[CraigS]

Originally Posted by renormalised

Problem is this, Craig....anything masking the Birkeland currents will cause them to collapse. You're effectively blocking the current flow. If you hide them from view (at all wavelengths), you're stopping any emissions from the plasma in which the Birkeland currents exist. Essentially, you're neutralising the plasma. Neutralise the plasma in any way, shape or form and it becomes nothing more than a cloud of gas.

From my rudimentary understanding, parallel filaments seem to feed off each other, also.

Also if there is no movement of the plasma, there is no current and thus no magnetic field and thus, no more filament (?). So if something blocks the path and its big enough (Like a planet or some other disruptive source), I can see that the filament would cease. Apparently, it puts up a fight, though, once its in existence.

But what if nothing blocks the path ?

This dark mode is a furphy. Essentially an engineering term for a bias current generated in a semiconductor detector or circuit without the input of an external source of energy, e.g. light, charge etc. In astrophotography parlance, it's the current that is generated in a CCD when no light is falling on the detector...hence the term "dark". It's called the bias and when taking your piccies, you have to account for it as it will increase the noise present in your piccies. This is a reasonably good explanation of what it is....Dark Current (Dark Mode).

It has nothing to do with astrophysics.

I have a feeling this 'Dark Mode' is not an electronics type of dark current. I'm pretty familiar with flows across doped junctions in semiconductors and I don't think this is what they're on about. The dark mode seems to come from a plasma voltage vs current density graph (verifiable in the lab) and is what precedes the 'Glow' phase which in turn, precedes the 'Arc' phase (as the current density increases). The plasma seems to behave differently, depending on the voltage applied and the density of the current/electric field.

What powers the transition from one phase to another seems to rely on an externally applied EMF. Presumably, this would be coming from a nearby source like a star.

Geezz .. I'm starting to sound like a plasma guy !!

There is a feedback mechanism in plasmas where a z-pinch can gain net energy from the plasma in the pinch (via elastic collisions and induction), but it still requires an input from somewhere to sustain it for any indefinite period. Otherwise, as you said, it becomes a perpetual motion machine.

Yep. This is the big question .. what is the physics of plasma over uninterrupted distances and distances separating the plasma from the driving potential difference ?

Interesting.

Cheers