Is lightning possible/visible in vacuum or not? I do understand that lightning is caused by electrons flowing, between two electrically charged regions in the air or the ground, thus causing electrostatic discharge and equalization.
https://en.wikipedia.org/wiki/Lightning
Though, on this site, there is no clear explanation on how these electrons flow, and if lightning is only possible in air or vacuum too, and whether it would be visible in vacuum. 
There are basically two explanations:


*

*Basically, lightning is visible because of the heat that is generated in the molecules of air, and the flow of the current is possible only because air atoms are ionized by the electric field, and the air molecules, in a plasma form, have atoms in excited state, and when these relax, they emit visible light. This is obviously not possible in vacuum.

*Electrons, flowing as the lightning propagates, are free electrons, not bound to nuclei, and if they accelerate, they can emit visible light. Thus lightning would be possible in vacuum, and it could be visible too.
I have read these questions:
Is lightning an example of energy emision from accelerated charge?
where G. Bergeron says:

The sort answer is no. Although the pulse of electric current will surely emit in the radio band as a normal antenna would via the process you're describing, emitting in the visible spectrum using the same process would require extreme acceleration of the electrons. This essentially the reason why you don't see visible light antennas. Most visible light is actually produced by electronic transitions in atoms or molecules.
  In the case of lightning, the current through the air is possible because the air is partly ionized by the electric field. This ionization is then maintained and maximized by the heat generated when the large current of a lightning bolt flows through the ionized channel. This heat is what produces most of the light. The heavily ionized air, now in a state of plasma, has many air molecules in an excited electronic state. When these excited states relax to their normal state, they emit light, much of it in the visible spectrum.

Can lightning happen in a vacuum?
where aquirdturtle says:

Depends on what you mean by "lightning". Yes, because charge can flow across vacuum, but no, because you won't see anything. The visual effect that you see in the sky is actually a luminescent plasma that is left in the wake of the charge moving through the atmosphere. No atmosphere, no plasma, no light.

Can electricity flow through vacuum?
where AV23 says:

The conductivity of the vacuum is not a very trivial issue. In fact, depending on how you look at it, it behaves in two different ways.
  Firstly, there is no retarding force on any charged particle with constant velocity in vacuum. To this extent, no extra work is required in maintaining a constant current through any surface in vacuum.
  Thus, the resistance of the vacuum is in fact, infinite, as long as we define resistance in terms of the response of the charge carriers of a material. In this sense, we might say that it is an insulator - there are no charge carriers.

What would it take to cause lightning to jump between the Moon and the Earth?

Since a "perfect vacuum" contains no charged particles, it normally behaves as a perfect insulator. However, metal electrode surfaces can cause a region of the vacuum to become conductive by injecting free electrons or ions through either field electron emission or thermionic emission.

https://en.wikipedia.org/wiki/Electric_current
So basically not only is the visibility of the lightning debatable, but the flow of the current itself in vacuum is contradictory too. 
Question:


*

*Is lightning possible in vacuum and could it be visible?

 A: First, lets stay away from the low vacuum of Paschen's law - this describes the breakdown of gases in the sub-Torr to 10's of Torr regime. Here, if you sustain the arc, you will definitely see light emission from the excited atoms in the gas phase. 
Instead we look to vacuum being $10^{-5}$ Torr or even better. Clearly one can propagate electrons (or ions) in such a vacuum all day long, as electron microscopes and ion accelerators demonstrate. But, if you look into the beamline where a well-controlled beam is propagating, you don't see anything with your human Mark I eyeball. Even a sensitive photodetector doesn't see anything. If you hit a phosphor (say zinc sulphide as Geiger and Marsden used) you will see light from the energetic beam exciting the phosphor. But, this isn't lightning either. 
So, we look for what is known about two phenomena well known to the electrostatic and pulsed power accelerator community: vacuum breakdown between metal surfaces, and insulator flashover. 
Flashover is well known, not least because it is important not just in vacuum, but in most of our power distribution grid. One of the highest cited papers in this area is Surface Flashover of Insulators, H. Craig Miller, IEEE Trans. Electrical Insulation 24(5) 765-786 (1989). Quoting from the abstract: 

Surface flashover of insulators
  in vacuum generally is initiated by the emission of electrons
  from the cathode triple junction (the region where the electrode,
  insulator, and vacuum meet). These electrons then usually
  multiply as they traverse the insulator surface, either as
  a surface secondary electron emission avalanche, or as an electron
  cascade in a thin surface layer, causing desorption of gas
  which had been adsorbed on the insulator surface. This desorbed
  gas is then ionized, which leads to surface flashover of
  the insulator.

And, yes, one can clearly see this flashover - you have ionized gas so you get recombination. One can dope the insulator surface with specific impurities and see them spectroscopically in the plasma above the surface during flashover. Extensive damage to the insulator, both surface and internal (tracking) may be observed.
The other area is vacuum breakdown, which occurs between two metal surfaces without an insulator in between them. This too has a long history, with one classic reference being Initiation of Electrical Breakdown in Ultrahigh Vacuum, D. Alpert et al., J. Vacuum Science and Technology 1 35-50 (1964). (Note that D. Alpert is known as well for the Bayard-Alpert Ion gauge.) Again quoting from the abstract: 

Based on field emission from sharp submicroscopic points, this picture predicts breakdown when the local electric field at the cathode reaches a critical value. The local field, which for broad area electrodes may be much larger than the average field, is deduced from observations of field emission prior to breakdown. 

In other words, with care they measured the field emission current before breakdown, and it follows a nice Fowler-Nordheim curve. All that is needed is that somewhere on an electrode surface there is some imperfection which locally amplifies the field. At a critical field it breaks down, emitting lots of electrons. This then can locally heat the region around emission (and where they impact), releasing material on the surface (contaminants or metal), leading to ionization and light emission from the now expanding plasma. Extensive damage to the metal surfaces is common, which then may need to be polished out to return to a reasonable state.
For those with experience 'conditioning' high voltage vacuum systems, the release of material into the gas phase from these sorts of events is well known, and easily seen on the vacuum pressure gauges that are usually well away from the actual breakdown event.
So, yes, one can get lightning in a vacuum chamber (and it can be quite a large BANG! when it happens). And, one gets light emission, but it is from the plasma formed during the process.
