A study of whistlers and related VLF phenomena.
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Date
2012
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Abstract
Whistlers are naturally occurring Very Low Frequency (VLF) phenomena which are the result
of lightning-radiated electromagnetic waves propagating in Earth’s plasma environment. Major
research into whistlers and their generation began in 1951 and since then much has been discovered
about them. This has allowed whistlers to be used as magnetospheric probes. Many issues
concerning whistlers are still disputed, however, such as the relationship between the lightning
location and the conjugate point of the receiver. A correlation between whistlers detected by the
DEMETER satellite above South Africa and lightning located by WWLLN was used to determine
the source region for these whistlers. The whistlers were found to originate from lightning strokes
as far away as 10000 km. This result is statistically significant.
During the course of this research an interesting observation of chorus was made on Marion
Island. Since this was the first observation of chorus made on the sub-antarctic Marion Island,
conditions surrounding the event were studied in great detail. This led to several interesting observations
about the nature of this observation. In particular, during the evolution of the emission,
it transformed to hiss, which makes this observation relevant to recent results suggesting that hiss
is generated by chorus. It was also found that Marion Island was close to the plasmapause during
the observation, which has further implications related to the chorus-hiss relationship.
A study of the occurrence of twin whistlers received at Rothera and SANAE IV was conducted.
These were whistlers which had propagated from a single ionospheric exit point to both receivers.
Rothera and SANAE IV share the same whistler source region, yet the average number of whistlers
received at Rothera is an order of magnitude greater than that received at SANAE IV. The twin
whistler analysis showed that the most probable reason for this disparity is that whistlers from
the source region enter the waveguide preferentially closer to Rothera, making it more likely for
them to be received at Rothera than SANAE IV. These results have implications on the nature of
sub-ionospheric propagation of whistlers, which is not the same as that of spherics.
Finally, a method for tracking tropical cyclones using lightning locations from WWLLN was
developed. During the course of this thesis, tropical cyclone Irena was the result of damage on the
east coast of South Africa. This presented an opportunity to investigate the ability of WWLLN
data to describe the passage of these destructive phenomena near South Africa. The details of
this new method are discussed. While the algorithm developed has room for improvement, its
performance was tested on the recent tropical cyclone Irina which occurred during 2012.
Description
Thesis (Ph.D.)-University of KwaZulu-Natal, Westville, 2012.
Keywords
Whistlers (Radio meteorology), Lightning., VLF emissions., Theses--Physics.