A Search for Optical Emissions From Narrow Bipolar Events

The radiofrequency signature of the Narrow Bipolar Event (NBE) has been known for a number of years, primarily from ground-based VLF recordings. Satellite instruments detect strong VHF pulses which are correlated with NBEs. The radio profile of NBEs can exhibit power orders of magnitude higher than that resulting from intracloud discharges. However, any corresponding optical emissions from NBEs or strong VHF pulses have been elusive; it is not clear whether this is due to a lack of intrinsic source luminosity or is a function of the source's location deep within clouds. We have fielded an all-sky photodiode detector system as part of the LANL EDOTX Great Plains Array. The photodiode records transient optical events in synchrony with the <500 kHz electric field change data provided by EDOTX. This allows us to combine optical measurements with geolocated RF signatures identifiable as having cloud-to-ground, intracloud, or NBE origin. With this instrumentation, we can statistically determine the optical power reaching the photodiode detector as a function of lightning type, current, range, and depth within cloud. By extrapolating from the relatively well-understood return strokes and intracloud events, we will be able to set upper limits on the source luminosity of the poorly-understood NBE phenomenon. This will constrain theoretical descriptions of NBE physics.