Same as 4Greens, I have seen it many times flying a civil aircraft (airbus). It was awesome and mesmorising the first time I saw it. Very cool. I believe it is the discharce of static built up by the friction of ice crystals rubbing together or on the airframe that causes it, and it is at least for me unpredictable. I guess it has to do with the different altitudes flown by the military guys due to different a/c that means they don't see it often, because I guarantee you that all the people I fly with see it quite regularly.......