A Review of Positive and Bipolar Lightning Discharges

Abstract

Characteristics of lightning discharges that transport either positive charge or both positive and negative charges to the ground are reviewed. These are termed positive and bipolar lightning discharges, respectively. Different types of positive and bipolar lightning are discussed. Although positive lightning discharges account for 10% or less of global cloud-to-ground lightning activity, there are five situations that appear to be conducive to the more frequent occurrence of positive lightning. These situations include 1) the dissipating stage of an individual thunderstorm, 2) winter thunderstorms, 3) trailing stratiform regions of mesoscale convective systems, 4) some severe storms, and 5) thunderclouds formed over forest fires or contaminated by smoke. The highest directly measured lightning currents (near 300 kA) and the largest charge transfers (hundreds of coulombs or more) are thought to be associated with positive lightning. Two types of impulsive positive current waveforms have been observed. One type is characterized by rise times of the order of 10 μs, comparable to those for first strokes in negative lightning, and the other type is characterized by considerably longer rise times, up to hundreds of microseconds. The latter waveforms are apparently associated with very long, 1–2 km, upward negative connecting leaders. The positive return-stroke speed is of the order of 108 m s−1. Positive flashes are usually composed of a single stroke. Positive return strokes often appear to be preceded by significant in-cloud discharge activity, then followed by continuing currents, and involve long horizontal channels. In contrast to negative leaders, which are always optically stepped when they propagate in virgin air, positive leaders seem to be able to move either continuously or in a stepped fashion. The reported percentage of bipolar flashes in summer storms ranges from 6% to 14% and from 5% to 33% in winter storms. Bipolar lightning discharges are usually initiated by upward leaders from tall objects. It appears that positive and negative charge sources in the cloud are tapped by different upward branches of the bipolar-lightning channel.

Asymmetries in bidirectional leader development of lightning flashes

“…This event demonstrates how negative leaders develop from retrograde negative leaders and move into the upper charge region, which occurs also in the events of Figure 5. We speculate that a bipolar flash (negative then positive charge transfer to ground [Rakov, 2003]) can occur if the negative return stroke channel to ground remains conductive while a new negative leader expands into positive charge. The LINET system often detects low peak current +CG strokes at the onset of such expansion.…”

Simultaneous records of current and 380-km distant electric field of a bipolar lightning flash

“…3a shows the overall current waveform which contains an Initial Continuous Current (ICC) of negative polarity which lasted for about 320 ms (from 880 to 1200 ms) and included more than 20 superimposed pulses with peak values of about 0.6 to 6.1 kA. After this initial current of negative polarity, the flash exhibits a polarity reversal of the current during its initial stage, therefore belonging to category I according to the classification proposed by Rakov [15]. It is worth noting that this bipolar flash can also be classified as being of Category III which involves return strokes of opposite polarity, assuming that the positive pulse can be treated as a stroke (which we do in this paper).…”

“…In this paper, we present and discuss simultaneous records of current and wideband electric field waveforms at a 380 km distance from the strike point associated with an upward bipolar flash initiated from the Säntis Tower. The flash was recorded on September 21, 2014 and it contained 23 negative strokes and one positive pulse superimposed on the initial-stage current and belongs to the category I bipolar flashes according to the classification proposed by Rakov [15]. In this paper, we will treat the positive pulse as a stroke.…”

Underlying mechanisms of transient luminous events: a review

“…The positive CG flashes preceding the sprite initiation may result in the extraordinary large charge transfer of the order of hundreds of coulombs (Uman, 1987;Rakov, 2000;Rakov and Uman, 2003). The positive charges can disappear for the short time about 1 µs that gives rise to the strong quasistationary electric field caused by negative charges predominantly located at the bottom of the thundercloud.…”

“…This plot corresponds to the thunderstorm charge q = 150 C. current (CC) in sprite-associated +CG lightning. This CC is normally much greater than that due to negative stroke, and its value amounts to 5-10 kA for a period 10-100 ms (Rakov, 2000). In this notation, the lines 1-3 can illustrate the enhancement of QE thunderstorm field that resulted from the thunderstorm charge accumulation during the CC.…”