I think you can't argue with the fact that the cause (voltage difference) of a bolt of lightning is DC; it is static electricity, caused by built-up positive or negative charges. The charges can either be cloud vs. ground, or cloud vs. cloud. What little I know about it is that the vast majority, 'normal' polarity (forgot what that was for lightning) resulted in 'mild' bolts, whereas a reversed charge was more rare, but also caused the most spectacular lightning, with the differnce in kilo-amps by a factor 10; normal lightning, say, 10 kA peak, and 'reversed' lightning at 100 kA, roughly speaking.
Anyway, now to complicate matters a little bit more.... the cause of lightning may be DC, but the bolt of lightning (plasma) itself is a strong radio transmitter of amplitude-modulated RF, as can be heard on any medium-wave radio. So an effect of the discharge of the built-up charges results in RF (electromagnetic radiation, which is AC).
For example, the first radio transmitters were spark-gap transmitters; and the experiments Rudolf Hertz did in the late 19th century involved DC discharges between two spheres. Definitely DC that was the cause of the arc. But it also produced high-frequency AC.... the RF radiation that he wanted. Same can be seen in Tesla coils, where the AC of a spark gap is transformed to insanely high voltages. And you can only transform AC, not DC.....
Not saying I understand it, far from it. Was just pondering a little and thought to throw this out in the group. Curious what others can make of it.... DC producing (high-frequency) AC....