Bimodal radio frequency pulse distribution of intracloud-lightning signals recorded by the FORTE satellite

[1] The FORTE satellite's radio frequency receiver payload has made millions of recordings of lightning discharges. The most commonly occurring such radio emission arises from intracloud (IC) electrical breakdown and is usually recognizable by a pulse followed by a delayed echo from the ground reflection. We show that these IC pulses have two polar opposite types that together account for much of the pulse population. One type is a very bright pulse characterized by extended width (>2 μs), deep random fading within the pulse, and lack of prior pulses within a flash to which it belongs. The other type of IC pulse is two orders of magnitude less intense and is characterized by narrow width (<0.1 μs), a simple pulse shape evidencing no random fading, linear polarization, and occurrence in close association with other such pulses within the same flash. We develop the characteristics of these two pulse types by extensive statistical analysis of FORTE data. We relate the two pulse types to prior observations by other instruments.