Currently, my desk lamp, a manual preheat luminaire from the 1970s, has one striating lamp. Weirdly enough, it only started a couple of minutes ago, after running for about two hours. The whole lamp is "flickering" but the striation appears to only be occurring on the right-hand side.

Any ideas? I've seen this in retail stores before, primarily in F32T8s on magnetic ballasts.

See here for a video of what I am currently seeing.

This is usually called swirling. Stirations are longitudinal instability instead, regular stripes often seen in cold or mercury starved lamps.

Swirling is thought to be caused by unintended impurities in lamp's volume. Usually goes away with some aging as impurities are becoming gettered/or decomposed.

Interestingly, neon makers often want swirling deliberately as a special effect in sign tubes, but it is almost impossible to reproduce reliably to appear stable for a long time!

Interesting, I’ve only seen this phenomenon on brand new fluorescent tubes having they’re first run in 😎

I see it frequently on tubes, that have been sitting without power for a few months, mainly when there has been cold...
Could be some residues, which tend to be hammered into the materials when in use and or influence things when mercury is missing (e.g. because condensed,...)

I have observed swirling mostly on brand new lamps and used lamps that have been put into storage for a long time.

IMO swirling occurs when the point of contact between the discharge and the filament is too small. This can be caused when the emitter is still in its initial state and the electrode is prone to building hot spots.
It also can occur when the filament is damaged or the emitter is worn out - this can often be observed in flyback-driven lamps (simple battery inverter style). They tend to swirl when the go EOL...

A small point of contact can also be caused when the lamp is mercury starved and the argon is the main carrier of the discharge current.

In my gold F20T12, it had heavy swirling when I turned it on for the first time. But after a few hours, it disappeared. It never came back since.
I think it was the burning of impurities in the lamp vacuum.

I have observed swirling mostly on brand new lamps and used lamps that have been put into storage for a long time.

IMO swirling occurs when the point of contact between the discharge and the filament is too small. This can be caused when the emitter is still in its initial state and the electrode is prone to building hot spots.
It also can occur when the filament is damaged or the emitter is worn out - this can often be observed in flyback-driven lamps (simple battery inverter style). They tend to swirl when the go EOL...

A small point of contact can also be caused when the lamp is mercury starved and the argon is the main carrier of the discharge current.

Swirling is certainly caused by impurities in the tube volume and is not related to cathodes behavior. It happens even in cold cathode lamps and even with non-activated electrodes like in old Soviet sign tubes.

Note that however because the tube is a closed volume at near vacuum, cathode problems can release impurities and this can start swirling.

I see your point, and it raises a question: why does swirling disappear when the lamp has been running for a few hours?

Talking about switling, I have a two lamp F20T12 fixture in the hallway of my apartment and one of the two lamps suddenly started swirling one day few weeks ago. The strange thing was that the lamp wasn't new. I had changed it about 5 months ago and the fixture is on about 12h per day. I let it do it's thing for the day and in next day it was back to normal.

I believe I've seen this in a bunch of 2x f36t8 fixtures lighting stairways at my local mall. It's not particularly cold here, only being about 10-15°c, and the tubes seem oldish. I'm thinking of pink lines swirling from one side of the tube to the other, maybe I'm talking about something completely different!