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Fine structure in solar radio spectra


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McConnell, David (1981) Fine structure in solar radio spectra. PhD thesis, University of Tasmania.

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Observations of the solar radio spectrum have been made with
high time and frequency resolution. Spectra were recorded over six 3
MHz bands between 30 and 82 MHz. The receivers used were capable of
time and frequency resolutions of 1 ms and 2 kHz respectively. A
large number of radio bursts exhibiting a variety of fine spectral
structure were recorded. The most interesting bursts recorded, referred
to as solar S bursts, were identified with radio emission previously
described by Ellis (Aust. J. Phys., 22, 177, 1969) who called them fast
drift storm bursts. Improved time and frequency resolution has allowed
a much closer study than did the original observations.
The bursts were observed throughout the 30-82 MHz frequency range
but were most numerous in the 33-44 MHz band and were very rare at 80
MHz. On a dynamic spectrum the bursts appeared as narrow sloping lines
with the centre frequency of each burst decreasing with time. The rate
of frequency drift was about 1/3 that of type III bursts. The variation
of frequency drift rate with wave frequency was consistent with their
being emitted from plasma radiation sources which were travelling out of
the solar corona with a constant velocity. Most bursts were observed
over only a limited frequency range (< 5 MHz) but some drifted for more
than 10 MHz. The durations measured at a single frequency and the
instantaneous bandwidths of S bursts was small; Δt = 49 ± 34 ms and
Δf = 123 ± 56 kHz for bursts observed near 40 MHz. A significant number
had Δt⍩20 ms. Flux densities of S burst sources were estimated to fall
in the range 10 -23 -5 x 10- nW M-2 Hz -1 .
A small proportion (1-2 %) Of bursts showed a fine structure in
which the burst source apparently only emitted at discrete, regularly spaced frequencies causing the spectrogram to exhibit a series of bands
or fringes. The fringe spacing increased with wave frequency and was
(δf = 90 kHz for fringes near 40 MHz. The bandwidths of fringes was
narrow, often less than 30 kHz and some cases down to 10-15 kHz.
The following conclusions are reached. Firstly, the bursts are
due to plasma radiation, probably at the fundamental. Secondly, the
fringed bursts may be indicative of a wave propagating in the corona
whose wavelength is λ = 10 3 km. This idea is strongly supported by
the observed correspondence between fringes in bursts which occur close
together in time.
Plasma radiation mechanisms are discussed in terms of their
ability to produce the observed intensities and bandwidths and the
small proportion of fringed bursts. A plasma radiation mechanism
necessarily includes a conversion process for transferring energy from
longitudinal plasma (Langmuir) waves to escaping electromagnetic radiation.
Four conversion processes are considered as possibilities for the
radiation of solar S bursts. Two of these, the scattering of Langmuir
waves by thermal ions and the coalescence of two Langmuir waves, are
rejected. Mode coupling across the plasma level and the coalescence of
Langmuir waves with low frequency waves are both retained as possible
conversion processes.

Item Type: Thesis (PhD)
Keywords: Solar cosmic rays, Spectrum, Solar
Copyright Holders: The Author
Copyright Information:

Copyright 1981 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, 1981. Bibliography: l. 168-173

Date Deposited: 19 Dec 2014 02:24
Last Modified: 25 Sep 2017 02:27
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