Formation

It was shown in Section 2.2.4 that electron avalanches can form when the E/N ratio exceeds the breakdown threshold of air at $\simeq$123 Td. It was also demonstrated that the electrons will collectively drift under the influence of an external electric field and this cloud of electrons will expand (see Figure 2.2.4a) due to diffusion and mutual electrostatic repulsion.

As the electron cloud expands and the number of electrons grows, the electric field produced by the electrons and positive ions will increasingly weaken the total electric field in the region between the electron and positive ion charge centers. In contrast, the electric field both ahead of the electron charge center and behind the positive ion charge center will be enhanced.

Figure 2.1 shows that the ionization and attachment frequencies ($\nu_i$ and $\nu_a$) are strongly dependent on the $E/N$ ratio. Thus, the alteration of the electric field by space charge in the electron avalanche can lead to a substantial alteration of the ionization rate, reducing the ionization rate in the region between the electron and positive ion charge centers and enhancing it ahead and behind of the respective charge centers. The enhanced ionization regions can develop and intensify into space charge waves which propagate in opposite directions away from the avalanche and have opposite polarities (Dhali and Williams, 1985; Loeb and Meek, 1940). These space charge waves are commonly referred to as ``streamers''.

There is a minimum radius of the avalanche region which is required for the streamer transition, as was shown by Pasko et al. (1998a) and Raizer et al. (1998). At 80 km altitude, the minimum radius is approximately 60 m (Raizer et al., 1998).