Radio Science, Volume 11, Number 12, pages985-990, December 1976 Effects of 200km propagation on Florida lightning return stroke electric fields M. A. Uman, C. E. Swanberg, J. A. Tiller, and Y. T. Lin Departmentof Electrical Engineering, University of Florida, Gainesville, Florida 32611 E. P. Krider Institute o[ Atmospheric Physics, University o[ Arizona, Tucson,Arizona 85721 (Received June 1, 1976.) The electric fields produced by lightning return strokes near Kennedy Space Center, Florida, have been measuredsimultaneously at distancesof about 5 to 25 km and about 200 km. Detailed records of the first 12 ixs of waveforms from four strokes are presented, as well as data on the initial field risetimes for 58 first and 92 subsequent strokes. The mean field risetime measured between the 10 to 90% points was about 1 ixs at the close station and about 2 ixs at the distant one. INTRODUCTION Johlerand Lilly [ 1961 ] have examined the effects of propagation on lightningelectric fields by analyz- ing waveforms from two Kansas return strokes observed simultaneously at about 60 and 400 km. The measuring system had an upper-frequency responsenear 100 kHz, and the observed waveform spectra were primarily below this limit. The mea- surements were made in an attempt to ascertain whether ground conductivity could be calculated from the differences in the spectra at the two stations. Electric field risetimes from strokes at distances of 0.5 km to 300 km have been measured recently by Fisherand Uman [ 1972], Lin and Uman [ 1973], Krider and Radda [ 1975], and Tiller et al. [1976] as part of an effort to determine lightning currents and other properties from remote electric and magnetic field measurements [Uman and McLain, 1970; McLain and Uman, 1971; Uman et al., 1973a,b; Uman et al., 1975a,b]. In this work it is importantto understand the effects of propaga- tion on the field wave shapes since accurate lightning propertiescan be determined only from undistorted waveforms. The upper-frequency response of the measuring systems used in theserecent studies was above one MHz. The published risetime data of Fisher and Uman [1972], Lin and Uman [1973], and Tiller et al. [1976] represent "zero" to peak measurements, Copyright¸ 1976 by the American Geophysical Union. with "zero" being chosen at the beginning of the abrupt field transition or "breakpoint" [Tiller et al., 1976] if the field prior to that point was small, and otherwise at an earlier point of relatively small slope. The intent was to identify only the field due to the return stroke and to exclude the presumed slower field changes due to the preceding leader or upward-going discharges initiated from the ground [Krider and Radda, 1975 ; Krider and Weid- man, 1976]. Electric field risetimes from strokes in the 100 to 200 km range over land are generally a few microseconds, with subsequent strokes in multiple-stroke flashes having slightly shorter rise- times than first strokes [Fisher and Uman, 1972; Lin and Uman, 1973]. For lightning within 20 km, the mean risetimes in Pennsylvania and in central Florida are not much different from distant data; whereas typical close lightning observed at the NASA Kennedy Space Center (KSC) has signifi- cantly shorter subsequentstroke risetimes than the close Pennsylvania and central Florida data but similar first stroke risetimes [Fisher and Uman, 1972;Lin and Uman, 1973; Uman et al., 1973a,b]. In the present study we examine the effects of propagation over 200 km of interior Florida on return stroke electric field risetimes. We choose to define risetime as the interval between the 10 and 90% levels of the total rise to peak in order to eliminate the arbitrary decision as to when the return stroke field starts. The relation between the 10-to-90% and earlier zero-to-peak risetimes will be discussed in the final section of this paper. 985