Space-Based Observation Of A Negative Sprite With An Unusual Signature Of Associated Sprite Current

We report the first observation of an unusual red sprite produced by negative cloud-to-ground (CG) lightning from the Imager of Sprites and Upper Atmospheric Lightning (ISUAL) in Colombia on August 15, 2012. The impulse charge moment change for this event is approximately -520 C center dot km, barely reaching the threshold for negative sprite production. However, different from all previously reported observations, this negative sprite contains a distinct "sprite current" feature that is unambiguously identified in the very-low-frequency magnetic field recorded at the 3,387 km range in Duke Forest. There is no evidence that this feature was produced by a fast discharging process associated with the causative CG stroke. Instead, the sprite current signal corresponds well to high-altitude optical emissions associated with the charge flow in the sprite body. The charge moment change caused by the charge transfer in the sprite region was estimated to be about -460 C center dot km. We attempt to reveal the possible causes of this atypical event from the perspective of the parent thunderstorm and the local ionosphere irregularities, whereas we believe that the most possible reason remains to lie in an extraordinarily long duration (more than 5 ms) of charge transfer to ground following the return stroke.