The transient ELF (~50-5000 Hz) magnetic field radiated by lightning discharges across North America was continously measured at Duke University during the summer of 2000. In total, 881 sprite-associated lightning discharge over 17 days were analyzed. We report in detail on 76 sprites for which we could reliably determine the lightning charge moment change from the ELF data at the time of sprite onset. The charge moment change for the initiation of a sprite is found to be as low as 120 C km. By folding together the charge moment distributions of sprite-producing lightning and all positive lightning, we find that the probability of sprite generation for lightning with >1000 C km charge moment change in less than or equal 6 ms is >90%, while the sprite probability for lightning with less than or equal 600 C km charge moment change in less than or equal 6 ms is less than or equal 10%.
Electric discharges;Magnetic fields;Probability;