Low frequency radio emissions from the galaxy
Creators: name: Hamilton, PA
Publisher: name: University Of Tasmania
Year: 2023

description: The thesis presents the results of three radio surveys of the southern sky together with an analysis of these and other surveys of the same region. A model of the emission and absorption of radio waves in the Galaxy is derived in which the free electron content of the interstellar medium Is consistent with the observations of related phenomena. The low resolution measurements of the radio spectra in the direction of the galactic poles are reviewed to obtain a reference spectrum for calibrating high resolution surveys. It is apparent that there are considerable differences between the emissions in the two galactic hemispheres, and it is concluded that the electron distributions responsible for the emission in each region have different energy spectra. The three surveys of the southern sky are described, at 4.7 MHz (beamwidth 4¬¨‚àû x I0¬¨‚àû), 10 MHz (beamwidth 4 ¬¨‚àûx 5¬¨‚àû) and 153 MHz (beamwidth 2.2¬¨‚àûcircular); lists of discrete sources measured are presented together with contour maps of equivalent aerial temperature. The results of three other surveys of the same region are summarised, and the six surveys are analysed in terms of a model in which the line of sight intensity Is the combined effects of a local region (called for convenience the \disk\" region) and a region remote from the sun (the \"extra-disk\" region). The processes of synchrotron emission and thermal absorption are assumed to occur in the disk region while there is emission and no absorption in the extra-disk region. The parameters of the model are the emissivity of the disk region the optical depth at each line of sight in the disk region and the total intensity at each line of sight from the extra-disk region. The extra-disk emission is found to be strongly anisotropic and the distribution is consistent with the existence of a galactic halo that is ellipsoidal in shape with axes of 18 Kpc and 20 Kpc. The assumption that the halo is dynamically stable implies that the mean magnetic field strength is 2x10\\(^{-6}\\) Gauss and the differential spectrum of the radiating electrons is n\\(_e\\)(E) ‚Äöv¢vñ 8x10\\(^{-2}\\) E\\(^{-2.3}\\) el cm\\(^{-2}\\) sec\\(^{-1}\\) ster\\(^{-1}\\) GeV\\(^{-1}\\). These electrons would interact with the 3 degK black body radiation field via the inverse Compton effect to produce x-ray photons in the 3-20 KeV range with intensities close to those observed. The properties derived for the emission from the disk region suggest a mean field strength of a few microgauss and an electron flux slightly higher than that estimated for the solar neighbourhood from measurements near the earth. The distribution of thermal absorption in the disk region suggests a uniform electron gas throughout the galactic plane with a somewhat greater mean density towards the galactic centre. For the line of sight at the south galactic pole we obtain ‚Äöv†¬¥ n\\(_e\\)\\(^2\\)/T\\(_e\\)\\(^{3/2}\\) dl ‚Äöv¢vñ 9x10\\(^{-6}\\) cm\\(^{-6}\\) degK\\(^{-3/2}\\) pc where n\\(_e\\) is the electron number density and T\\(_e\\) is the electron kinetic temperature. It is concluded that a model of the intersellar medium in which the intercloud region has a density of 0.5 cm\\(^{-3}\\) a temperature of 1000 degK and an ionization of 6% will explain the observations of hydrogen-line emission Faraday rotation pulsar dispersion and low frequency absorption."