I'm fairly sure that most of the arc is caused by the capacitive circuit formed between the line, the helicopter, and earth/other lines, combined with corona discharge current from the helicopter to the air around it, and again from there to the other lines or earth.
I estimated the capacitance of a helicopter as being equivalent to a sphere 3 meters in radius. This is larger than the body, smaller than the rotor disk, and of the wrong shape...but it should give an order of magnitude approximation. I got a value of about 330 pF. The 500KV current into a 330pF capacitance at 60Hz is about 60mA, more than enough to get your attention. That 60mA will be relatively constant current; it doesn't matter if you have 50 or 100kV to make an arc jump, or direct bonded contact with no voltage drop at all; most of the voltage drop is across the capacitor, and the capacitor is what limits the current.
I don't know how to figure corona discharge from a helicopter at 500KV
I did some looking; helicopters can charge up to very high voltages, but it looks like the current is less than 1mA even in really bad conditions. One interesting point which I came across but did not look into enough to confirm. It seems that most of the electrostatic current doesn't come from the spinning blades, but from other sources, eg. engine exhaust.
My guess: The capacitive current dominates the arc that we see. We see an arc when the circuit is made (closing the switch) and when it is broken, and at other times the same 60mA is flowing through the fat bonding cable. Just my best guess at the present time.
-Jon