Unipolar and bipolar pulses emitted during the development of lightning flashes

Abstract

Both unipolar and bipolar magnetic or electric field pulses have been observed during preparatory stages of a lightning flash. We introduce a new simple analytical model to describe both kinds of pulses. We show how the polarity overshoot depends on the parameters of the model, including the propagation velocity of the current pulse, the step length, and the injected current waveshape. We observe that the expression for the radiation part of the magnetic field can be decomposed into two time-shifted terms with opposite polarities. The time shift of the two terms is determined by the overall propagation time of the current pulse. The model well corresponds not only to observations of the bipolar preliminary breakdown pulses at time scales of tens of microseconds but also to both unipolar and bipolar dart-stepped leader pulses at submicrosecond time scales.