Electromagnetic activity before initial breakdown pulses of lightning

Abstract

AbstractLightning flash initiation is studied using electric field change (E‐change) measurements made in Florida. An initial E‐change (IEC) was found immediately before the first initial breakdown (IB) pulse in both cloud‐to‐ground (CG) and intracloud (IC) flashes if the E‐change sensor was within 80% of the reversal distance of the IEC. For 18 CG flashes, the IECs had an average point dipole moment of 23 C m and an average duration of 0.18 ms; these parameters for 18 IC flashes were −170 C m and 1.53 ms. The IECs of CG flashes began with a change in the slope of the E‐change (with respect to time) from zero slope to a positive slope, consistent with downward motion of negative charge and/or upward motion of positive charge. For IECs of IC flashes, the beginning slope change was from zero to negative slope, consistent with upward motion of negative charge and/or downward motion of positive charge. During an IEC, the E‐change monotonically increased for CG flashes and monotonically decreased for IC flashes. In 14 of 36 cases, the IEC beginning was coincident with a discrete, impulsive source of VHF radiation; another 13 cases had at least one VHF source during the IEC or the first IB pulse. Before the IECs, there were no preliminary variations detected in the 36 flashes. It is hypothesized that lightning initiation begins with an ionizing event that causes the IEC and that the IEC enhances the ambient electric field to produce the first IB pulse.