Statistical Characteristics of Magnetic Field Produced by Tall-Object Lightning in Guangzhou During 2011-2012

With the development of society and economy, more and more tall objects, such as tall towers, skyscrapers and other kinds of high buildings are erected in China. It is a commonly used method to study physical mechanisms of lightning discharge by measuring the electromagnetic fields produced by the lightning occurring on tall objects. Characteristics of electromagnetic fields and the influence on the electromagnetic environment induced by lightning flashes occurring on or around the tall objects are also widely studied. Since 2009, a field experiment is conducted to study the physics process of lightning flashes striking on tall objects in Guangzhou. The Tall-Object Lightning Observatory in Guangzhou (TOLOG) is established on the top of a building with a height of approximately 100 m that belongs to Guangdong Provincial Meteorological Bureau to observe lightning flashes striking on tall objects with different heights in Guangzhou. In this experiment, the height of observed lightning striking point is found to be within 90-600 m, while the distance between the lightning striking points and the observation point is within 140 m-3.3 km. Magnetic field data for 40 negative lightning flashes obtained during 2011-2012 are analyzed. Statistical results show that tall objects have an enhancing effect on the magnetic field induced by the lightning flashes striking on them. The taller the object is, the larger the enhancing effect will be. The geometric mean (GM) value of the magnetic field peak values induced by the lightning flashes to the objects taller than 200 m is 2.4 times of that induced by lightning flashes to the objects lower than 200 m. Waveforms of the lightning magnetic field always exhibit multi-peak behavior. Regarding the magnetic field waveforms of the first return stroke, 13 out of 20 cases in which the lightning flashes strikes objects lower than 200 m have the subsequent peak value that is greater than the initial peak value; 8 out of 14 cases in which lightning flashes that strike tall objects higher than 200 m exhibit the same characteristics. The GM value of inter-stroke intervals of all of 135 return strokes is 69.1 ms. Among them, the GM value is 65.0 ms for the inter-stroke intervals of the 53 return strokes occurring on the objects taller than 200 m, and 71.5 ms for the inter-stroke intervals of the 82 return strokes occurring on the objects lower than 200 m. In addition, 10 lightning flashes (45%) among 22 multi-stroke negative lightning flashes are found that the magnetic field peak value induced by subsequent return stroke is greater than that induced by the first return stroke.