Horizontal electric fields from lightning return strokes

Abstract

Measurements are presented of simultaneous horizontal and vertical electric fields from 42 lightning return strokes in 27 flashes at distances from 7 to 43 km. The data were obtained at the NASA Kennedy Space Center, Florida, using an elevated spherical antenna having a system bandwidth of 3 Hz to 4 MHz. The strokes studied occurred in a 6‐min interval from 1850 to 1856 UT on August 11, 1984. The 42 measured horizontal fields exhibited initial peaks having a mean half width of 0.52 μs. The ratio of the amplitude of the peak horizontal field to the associated vertical field peak had a mean value of 0.030, with a standard deviation of 0.007. The measured horizontal field wave shapes are compared with those obtained from calculations using the measured vertical fields and theory (the “wavetilt” formula) that models the fields as plane wave radiation over a homogeneous ground of uniform conductivity and permittivity. The local ground conductivity, measured as a function of depth using the Wenner four‐probe method, was found to increase with depth within the region relevant to this study, from the surface to a depth of a few meters: the conductivity was 8.2×10−3 mho/m between the surface and a depth of 2.2 m, and 3.1×10−2 mho/m below this layer to a depth of 11 m. If a ground conductivity equal to that of the top layer is used in the wavetilt formula, the calculated horizontal field peaks are found to be, on average, about 33% wider and larger than the measured field peaks. Errors and compensation procedures for tilt of the spherical antenna, for electronic system distortion, and for electric field distortion at the antenna due to proximity of the ground and of the antenna support structure are described.