Abstract:
Temporal variability of the optical emissions of several selected spectral lines was
studied together with the discharge current for four different laboratory discharges.
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 weak correlation was
evident between the current and optical peaks, indicating the possibility of remote
sensing the discharge channel peak current and its rise times using these wavelengths
to a good accuracy. A verification with ten triggered lightning return strokes indicate
the current / optical-intensity relationship holds till current reaches its peak for the
four spectral lines investigated. 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. The peak optical intensities of
these spectral lines did not show any significant correlation to the peak return stroke
current probably due to varying atmospheric conditions at the time of the return
strokes. Two other spectral lines (656 nm and 778 nm) indicated a poor correlation to
the current in the case of laboratory discharges, reaching their peaks at a later time
than the cunent. 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.
