On the Optical Emission of Intracloud Lightning Leaders

Abstract:

The purpose of this study is to clarify and examine the characteristics of intracloud (IC) lightning leaders. IC leaders have been characterized in the literature in terms of the temporal evolution, the most active portion of the IC leader and electric field waveform characteristics. ICs are noted to vary widely in both waveform pulse characteristics and pattern; thus further characterization with supplementary data yields more insight to the physical characteristics of IC discharges.

Optical emission by IC lightning has not been extensively studied - individual pulse contributions to optical emission are not well characterized and little has been concluded about overall IC optical emission. To address this we explore IC lightning and leader processes in the context of optical emission. Our study includes VLF measurements from the Huntsville Alabama Marx Meter Array (HAMMA), VHF measurements from the North Alabama Lightning Mapping Array (NALMA) and optical measurements from the Lightning Imaging Sensor (LIS).

The data set compiled in this study consists of 345 IC radiation field pulses from 19 IC lightning flashes occuring on Oct. 25th, 2010 within 70km of the center of HAMMA. Waveform characteristics complied include initial half cycle width, rise time, pulse duration, peak amplitude (normalized to 100km) and LIS group associated within 2ms. With this data set we find that large amplitude (>1.0V/m) pulses are consistently associated with a LIS group; however small amplitude pulses (<0.5V/m) constitute 70% of radiation field pulses studied and are not always directly associated with a LIS group. Despite this, a LIS group can be associated with groupings of small amplitude pulses. We seek to understand the cumulative optical emission resulting from small and moderate amplitude pulses; this cumulative emission can exceed the LIS threshold. We compare waveform characteristics of the small amplitude pulses, particularly in terms of the large amplitude pulses which do result in a LIS group. Further, we examine the electrostatic field from flashes within 15km of the HAMMA array. Utilizing nearby IC ashes and LIS groups associated with electrostatic changes, a more complete picture of IC discharges is developed using charge neutralized, the resulting waveform and the related detectable optical emission and radiance.