|dc.description.abstract||Wave induced electron precipitation (WIEP) can modify the ionosphere above
a sub-ionospherically propagating VLF signal in such a way as to perturb the
amplitude and phase of the signal: The "Trimpi Event".
In this thesis trimpi events are used in a study of WIEP events and in the responsible
mechanism: The gyroresonant interaction. Trimpi activity at middle latitudes
(SANAE, Antarctica, L = 4.02) and low latitudes (Durban, RSA , L = 1.69) together with the corresponding theory for the gyroresonant interaction is examined
A newly developed computerised system for the detection and analysis of trimpi
events has been developed in Durban. This system has been used to analyse
tape data recorded at SANAE. Trimpi events were found on various transmitter
paths to SANAE and a complete study of 1982 data has led to the establishment
of trimpi characteristics as seen at SANAE: an absence of positive events
and causative whistlers, a preference for short duration events (t < 25s), the occurrence
of some very large events (up to 90% signal attenuation) , two minima
in occurrence near 0015 and 0400 h Local Time, low occurrence and occurrence
rate of events and evidence that interactions with non-ducted whistlers are of
The computerised sytem was then extended to collect data at Durban simultaneously
from up to 20 transmitters worldwide. Examination of data from this survey
showed very low occurrence rates of trimpis but yielded some daytime events
for which the effectiveness of the gyroresonance interaction, which successfully
explains the trimpi event at middle and low latitudes, had to be questioned.
Thus a fully relativisic test particle simulation of the gyroresonant interaction
was used to examine the effectiveness of gyroresonance at low L for producing
trimpi events. This simulation was run for a wide range of interaction parameters
and yielded the following constraints for effective pitch angle scattering (and hence
precipitation) of electrons at low L: wave intensities in excess of 150 nT, wave
frequencies in excess of 10 kHz and background electron densities at least one
order of magnitude higher than normal.
First data from the OMSKI project, a sophisticated VLF receiver operated at
Durban as part of an international project, shows further evidence of low-latitude
trirmpi activity. A survey of one month's continuous data is presented.
In face of the evidence that trimpi events that occur at low L have the same
signature as those at middle L but that the standard gyroresonance interaction is
insufficient to cause them, alternate scenarios that could enhance the interaction
were sought. In particular distortions in the ambient magnetic field (eg. PC-5
pulsations) were modelled using a new dipole-like background field model. This
simulation showed that distortions which tend to reduce magnetic field curvature
along field lines can significantly enhance the gyroresonant conditions and hence
the interaction. A new set of conditions for effective gyroresonance at low L is
thus established and contrasted with the more lenient conditions at middle L.
A study of "frequency tracking" as a means to prolong resonance showed that
natural whistlers do not posess the required frequency /time characteristics for this
mechanism, and that artificial waves in a narrow range around the equatorial
resonance frequency would ~ well suited for this purpose.
An overview of the status of worldwide Trimpi detection networks together with
the S.P.R.I. 's role in this regard is presented.||en