The Origin of Radio Scintillation in the Local Interstellar Medium

We study three quasar radio sources (B1257-326, B1519-273, and J1819+385) that show large amplitude intraday and annual scintillation variability produced by the Earth’s motion relative to turbulent-scattering screens located within a few parsecs of the Sun. We find that the lines of sight to these sources pass through the edges of partially ionized warm interstellar clouds where two or more clouds may interact. From the gas flow vectors of these clouds, we find that the relative radial and transverse velocities of these clouds are large and could generate the turbulence that is responsible for the observed scintillation. For all three sight lines the flow velocities of nearby warm local interstellar clouds are consistent with the fits to the transverse flows of the radio scintillation signals. Subject headings: ISM: atoms — ISM: clouds — ISM: structure — line: profiles — ultraviolet: ISM — ultraviolet: stars 1. OBSERVATIONS OF FAST RADIO SCINTILLATION Interstellar scintillation (ISS) is the apparent variation in flux density of very compact radio sources due to propagation through the irregular refractive index of the ionized interstellar medium. The phenomenon was discovered and investigated through its 100% variations in pulsar signals seen on time scales from minutes to days. Most extragalactic radio sources have angular diameters much too large to scintillate, in the same way that JILA, University of Colorado and NIST, Boulder, CO 80309-0440 Department of Electrical and Computer Engineering, University of California San Diego, La Jolla, CA 92093-0407 Department of Astronomy and McDonald Observatory, University of Texas, Austin, TX 78712-0259 Hubble Fellow