Hot near-vacuum gases often need molecular approaches to make models. "Sound" itself needs careful definition in such atmospheres. They're talking about jets of air pushed around by huge electric fields. "Speed of sound" is just a good way to indicate scale, it probably doesn't have much other useful meaning.
Yeah, that's pretty much what I'm getting out of the article... the problem is I still have no idea what that scale is, do you? I'm sure the term 'supersonic' was meant to impress and sound fast, but there's at least an order of magnitude uncertainty.
> "Sound" itself needs careful definition in such atmospheres.
Nothing to say here except I love that sentence.
How much thermal energy ends up as kinetic energy in translation effects the proportion of T to speed of sound.
The storage also depends on temperature, how much energy goes into each degree of freedom (rot, trans, vib) changes with temperature but is fairly static around the human experience scale of things.
So according to the above graph, supersonic speed in the ionosphere is ~280-300 m/s, lower than at sea level. Temps are lower on average too. But a plasma jet punching through the ionosphere at 10k C would mean the plasma itself is creating the condition for supersonic speeds on the order of 2000 m/s. (Not sure if the research is saying this, or saying the jets are moving at supersonic relative to the surrounding atmosphere...)
Just found this super-helpful link: http://www.sengpielaudio.com/calculator-speedsound.htm
I had forgotten about speed of sound being primarily governed by temperature until @djaychela reminded me. But I still find it completely unintuitive, even with your explanation... which is really interesting!