On the Causative Strokes of Halos Observed by ISUAL in the Vicinity of North America

The analysis of impulse charge moment change (iCMC) for the parent lightning strokes of 497 halos observed by the Imager of Sprites and Upper Atmospheric Lightning near North America in 2004–2015 indicates that the majority were produced by negative cloud-to-ground strokes predominantly produced in oceanic and coastal thunderstorms. Positive halos are almost always accompanied by sprites, and negative sprites are usually associated with halos. There are limited observations of positive pure halos with supercritical iCMCs (> +320 C km), but there are many negative pure halos with supercritical iCMCs (> −500 C km), suggesting a critical role of impulse charge transfer duration in the formation of streamers. The halo-producing threshold of lightning strength does not considerably depend on the polarity. Due to the dependence on the timescale of impulse charge transfer for streamer development, many negative cloud-to-ground strokes with iCMCs exceeding the threshold for sprite production actually produce halos instead.