Broadband Positive Breakdown Characteristics of Natural Positive Lightning Leaders

Using an upgraded short-baseline VHF interferometer with 200 MHz bandwidth and improved sensitivity, radio emissions from weak positive breakdown of an intracloud (IC) positive leader tip and strong positive breakdown of a cloud-to-ground (CG) positive leader tip are definitively imaged.

The weak IC positive emission signatures are identified when negative breakdown is inactive, including the needle discharge. Comparing to the negative needle breakdown occurring in large scale along the positive leader body, the IC positive breakdown concentrates at the leader tip, and has essentially similar spectral shape and pulse time scale as negative breakdown pulses but at least 10 dB weaker. For this reason, the IC positive leader breakdown is undetectable in most cases due to low SNR and a mask by concurrent negative breakdowns. Although we are not able to identify positive leader tip breakdown when the needles are active, it should be noted that the positive leader tip and the propagating needle front extend at a same 2D speed of 3×10^4 m/s in the case of our study.

In addition, strong positive breakdown emission produced by a downward CG positive leader tip is also determined. The CG positive leader breakdown produces VHF emission as intensive as a negative leader at low frequencies (<100 MHz) that is easy for interferometric imaging, but the VHF power decreases more than negative leaders at high frequencies and again gets masked by negative breakdown during 100-200 MHz. The CG positive leader tip propagates continuously in the 2D imaged map at a speed around 2×10^6 m/s but produces impulsive "stepped" VHF radio bursts. Each pulse burst lasts about 5-10 μs and contains ~10 narrow VHF pulses of equivalent magnitude and similar pulse interval ~1 μs. These bursts are seen as stepped pulses in the low frequency (LF) magnetic field data of step interval ~20 μs, which is similar to a negative stepped leader. We then compare these bursty emissions to stepping negative emissions to gain some insight into how they are similar and how they are different.