Why is Lightning referred to as High frequency source?

Lightning strike is categorized as DC because of its electrostatic nature. The negative flash/stroke is found to have 200KA of current with a voltage close to 300KV (cloud to ground). But why is it referred to as a High frequency source, since it does not have a frequency component? In an electric circuit diagram will lightning be represented as a voltage source or current source?

3 Answers

Of course lightning "has a frequency component". If it didn't have non-zero frequencies, it could never start or would last indefinitely.

Lightning is a huge current pulse over a short time, usually several pulses over a few milliseconds. But more importantly, the current is started and stopped very abruptly, which by necessity means it has a broad spectrum containing significant high frequency content.

Spark gaps in general are infamous for being broad band frequency sources well into the radio range. Some early radio transmitters harnesses this by connecting a spark gap to a resonant circuit. Since the spark gap produced a wide spectrum, it also produced the frequency of the resonance. However, the other frequencies aren't sufficiently attenuated, so spark gap transmitters are specifically banned in the US (probably most countries).

I think your question really comes down to a unawareness of Fourier analisys. That would be a good search term to look up additional information.

According to wikipedia, average duration of a lightning is $30\,\mu s$. If we take a gaussian current splash with $\sigma=30\,\mu s$, its spectrum will be a gaussian with $\sigma_k\approx33\,\text{kHz}$. This doesn't look like DC.

I'm not sure what's meant by a "gaussian current splash". But per various sources, an individual lightning stroke (ignoring repeat strikes, branching, etc) consists of an essentially instantaneous rise from 0 to perhaps millions of amps, followed by exponential decay with a half-life of around 5 microseconds (although there's quite a lot of variance in that number depending on where you read). I made up some data like that and ran it through an FFT. The answer was that you do get quite a lot of DC and low frequency AC, but there's also significant energy put out in the 5 KHz and above range.