Rise times of impulsive high-current processes in cloud-to-ground lightning

Measurements are presented of electric-field derivative (dE/dt) waveforms that were radiated by first and subsequent return strokes, stepped, and dart-stepped-leader steps just before return strokes and "characteristic pulses" in normal (negative) cloud-to-ground lightning under conditions of minimal distortion due to ground-wave propagation. The main dE/dt peaks produced by the fast-rising portions of all of these processes are found to have similar durations [mean full-width at half-maximum (FWHM) ranging from 79/spl plusmn/20 ns for subsequent strokes to 54/spl plusmn/17 ns for stepped-leader steps], although widely differing absolute magnitudes (spanning nearly a factor of four). Field-change (E) signatures of first strokes are examined in greater detail after eliminating the 39% of events with multiple dE/dt peaks during their fast-rising portions. The "slow fronts" beginning these waveforms had durations of 3.7/spl plusmn/1.2 /spl mu/s and amplitudes 50%/spl plusmn/10% of peak E. The latter ratio was uncorrelated with either peak E or peak dE/dt. The range-normalized peak magnitudes of the remaining fast-rising portions of these field changes were well correlated with those of the corresponding dE/dt signatures, whereas the values of FWHM of dE/dt were uncorrelated with peak dE/dt and only poorly correlated with peak E.