What the LMA can tell us about lightning initialtion processes

We are studying the details of the initial lightning behavior using the New Mexico Tech lightning mapping array (LMA). The LMA includes an array of receivers that each accurately finds the power and the arrival time of RF (radio frequency) impulses at 60 MHz. This information is used to locate the source of many of the RF impulses in the thunderstorm. In a large lightning discharge we are able to locate the source of several thousand impulses. The locations give a 3-D picture of the discharge. Most of the impulses are generated as the negative leader extends in a stepping manner. If there are RF impulses from the positive leader they are much weaker and we have no evidence that they are seen by the LMA. Laboratory experiments clearly show that positive and negative leaders behave very differently. Runaway electron avalanches and breakdown (RB) may enhance the electrical field in the storm which can then initiate conventional breakdown. We have looked for evidence of this in several ways. First we found that the initial leader speed seems to be enhanced by the conditioning of the atmosphere by the RB. Second we have looked for evidence of RF impulses generated by the RB rather than leader extension; the results are not conclusive. If cosmic ray generated extensive electron showers are needed for the RB to initiate lightning, then the lightning rate cannot be greater than the rate of high energy cosmic rays. We have found areas in some storms where the rate of lightning initiation is at least 10 times that for 1016 eV cosmic rays.