angular size proportional to the first power of the wavelength. Thus, observations at a longer wavelength will always be more sensitive to scattering than observations at higher frequencies. In this article we consider in detail two issues. (1) The effect of turbulence in the interplanetary medium is considered. We conclude that interplanetary scattering will limit the effective sensitivity of a low-frequency interferometer to far above its theoretical value. Any serious design considerations for a low-frequency, space-borne interferometer must include a detailed investigation of the effect of the interplanetary medium. (2) If the limitations imposed by the interplanetary medium can be overcome, a low-frequency interferometer could be used to search for turbulence near supernova remnants. Such turbulence has been hypothesized to play an important role in the acceleration of the cosmic rays, and observational evidence for its existence would be a major accomplishment. Finally, we discuss a couple of additional, more minor topics which could be addressed with such an instrument, such as studies to verify if the low-frequency variability of extragalactic radio sources is a scintillation phenomenon.