Stinebring - How Porous is the Ionized Interstellar Medium? Results from a Scintillation Arc Survey

Abstract: Decades ago, astronomers viewed the warm ionized interstellar medium (WIM) as a turbulent, space-filling plasma with a mean density of about 0.03 electrons/cm^3 in the Galactic plane. How accurate is this picture? In particular, is there evidence for a more bubble-like (porous) distribution of the WIM?

Techniques developed over the last twenty years, broadly referred to as scintillometry, allow us to make new progress on this question. By giving us the ability to locate scattering centers along the line of sight to pulsars, evidence is emerging that a bubble-like topology is appropriate for the WIM. I will present results of a survey of 22 relatively nearby pulsars, observed with the Green Bank Telescope and the Arecibo Telescope. Most show one dominant scattering screen along the line of sight, but higher sensitivity observations reveal larger numbers of scattering screens. What view of WIM topology is implied by these observations?