On negative Sprites and the Polarity Paradox

Energetic positive and negative cloud-to-ground (CG) flashes are both capable of producing sprites. Negative CGs typically outnumber the positive ones by 10 to 1. However, >99.9 % of reported sprites were found to be initiated by positive CGs-thus the polarity paradox. Here, sprites recorded by the Imager of Sprites and Upper Atmospheric Lightning (ISUAL) mission were analyzed along with extremely low-frequency band magnetic field data to resolve this paradox. Approximately twenty-five percent of the sprites are found to be associated with negative CG lightning. "Negative" sprites mainly congregate in the latitudinal regions below 20 degrees, while positive sprites scatter up to 50 degrees. The ISUAL negative sprites are evidently beyond the observable ranges of the ground sites reported in previous studies. Hence, the sprite polarity paradox is likely a selection effect of the middle- to high-altitudinal observation sites. The charge moment changes and accompanying transient luminous events of sprites were also examined and found to be polarity dependent. Plain Language Summary Sprites are lightning-induced optical transient emissions in the mesosphere. In the 1920s, C.T.R. Wilson proposed that both intense positive and negative cloud-to-ground (CG) lightning can cause dielectric breakdown in the mesosphere and induce the sprites as we call them today. The occurrence of negative CGs typically exceeds the positive ones by a ratio of 10 to 1. However, more than 99.9% of sprites reported so far were produced by positive CGs and this leads to the polarity paradox. The rarity of negative sprites also inhibits the effort to elucidate their general characteristics. In this study, sprites observed by the Imager of Sprites and Upper Atmospheric Lightning (ISUAL) mission were analyzed along with two sets of magnetic field data recorded at the extremely low-frequency (3 Hz-3 kHz) band to resolve this paradox. In all, 127 negative CG sprites, similar to 25% of all the polarity-identifiable sprites, were recognized unambiguously. The observed negative sprites mainly congregate in the tropical regions below 20 degrees latitudes, but positive sprites scatter over wider areas up to 50 degrees. The observed ISUAL negative sprites were mostly beyond the observable ranges of the ground sites reported in previous studies. Hence, the sprite polarity paradox is likely a selection effect of the observation sites.