The Structure of the Ionized Component of the Local Interstellar Medium

Abstract

The possibility of obtaining information about small-scale inhomogenities of the electron component of the local interstellar medium (ISM) is investigated using interstellar scintillation of extragalactic radio sources. We analyze Culgoora array variability data at 80 and 160 MHz for 190 extragalactic radio sources distributed over most of the sky. The variability on time-scales of 1 month-15 years is interpreted as interstellar scintillations in rapidly-moving nearby (less than 150 pc) hot gas near shock waves in the local ISM. All-sky maps of scintillation index m averaged over 3-5 neighbouring sources and over m for time-scales of 1 month (m1) and one or several years (m12) show several maximum values for m. Locations of the maxima are insensitive to the method and number of points used for averaging. The positions of the maxima obtained in different ways agree to within 15-30 degrees on the sky; this is the angular resolution of this method. Two of the three most certain maxima are probably associated with Loop I, and the third coincides with a soft X-ray (0.1 - 0.3 keV) background maximum near the South Galactic Pole. Other less certain scintillation index maxima probably correspond to the Orion starformation region and to a soft X-ray maximum near the North Galactic Pole. A tunnel that is free of gas in the direction l = 240° is indicated by low values of m. The estimated time-scale for interstellar scintillations in these structures in the local ISM is in agreement with the time-scale for the observed radio source variations.