The entire process usually occurs in less than a half-second. A CG lightning discharge typically initiates inside the thundercloud. When enough electrons collect in the bottom of the cloud, a very faint, negatively charged channel, called the stepped leader, emerges from the base of the cloud. When the stepped leader approaches the ground, its strong, negative electric field repels all negative charge in the surrounding ground, while attracting all positive charge. An electric current wave then propagates up the channel as a bright pulse This discharge process takes less than 100 microseconds and is called the return stroke.
Electric charge flows up the channel behind the wave front and produces a ground level current.
This current has a peak value of about 30,000 amperes, though it can be as high as 300,000 amperes. It takes about 1 microsecond for the current to reach its peak value, and about 50 microseconds to decay to half that value. As the leader charge flows down the channel to the ground, electric and magnetic field changes are produced that propagate outward from the entire length of the channel. These field changes have rapid variations that follow the channel of the stepped leader. The field changes have electrostatic, inductive, and radiative components, and each of the components has fluctuations of different frequencies that have different attenuation characteristics as the fields propagate from the lightning channel.