Influences of the Earth Conductivity on Electric Fields Radiated from Lightning Strikes to Tall Towers

Waveforms of electromagnetic field radiated from lightning return strokes to tall towers include oscillations due to current reflections at the top and bottom, thus differing from those radiated from return strokes to flat earth. Further, because of differences in conductivity between sea and ground, waveforms of electromagnetic field radiated from lightning return strokes to an offshore wind turbine tower might be different from those radiated from lightning return strokes to an inland tower. We have studied the influences of earth conductivity on vertical electric field waveforms at distance 200 km, radiated from lightning return strokes to flat earth and a <tex xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">$180-\mathrm{m}$</tex> tall towers, with the help of the finite difference time domain method. It follows from the computed results that electric fields in the vertical direction at far distances are attenuated and distorted when the earth conductivity is lower, particularly for lightning return strokes to tall towers. We have estimated vertical electric field waveforms for perfectly conducting earth from those for lossy earth.