Abstract:
The work performed on the two branches of 'atmospheric electricity', namely
'fair-weather electricity' and 'thunderstorm electricity' are presented with much
emphasis on the building up of an atmospheric electricity research station in Sri
Lanka. Reviews of previous investigations and introductory remarks on both the
above branches are made with particular reference to the work presented here.
The first part of this thesis deals with fair-weather electricity. Instruments that
have been constructed to measure atmospheric vertical electric field, space charge
density, small ion number density and conductivity in Colombo are described. The
average values of those electrical parameters, measured for the first time in Sri Lanka,
are presented.
It is shown that theoretically there is a relationship between temperature and
space charge density fluctuations associated with the turbulent mixing of the planetary
boundary layer. The results of the affiliated experiment confirm the theory and also
give evidence for the existence of the turbulent electrode effect.
Errors that may occur due to distorted atmospheric electric field lines caused by
nearby grounded conducting objects, earth’s orography, or elevated measuring
instruments themselves ip measuring atmospheric electrical parameters are discussed.
It is shown that the measured atmospheric polar ion number density values can
deviate by as much as 16 per cent from their real values due to the proximity of a
grounded conducting rod of height 6 m. Rough criteria are presented for choosing a
site in the proximity of a grounded conducting object for the measurement of
atmospheric electrical parameters within an error of 5 per cent.
The second part of this thesis mainly supplies more information about lightning
discharges in the tropics, a region with the most active thunderstorms and yet the least
investigated.
Oscillograph pictures of radiation field waveforms from distant lightning
flashes transporting negative charge to ground were recorded using a video camera
set-up. Four different types of first return-stroke waveforms and the most common
type of subsequent stroke waveforms observed in Sri Lanka are presented. The high
average number of branches per channel and the low percentage of spatially separate
channels observed are explained. Statistical distributions of first and subsequent
return stroke zero crossing times, multiplicity and interstroke time interval of negative
ground flashes are presented with the mean values of those parameters. A new
hypothesis is introduced to classify lightning parameters according to the
climatological zones of the world, instead of the unsuccessful attempts that have been
made to classify them according to the geographical latitude.
Simultaneous recordings were made on the fast electric field and high
frequency (HF) emissions at 3 MHz at times close to the first and subsequent return
strokes. It is shown that steps are the source of HF generation at 3 MHz in the leader
phase of ground flashes. In the return stroke phase, it is suggested that the tiny subbranches
of the order of 10 to 100 m in length, which are probably created during the
formation of the corona sheath, are the source of HF at 3 MHz.
A numerical procedure for calculating the transformation of the electromagnetic
pulses generated by a lightning return stroke over the sea as it crosses the sea-land
boundary is presented.