From electric field records of 27 ground flashes near Tampa, Florida, and 19 ground flashes at the NASA Kennedy Space Center (KSC) the occurrence and waveshape of microsecond-scale electric field pulses associated with both millisecond-scale steplike K changes and millisecond-scale hook-shaped M changes are examined to test and disprove the following two hypothesis: (1) that K changes contain a microsecond-scale pulse component which can be described by the characteristic pulse waveform proposed by Arnold and Pierce (1964) and (2) that there is essentially no difference between K and M processes, as argued by Kitagawa et al. (1962). Microsecond-scale electric field variations exceeding by at least 50% the system noise level were observed in 23% of 135 K changes from Tampa and in 25% of 128 K changes from KSC, while such field variations were found in 44% of 88 M changes from Tampa and in 77% of 30 M changes from KSC. In the majority of the K changes having microsecond-scale pulse activity, that activity did not occur at the beginning of the K step, while in most cases the pulses associated with M changes occurred at the initial portion of the M hook.

These results can be interpreted to imply that K changes and M changes are associated with dissimilar physical processes, in refutation of hypothesis (2) above. The microsecond-scale pulse activity during K changes and M changes was highly variable and sometimes irregular in waveshape. Not all the pulses had the same polarity as the K step or the initial portion of the M hook on which they were superimposed. The relation of the microsecond-scale variations to the overall K changes in ground flashes as regards the frequency of occurrence, the position of pulses within the slower field change, and the shape of the pulses is similar to that reported by Bils et al. (1988) for K changes in cloud flashes. The observed microsecond-scale field variations associated with K changes are not consistent with the characteristic electric field pulse waveform attributed by Arnold and Pierce (1964) and some other investigators to K changes, in refutation of hypothesis (1) above. No relation was observed between the magnitude of a K change and the presence or absence of corresponding microsecond-scale field variations. M changes during continuing-current field changes of relatively short duration (less than 20 ms or so) are more likely to have pulses than M changes during continuing-current field changes of longer duration. ¿ American Geophysical Union 1992