Introduction

Downward negative lightning discharges, that is, discharges that are initiated in the cloud, initially develop in an overall downward direction, and transport negative charge to ground, probably account for about 90 percent of all cloud-to-ground discharges (Section 1.2). The overall cloud-to-ground lightning discharge, termed a flash, is composed of a number of processes, some of which involve channels that emerge from the cloud while others involve channels that are confined to the cloud volume. Only processes occurring in channels outside the cloud render themselves to optical observations that can be used to determine channel geometry, extension speed and other pertinent features of those channels. All lightning processes are associated with the motion of charge and, therefore, can be studied via measurement of the electric and magnetic fields associated with that charge motion. VHF–UHF radiation associated with breakdown processes, particularly those involved in the formation of negatively charged leader channels, can be used to track channel extension, as discussed in subsections 17.4.2 and 17.6. Major insights into downward negative lightning processes that have been gained from triggered lightning experiments are included in this chapter; a detailed discussion of triggered lightning will be found in Chapter 7.