On our last Atomic physics lecture, we experimented with gas discharge tubes, and the dark-light zones. During the experiment, bright, white flashes of light appeared on the cathode of the tube. The number of them increased with the anode-cathode voltage, and didn't seem to be related to the pressure inside the tube. I asked the professor and the TA presenting the experiment about them, but at the end we could not come up with a statisfying explanation.
The phenomena is observable on the following video I took after the lecture:
Flashes on the anode of a gas discharge tube (YouTube)
A frame from the video (though the video gives a much better insight):
The flash rate seems to be consistently 3-4 flash per frame at 29.7fps, giving a rate of approx. 104 flashes per second.
The tube was an approximately 1m long, 5cm diameter cylinder filled with air, connected to a vacuum pump and an adjustable valve to dynamically sustain an adjustable degree of vacuum. The anode and the cathode are both approx. 1cm wide aluminium cylinders with a small hole in the middle. Their sides are exposed to the environment outside of the tube. The anode-cathode voltage is adjustable between 1-6kV, but the exact voltage was not noted.
What is the explanation of these flashes?
They are caused by the dust in the air, turned into plasma (source: the professor)
They are sparks on the anode's surface (source: TA)
They are caused by the electrons when leaving the surface (my idea)
It is some kind of a scintillation effect (my idea)
NOTE: a previous version of this question wrongly stated that the flashes are happening on the anode. Older answers and comments are reflecting this.