What is lightning channel conditioning?*

A cloud-to-ground lightning flash can last as much as a second and consist of one to 20 return strokes. Some return strokes retrace channels first created by earlier strokes. The leader for an initial channel is called a stepped leader and has a propagation velocity of $\simeq5\times10^5$ m/s. If another leader retraces that channel within 100 ms, it progresses much more rapidly, up to $\simeq1\times10^8$ m/s. It is said that the difference between a stepped leader and a dart leader is that the dart leader proceeds on a channel which has been ``conditioned'', by the prior stepped leader. What is conditioning? One might assume that the channel is still hot and thus has a high population of free electrons. Radar studies by Holmes that averaged over tens of km of space found electron recombination time constants of 3--20 ms. Spectroscopic studies have also suggested channel cooling times in the millisecond range. How then, can a dart leader occur 100 ms after a stepped leader? One idea is that the free electrons recombine with Oxygen atoms creating a temporary population of ${O_2}^-$ ions. The ionization potential of ${O_2}^-$ is less than 1.5 eV, compared to 12 eV for $O_2$. Thus, conditioning is perhaps a "chemical" rather than a thermal process.