Airborne Measurements of the Risetimes in Lightning Return Stroke Fields

Abstract : A WC-130 aircraft instrumented with electromagnetic field sensors was flown in South Florida in close proximity to thunderstorms. Electric and magnetic fields were measured over one thousand lightning return strokes at distances between 4 and 35 km from the aircraft. These field waveforms were recorded with a Digital Transient Recorder with a 25 MHz bandwidth which consists of 8192 samples in 20 nsec intervals. The risetimes for over two hundred return stroke magnetic fields have been calculated. The average risetime (10 to 90%) of the entire waveform for the first return stroke was 1.78 microsec with a standard deviation of .87 microsec. The average risetime for the fast portion of the first return stroke was 356 nsec with a standard deviation of 141 nsec. The average risetime for subsequent return strokes was 405 nsec with a standard deviation of 148 nsec. From the far-field data, the average rate of rise of the current in the return stroke channel for 38 strokes was estimated as 4.39 x 10 to the 10th A/sec with a standard deviation of 2.39 x 10 to the 10th A/se.