As seen on this video posted by Elon Musk
https://twitter.com/elonmusk/status/1154629726914220032?s=19
As seen on this video posted by Elon Musk
https://twitter.com/elonmusk/status/1154629726914220032?s=19
That's a shock diamond. Commonly seen in jet and rocket exhaust.
There's a good explanation of the physics behind the diamonds here. Short summary: If the pressure at the exit of the engine doesn't match the ambient pressure, then shock waves will form and reflect throughout the plume, causing the diamond patterns.