A REEXAMINATION OF THE PEAK CURRENT CALIBRATION OF THE NATIONAL LIGHTNING DETECTION NETWORK

The peak current calibration of the National Lightning Detection Network (NLDN) recently reported by Orville (1991) has been reexamined with a roughly threefold larger data set of 57 directly measured stroke peak currents, I(Peak) (kiloamperes), and their corresponding NLDN mean normalized magnetic signal strengths, M(peak)BAR (LLP units). These 57 lightning strokes originated in 36 triggered lightning flashes initiated at the Kennedy Space Center (KSC), Florida, during the research campaigns of 1985-1991. Identification of corresponding I(peak) and M(peak)BAR measurements was verified through accurate coincidence in absolute time of the two independent data sets. The I(peak)-M(peak)BAR data (with one point excluded as an outlier) is apparently linearly related with a correlation coefficient of 0.88 1, consistent with that predicted by application of the transmission line model of the lightning return stroke. The regression equation for prediction of I(peak) from NLDN M(peak)BAR measurements is I(peak)=20+0.171M(peak)BAR where the slope is expressed in kiloampere/(LLP units). Examination of the overall I(peak)-M(peak)BAR data set for the possible influence of two different models of signal strength attenuation with distance, D, (power law, D(beta), and exponential, [exp(alphaD)D]-1) indicates negligible sensitivity to the proposed variations; other larger error sources Likely mask the true attenuation effect. Twelve flashes were detected with four or more direction finders; a power law fit to the direction finder signal strength variation with distance of these individual flashes yields a mean beta value of -1.09. Examination of the overall I(peak)-M(peak)BAR data set for the possible effect of a nonlinear relation between the source stroke peak current and return stroke propagation speed indicates no obvious influence. The 95% confidence bounds for peak current prediction in the mean suggest a percent uncertainty of 10-15% for peak currents between 15 and 60 kA. Similar accuracy is expected for peak currents greater than 60 kA provided care is taken to minimize possible nonlinear amplification effects in the NLDN data. Below 15 kA, the percent uncertainty rapidly increases suggesting that the inferred distribution of peak current values less than this limit may be quite unreliable.