In 2019 Aug 27's test flight, during the final 8 seconds before landing (T+00:45), Starhopper's plume was suddenly much brighter. Why?
The Raptor's thrust can't have changed much then, because Starhopper's speed and trajectory hardly changed. A different fuel mix? (Again, why bother?)
The glow starts about when the visible plume touches the landing pad, which had already started to glow. But how could that touch affect the appearance of the plume itself? Burning concrete dust (or whatever) can't propagate up towards the engine.