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Low frequency radio emissions from the galaxy


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Hamilton, Philip Alexander 1969 , 'Low frequency radio emissions from the galaxy', PhD thesis, University of Tasmania.

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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° x I0° ), 10 MHz (beamwidth 4 °x 5°) and
153 MHz (beamwidth 2.2°circular); 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
ne (E) = 8x10 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
∫ n e 2 /Te 3/2 dl, = 9x10-6 cm-6 degK-3/2 pc,
where ne is the electron number density and Te 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.

Item Type: Thesis - PhD
Authors/Creators:Hamilton, Philip Alexander
Keywords: Radio waves
Copyright Holders: The Author
Copyright Information:

Copyright 1969 the Author - The University is continuing to endeavour to trace the copyright
owner(s) and in the meantime this item has been reproduced here in good faith. We
would be pleased to hear from the copyright owner(s).

Additional Information:

Thesis (Ph.D.) - University of Tasmania, 1969. Includes bibliographies

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