Calculations are pesented to show how the return stroke electromagnetic fields are influence by the variation of amplitude of the current waveform, the variation of risetime of the current waveform, and the variation of return stroke velocity along the return stroke channel. Calculations clearly show that these parameters play a dominant role in determining the characteristics of the return stroke electromagnetic fields. The narrow initial peak observed in first and subsequent stroke electromagnetic field can be produced either by a narrow initial peak present in the current waveform, a rapid decrease in the return stroke current amplitude during the first hundred meters, a rapid decrease in the return stroke current amplitude during the first hundred meters, a rapid decrease in the velocity of the return stroke over the first few hundred meters, or the variation of return stroke current risetime with height. The subsequent peak often observed in return stroke radiation field can be generated either by a similar feature in the return stroke currnet waveform or by the variation of return stroke current risetime with height.

The rate of decay of the radiation field after the initial peak depends on the rate of decay of the current waveform after the initial peak, the variation of the amplitude of the current waveform with height, and the variation of return stroke velocity with height. Calculations are also presented to show how the risetime of the current waveform affects the radiation field generated by a tortuous return stroke channel. These calculations provide an explanation for the lack of fine structure in the subsequent return stroke radiation fields though the subsequent return stroke channel is tortous. The calculations presented in this paper demostrate the importance of including the variation of return stroke current amplitude, variation of return stroke velocity with height, and variation of risetime of the current waveform with height in the return stroke models which are used to derived the return stroke current parameter from the return stroke electromagnetic fields. ¿American Geophysical Union 1990