Electric and magnetic fields at very close range from a lightning strike to a tall object

We show that the electric field generated by a lightning return stroke to a tall structure can change polarity at very close range, typically at distances of about one tenth of the height of the struck object. Such an inversion of polarity is theoretically explained using the equation derived by Baba and Rakov for the case when the return stroke wave front speed is assumed to be equal to the speed of light and the reflection coefficient at the top of the tall structure is zero. A simple equation is derived which provides an estimate of the critical distance below which such an inversion of polarity might occur. It is also shown that the inversion of polarity depends on the value of the reflection coefficient at the base of the tower and disappears for reflection coefficients close to 1. In other words, the inversion of polarity occurs only for poor tower grounding. Other parameters such as the return stroke speed, the reflection coefficient at the top of the strike object, and the adopted return stroke model seem not to have an appreciable impact on the inversion of polarity.