Many differences exist between positive and negative cloud-to-ground (CG) lightning flashes, though these differences are not well understood. Observations show that negative CG flashes tend to have multiple return strokes with little continuing current from each stroke, whereas positive CG flashes tend to have only one return stroke with continuing current. Though the reason for this polarity asymmetry in terms of multiple strokes has not been identified, the leading hypothesis is related to the idea that the return stroke channel needs to be maintained by infusion of current from within the cloud. Insufficient infusion of current in the initial return stroke may lead to channel decay, thus resulting in a successive stroke. However, if this infusion of current is sufficient enough to maintain the initial stroke channel, then there may be large continuing current down the initial channel and no subsequent strokes. One possible factor involved in this process is the difference in the altitude of the charge source region of positive CG versus negative CG flashes. If the source region for negative CG flashes is further from ground, then negative CG flashes would require a longer channel to the ground. A longer channel length may require a larger infusion of current to maintain it and thus be more likely to decay and require a subsequent stroke. Our hypothesis is that, regardless of polarity, longer channel lengths lead to multiple strokes, whereas shorter channels are more likely to be single-stroke with continuing current.

We will test this hypothesis by using the National Lightning Detection Network (NLDN) and Lightning Mapping Arrays (LMAs) to determine the relationships among polarity, number of strokes, and source altitude. Preliminary results indicate that source altitudes for negative CGs are between 5 and 6 km, whereas source altitudes for positive CGs are between 8 and 9 km. Though these initial results are contrary to our hypothesis, we will analyze a larger, more diverse data set before making any conclusions.