Abstract
Abstract. Certain intracloud lightning discharges emit energetic, multi-microsecond pulsetrains of radio noise. Observations of this distinctive form of lightning date from 1980 and have involved both ground-based and satellite-based radio recording systems. The underlying intracloud lightning discharges have been referred to as "Narrow Bipolar Pulses", "Narrow Bipolar Events", and "Compact Intracloud Discharges". An important discriminant for this species of radio emission is that, in the range above ~30 MHz, it consists of several microseconds of intense radio noise. When the intracloud emission is viewed from a satellite, each radio pulsetrain is received both from a direct lightning-to-satellite path, and after some delay, from a path via ground. Thus one recording of the radio emission, if of sufficient length, contains the "view" of the intracloud emission from two different angles. One view is of radiation exiting the emitter into the upper hemisphere, the other for radiation exiting into the lower hemisphere. However, the propagation conditions are similar, except that one path includes a ground reflection, while the other does not. One would normally expect a stereoscopic double view of the "same" emission process to provide two almost congruent time series, one delayed from the other, and also differing due to the different propagation effects along the two signal paths, namely, the ground reflection. We present somewhat unexpected results on this matter, using recordings from the FORTE satellite at a passband 118–141 MHz, with simultaneous data at 26–49 MHz. We find that the 118–141 MHz pulsetrain's detailed time-dependence is completely uncorrelated between the two views of the process. We examine statistics of the 118–141 MHz pulsetrain's integrated power and show that the power emitted into the lower hemisphere, on average, exceeds the power emitted into the upper hemisphere. Finally, we examine statistical measures of the amplitude distribution and show that the 118–141 MHz signal emitted downward is slightly more dominated by discrete, temporally-narrow impulses than is the signal emitted upward.
Highlights
The most powerful High Frequency (HF; 3–30 MHz) and Very High Frequency (VHF, 30–300 MHz) emissions from lightning were observed 30 years ago, and were inferred to be due to intracloud discharges (Le Vine, 1980)
We examine statistical measures of the amplitude distribution and show that the 118–141 MHz signal emitted downward is slightly more dominated by discrete, temporally-narrow impulses than is the signal emitted upward
Le Vine observed that these HF/VHF bursts occurred simultaneously with “sferics”, or Very Low Frequency (VLF, 3– 30 kHz)/Low Frequency (LF, 30–300 kHz) radiated electric field pulses from transient charge redistribution
Summary
The most powerful High Frequency (HF; 3–30 MHz) and Very High Frequency (VHF, 30–300 MHz) emissions from lightning were observed 30 years ago, and were inferred to be due to intracloud discharges (Le Vine, 1980). The opportunity to test these predictions on pulse separation was provided by the FORTE satellite (launched in 1997), and the evidence was grossly consistent with the ground-reflection hypothesis (Jacobson et al, 1999) The more numerous leader steps allow a further test of ground reflectivity: The polarization state of coherent lightning emissions in the 38-MHz band has been measured with the dual, crossed antennas on FORTE (Shao and Jacobson, 2001, 2002). For those intracloud discharges whose first www.ann-geophys.net/29/1587/2011/. This could not happen if the ground reflector were substantially Lambertian
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