Return-Stroke Modeling Based on Observation of Close Electromagnetic Fields

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An electromagnetic return stroke model of negative cloud-to-ground lightning is newly proposed. This model is aiming at studying the difference of lightning current parameters between those of lightning strokes hitting directly ground and of those hitting a tall structure. The analyzed results predict that the influence of a tall structure hit by lightning on associated electric field is quite different from that on associated magnetic field within a few hundred meters from the hitting point. If simultaneous measurement of lightning current, close electric field and close magnetic field is achieved, the proposed model will be evaluated. A verified model will conclude the argument on the discrepancy in the statistics of lightning currents measured on instrumented towers and those estimated from remote electromagnetic field.Regarding subsequent return strokes, a new engineering model which can well reproduce published observation results on current and close fields associated with rocket-triggered lightning has been developed. The current waves in a subsequent return-stroke channel start to travel upward and downward from the junction point of connecting leaders, a few meters above the ground. The velocity of the current waves is estimated to be less than a half of the speed of light, and the downward traveling wave is reflected at the ground.The newly proposed return-stroke model for a first return stroke also predicts that the relationships between the peaks of lightning current and remote electromagnetic field are not the same for first and subsequent strokes. The difference comes from the well-known fact that the rise time of a first return stroke is generally much longer than that of a subsequent stroke. Verification of this point by reviewing accumulated observation data will clear the unresolved problem on the discrepancy between the statistics of lightning current obtained by direct measurement and of those estimated electromagnetically.