Black to white gradient

Render Difference Clouds. Repeat a few times while holding down the Alt key to get a decent effect.

Use levels to do the Photoshop Lightning effect

Erase the excess and feather the edges, and define brush preset

A bit of Size Jitter
Maximum Angle jitter
Maximum Roundness jitter

Set the foreground and background colours
And Maximum Foreground/Background jitter
Check Apply Per Tip
Check Build Up

Stamp down with the mouse, and hold the left click till done

It would be good if we had flip jitter, to flip the brush on a 'per tip' basis, but I don't know ho to do that.
If you wanted a radial gradient, just clip it on a layer above.
This is a bit better. I added a wee bit of Scatter jitter, and Checked Both Axis, and that seems to have broken the pattern repetition.
