Fluorescent "barber poleing"

[lasagafield]

Today I got a 5ft fitting and shoved one of my 65/80's in it and it started doing something unusual.
There was a patch of brightness that was "helter skeltering" it's way down the tube.
Say you have the cylinder of the tube and then this spinning barber pole pattern of brightness that was spinning it's way around the tube.
It's absolutely mesmerizing to watch!
It's like the arc inside has tightened and was rolling around inside the tube and slowly rippling it's way down.
My camera won't pick it up so that's the best I can describe it.
Does anyone know what causes this?

[fluorescent]

It's known as swirling, I believe it's just the gasses in the tube that have separated, it'll settle down after a while.

[Slyspark]

Take tube out, rotate it 180 degrees and put it back in. It'll stop then

[Kev]

Had an 8’ do it really bad once one end was completely mercury starved the other was dim but one patch was really bright it did eventually sort it’s self out!

[lasagafield]

I switched it off for an hour and put it back on and now its fine.
It's a cool effect though. I wonder if you can change the gas mixture inside to increase the effect?

[Zelandeth]

Quite common on T12 lamps either the first time they're used (or the first time they're used on a substantially different gear type sometimes). Usually settles down after a bit of fun time.

I don't have sufficient knowledge to explain exactly what it is that's happening or why sadly. Have wondered myself though.

It can happen with T8 tubes as well, but in my experience this seems far less common.