Sprites in rapid succession - ELF electromagnetic signatures and current moment waveforms

Abstract:

An intense thunderstorm that passed over Central Europe on August 6 and 7, 2013 generated a large number of sprites. Some of these sprites occurred in rapid succession - a few sprites within a 1-second interval. We have analyzed their ELF electromagnetic signatures recorded at the Hylaty ELF station, which we operate in a sparsely populated area of Poland. The station is equipped with two orthogonal magnetic antennas, one facing north-south and the other east-west. The thunderstorm was located only a few hundred kilometers away from the ELF station, which allowed us to obtain high quality waveforms. The video images were registered even closer, in Nydek, Czech Republic. 
Based on the recorded signal we have reconstructed the current moment waveforms and calculated the charge moment changes of the most interesting cases of sprites in rapid succession. We have divided them into two categories depending on the evolution of their current moment waveform, that is, sprites in sequences and dancing sprites. In this work we present examples that illustrate the difference between these two categories. We also show evidence of an ultra low frequency waveform associated with each sprite event that, surprisingly, was observed in the antenna that was practically parallel to the direction of the sprite associated discharges and could not have recorded the electromagnetic waves generated directly by the discharge current.