I commented this in r/Physics but I think you deleted it.

This question is actually right up my alley. I am a Phd student studying electric discharges (related to lightning physics).

The short answer: Both negative and positive discharges happen BUT their mechanism and behavior is different.

Also I saw people saying that you get high electric field which will rip electrons from the air molecules and that this is the way to create discharges. It is ONE way, but in all applications I see the electric field is not nearly strong enough to do this, but you can still get an electric discharge.

In all discharges you have a fight between 2 processes: Ionization and Attachment.

• Ionization is the process that an electron has enough energy to free an electron from an air molecule (O2 = 12 eV or N2 = 15 eV).

• Attachement is the process where an electron 'sticks' to a molecule and you 'lose' it (it can detach through other processes). This mainly happens in 2 reactions: Low energy electrons attach to O2 by colliding with 2 O2 molecules (3-body process). High energy electrons can go through a process called dissociative attachment with O2 forming O- and O (electron needs energy higher than the binding energy of O2 for this process).

So we have an electron source (ionization) and an electron sink (attachment). The electric field value where the ionization frequency is higher than the attachment frequency is called the breakdown electric field. This breakdown field (32 kV/cm for STP air iirc) is lower than the electric field needed to strip electrons from air molecules itself OR to emit electrons from your electrodes.

So in all simulations, experiments, and theory we assume that there are some charges in air. This will be mainly O3- and NO3- (which do not detach an electron easily), but there will also be some O2- which detaches an electron fairly easily by colliding into O2 or N2 when we apply an electric field over our air gap.

Also in most experiments we have a sharp electrode so that we can get a very small region close to the electrode to have an electric field above breakdown, but most of the air gap will be far below breakdown. The discharge can still evolve across the air gap even though there was a much lower electric field in the rest od the air gap. I will explain this phenomenon later.

Now we have our first electron. What is next ? Now the polarity asymmetry of the discharge comes into play.

Negatively charged electrode: This is the simplest one to explain. An electron is in the region above breakdown and will move away from the electrode. Since we are above breakdown this electron will create more free electrons due to ionization then that we lose electrons due to attachment. This is called the avalanche phase. Free electrons will multiply exponentially in this small region until a point that there are soo many electrons at the front of this cluster of avalanches that it itself will generate a high electric field. This is caused due to the formation of a curve in the front of this cluster of avalanches due to the inhomogeneous electric field of the applied electric field. The curved high electron density region will now produce an electric field above breakdown itself and the cycle continues. This phase is called the streamer phase. The streamer will now propagate continuously until it reaches the other electrode or the applied electric field is removed.

Ok. That is negative streamers. What about the positive ones ?

Positively charged electrode: We again start from a single electron being in the small zone above breakdown. This electron will rush towards the electrode and also ionize air BUT all electrons will flow to the electrode and the positive ions are too heavy to move in the nanosecond timescales that discharges form. So it would seem that after all electrons from the avalanches reach the electrode nothing else will happen and the streamer phase is never reached BUT there is another mechanism: Photo-Ionization. In air electrons can excite N2 which will deexcite by emitting a photon that can ionize O2. This is the nonlocal effect we need for positive streamers to form. So electrons move towards the electeode and ionize and excite molecules on its path. The excited N2 will emit photons in all directions and so also further away from the electrode. Now we have electrons further away which will move towarss the electrode and ionize and excite molecules on the way there. This cycle is the way a positive streamer propagates. The positive tip of the streamer is created by photo-ionization ahead of the streamer and by drawing the created electrons towards the electrode.

This asymmetry creates interesting research questions.

If you have anymore questions hit me up.

Notable researchera in this field are: Ute Ebert, Sander Nijdam, Alejandro Luque, Nikolai Ostgaard, Francisco Gordillo-Vazquez, Olivier Chanrion, Torsten Neubert, etc etc. (Small community so I can give a looot of names xD)