Ph.D. Thesis, Department of Physics, University of Colombo (2007)

Supervisors: V. Cooray (vernon.cooray@hvi.uu.se), I. M. K. Fernando (fernando@phys.cmb.ac.lk), D. U. J. Sonnadara (upul@phys.cmb.ac.lk) and T.R. Ariyaratne (ari@phys.cmb.ac.lk)

Correlation between optical and current signatures of electrical discharges with special attention to remote sensing of lightning flashes
J. P. Liyanage (prasanna@sab.ac.lk)

Temporal variability of the optical emissions of several selected spectral lines was studied together with the discharge current for four different laboratory discharges. The studied discharges were electrostatic discharges, discharges due to a step like voltage pulse, discharges with a 10/350 s current impulse and long laboratory sparks. The types of discharges studied differ by their peak currents, current rise times, wave shapes, gap lengths and electrode geometries etc. It is shown that for all four types of discharges, the optical emissions corresponding to the spectral lines 486, 500, 554, 510, 554, 559, 568, 594 and 799 nm are correlated to the current with a coefficient of correlation either exceeding or close to 0.9 during the rising portion.

Four of these wavelengths, 500, 510, 517 and 568 nm were analysed further with two types of laboratory discharges to study the correlation of the peak value of the discharge current signal to that of monochromatic optical signal. A linear relationship was evident indicating the possibility of remote sensing the discharge channel current and its rise times using these wavelengths to a good accuracy. A verification made with 10 triggered lightning return strokes indicates the possibility of extending this to infer the temporal variability of lightning return stroke currents till it reaches its peak. After reaching the peak, these spectral lines decayed at a faster rate than the current. The rate of decay was dependent on the emission, with some spectral lines having a faster decay than the others. This indicates that the processes that are responsible for generating some spectral lines are only associated with the initial growth mechanisms of the channel.

Two other spectral lines (656 nm and 778 nm) indicated a poor correlation to the current, reaching their peaks at a later time than the current. It is suggested that the longer decay times of the broadband optical signals observed in other studies could be due to this type of spectral emissions that is embedded in the broadband signal. It was not possible to predict the variation of the discharge current beyond the current peak, using the spectral emissions considered.