Apologies in advance for the laymans language.....
There has been alot of research done, and I think the conclusion is that the strikes tend to happen around the 0°C air band (+/- a bit!) So, this puts helicopters smack in the middle of this region in the North Sea for much of the year. Basically, the charge is developed from friction between slightly wet water crystals rubbing together as they change levels in the cloud and you end up with one set of charged crystals in the region just mentioned, and the opposite charged crystal higher. Normally this continues until the electrical potential is large enough to bridge the gap, unless some other helpful initiator passes by - like an aircraft! Aircraft are themselves highly charged, and are also leaving an ion trail from their exhausts - they may as well tow a line of copper wire!
So, in the North sea the helicopter actually causes the lightning and then attracts it, whereas in the tropics the lightning has occurred 'naturally' at a high level and it is more or less bad luck to be in the way as it passes to earth. Or, the aircraft is on the ground and acts as a handy lightning conductor!
Of course, it's not only helicopters that get struck - turboprops are very succeptible in warm climates as they stagger slowly through the 0°C levels, and FW in general do suffer strikes on their way up and down. The big difference is that helicopters are more vulnerable to the potential damage that results.
Having spent the last 15 years flying in equatorial lattitudes, I can vouch that the lightning is pretty incredible, and I'm very glad that this characteristics is true
