JOURNAL OF GEOPHYSICAL RESEARCH, VOL. 99, NO. D5, PAGES 10,641-10,652, MAY 20, 1994 Propagation effects on the lightning-generated electro- magnetic fields for homogeneous and mixed sea-land paths Vernon Cooray and Ye Ming Institute of High Voltage Research, University of Uppsala, Sweden Abstract. The influences of the path of propagation on the shape and amplitude of the electric fields andelectric field derivatives generated by lightning return strokes wereinvestigated. Results arepresented for the cases where(1) boththe lightning return stroke andthepointof observation are located over finitely conducting, homogeneous ground and (2) the lightningreturn stroke is located over seaand the point of observation is located over land at some distance from the sea- landboundary. For propagation paths over finitely conducting ground the electric field derivative can decrease significantlyin propagation distances as small as 1000 m. When the path of propagation is partlyoversea and partlyoverland,thepropagation effects on the electric radiation field derivative are significant unless the width of thelandportion of the propagation path is less than a few tens of meters. 1. Introduction Protection of structures and electrical systemsfrom lightningrequires knowledge concerning the characteristics of electromagnetic fields generated by lightning and the statistical distribution of lightningcurrentparameters. The statistical distributions of lightning current parameters canbe obtained by recording the currentsin lightning flashes striking high towers [Berger, 1967]. However, the presence of the towermay distort these distributions to some degree, andthereis always the unresolved question of whether these distributions are valid for lightning flashes striking plane ground. On theother hand, some of the information necessary for determining the characteristics of currents in lightning return strokes is embedded in the lightning-generated electromagnetic fields. However,in propagating from the source to the measuringstation, the characteristics of the electromagnetic fields will change in various degrees depending on the geometry and electricalcharacteristics of the propagation path. For example,in propagating over finitely conducting ground, the electromagnetic fields will lose their higher-frequency components, and as a result,the amplitude of the electromagnetic field decreases and its risetime increases with increasing propagation distance over land [Le Vine et al., 1986; Urnan et al., 1976;Gardner, 1981; Cooray and Lundquist, 1983; Cooray, 1987].Therefore any attempt to derive returnstrokecurrent parameters from the measured fields without correcting for thesepropagation effects may lead to significant errors. Recently, Cooray [1987] analyzed the propagation effectson the risetimes and initial pea• of lightning return stroke radiation fields as they propagate over finitely conducting ground. The results given in that papercouldbe usedto estimate the attenuation of the initialpeakof the electric field for distances larger thanabout 5 km. Furthermore, the results given in thatpaper show that the attenuation of the initial peak of the electric field is not very largefor distances smaller than 5 km. For example, at 5 Copyright 1994 by the American Geophysical Union. Paper number 931D03277. 0148-0227/94/931D- 03277$05.00 km the attenuation of the peak is less than 5% for a ground conductivity of 0.01 S/m and is lessthan 20% for a ground conductivity of 0.001 S/m. However, in that study no attempt was made to determine the propagation effects on the derivatives of the return stroke fields. Therefore we were interested in investigating the propagationeffects on the electric field derivatives of lightning-generated electro- magnetic fields. To avoid the propagationeffects caused by finitely conducting ground, many researchers have measured the electromagnetic fields from lightning using techniques that minimize propagation effects. This was achieved, for example, by measuring electromagnetic fields from lightning flashes striking the sea,so that the pathof propagation of the electromagnetic fields is over sea-water[Weidman and Krider, 1980; Cooray, 1986; Willerr et al., 1988, 1990; Le Vine et al., 1989]. Since seawater is a better conductor than soil, ectromagnetic fields propagating over seawater are subjected to much less severe propagation effects compared with electromagnetic fields propagating over land. Recently, Ming and Cooray[1993] analyzed thepropagation effects caused by a finitely conducting and rough sea on the lightning- generated electric fields. In some of the experimental investigations mentioned above, the electromagnetic field recordingsites were situatedseveral tens to severalhundreds of metersinland [Cooray, 1986; Willerr et al., 1988, 1990; Le Vine et al., 1989]. In such situations the path of propagation of electromagnetic fields generated by lightning flashes strikingthe, sea is entirelyover seawater except for the last few tens or hundreds of meters. It would therefore be of interest to investigate the propagation effects on the electromagnetic fields caused by the presence of this stripof land in the path of propagation. In this studywe calculated the propagation effectson the lightning-generated electric field derivatives with special attention to distances of propagation less than about 10 km. Furthermore, resultsare also presented for the propagation effects on the lightning return stroke fieldsas theypropagate along a mixed sea-land path. These results allowed us to interpret the experimental observations obtained by Le Vine et al. [1989] and Willerr et al., [1988, 1990] where the recording station was situatedseveral tens to severalhundreds of meters from a sea-land boundary. 10,641