Calibration of a magnetic direction finding network using measured triggered lightning return stroke peak currents

Abstract

Peak currents from 18 triggered lightning return strokes lowering negative charge have been measured at the NASA Kennedy Space Center, Florida. These strokes are correlated with simultaneous measurements by six Lightning Location and Protection (LLP) Inc. wideband magnetic direction finders (DFs) in Georgia and Florida operating at high gain from 1985 through 1988. The following have been determined: (1) The normalized signal strength amplitudes, in LLP units, can be used to estimate the peak current in lightning return strokes by the relation, I(kA) = 2.3+0.19(Normalized Amplitude), where the amplitude is range‐normalized to 100 km. An approximation of the form, I(kA) = 0. 2(Normalized Amplitude) is also valid, introducing errors of typically less than 6 %. (2) Range‐normalized signal strength amplitudes, in LLP units, for individual DFs have been examined as a function of the measured return stroke peak currents. It is found that a DFs normalized amplitude has a higher correlation with peak currents, the further the DF site is from the triggered lightning. The correlation coefficients range from 0.53 to 0.93, At a DF site the standard deviation of the normalized amplitudes from the best fit line to the measured peak currents are independent of the DF site's distance from the triggered lightning, averaging 28 LLP units, which corresponds to 6 kA. (3) Signal strength amplitudes, which are proportional to the peak magnetic radiation fields, have been examined for seven triggered lightning return strokes. The signal strength amplitudes decay with distance, D, as D−1.13. (4) An evaluation of the error in the mean range‐normalized signal strength amplitude caused by lightning location errors reveals that the mean amplitude is typically in error by less than 2% from the value calculated using the exact location of the triggered flash.