What conditions are necessary for Birkeland currents?

[celeste]

If charged particles flow through non-magnetised space will they form Birkeland current? Or can such currents only form along existing magnetic field lines?

A question in return: In the water bridge experiments, we have what looks (and works?) like a current filament, yet it runs between the two beakers. It does not seem to matter if the water bridge is aligned with the background field in the room, correct?

Not sure, but yes, if you say so. What is the significance of this observation?

The water bridge does not need to be aligned with the background magnetic field. Your second question was "can such currents only form along existing magnetic field lines?"

[celeste]

In the water bridge experiments, we have what looks (and works?) like a current filament, yet it runs between the two beakers.

Does it really act like a Birkeland filament?

Can water bridges do this (observe the helical winding in pairs of Birkeland filaments)?

https://upload.wikimedia.org/wikipedia/ ... Nebula.jpg

With the water bridge, they found ions spiraling around the surface. This is the same type of motion our solar system has as it travels along the surface of the local interstellar cloud. And it is this radial charge separation, that holds the filament together.

I have not seen experiments where they bring two distinct water bridges together. That would be interesting to see.