4.6.1 Preliminary breakdown process

Experiments conducted in long gaps with metal electrodes under atmospheric conditions show the occurrence of several corona bursts before a self propagating leader discharge is launched from the high voltage electrode to the earthed electrode ([7], see also Chapter 3). On the basis of this experience one may expect some form of electrical activity inside the cloud before the stepped leader is launched. This electrical activity in the cloud can be much more complicated than that in the laboratory studies, mainly for the following two reasons. First, in contrast with the case of metal electrodes, the electrical charges involved in thunderclouds reside on cloud droplets and precipitation particles and an efficient mechanism is needed to accumulate the charge necessary for the initiation of the stepped leader. Second, discharges are of several hundreds of metres to several kilometres in length and they occur within a complex environment of cloud particles and reduced pressure. The best means available today to deduce the breakdown mechanism inside the cloud is to make measurements of the electromagnetic fields generated by the processes inside the cloud at several stations and to combine the information gathered with the data from VHF time of arrival and VHF imaging techniques ([1 6], see also section 4.11).