They last longer than normal lower stratospheric discharges, which last typically a few milliseconds, and are triggered by the discharges of positive lightning between the thundercloud and the ground.
Early and late polar breakup episodes have occurred, due to variations in the stratospheric flow structure and upward spreading of planetary waves from the troposphere.
Though the same chemical substance, ground-level ozone should not be confused with stratospheric ozone, or the ozone layer, which protects the earth from harmful ultraviolet rays.
