Long impulse currents associated with positive return strokes

Long distant electric fields (400-500 km), generated by 26 positive cloud-to-ground flashes, were analysed, These flashes consist of well detectable long impulse fields subsequent to the initial peak. These hook-shaped slow fields are of considerable amplitude and have a mean duration of 1.24 Ins. The amplitude of the long impulse field and the initial peak of the held show an approximately linear relationship. The long impulse current pertinent to positive return strokes which give rise to the measured long impulse fields were estimated. Flashes with these slow fields lower a mean charge of 50 C within the first 3 ms (excluding the first 100 mu s which contains the initial peak), whereas the maximum charge lowered is 124 C. The mean of the ratio between the peak of the long impulse field and the initial peak is 41%. We also estimated the fields that will be generated by the long impulse currents at distances of 1000 km, 3000 km and 5000 km fi om the strike. The estimated peak magnetic fields at 5000 km have;3. mean of 52 pT. The peak magnetic fields observed at distances of about 5000 km from positive lightning flashes, which were associated with red sprites, are in the same range as the peak magnetic fields that we have calculated for the above 26 flashes. Hence we conclude that the observed Q-bursts which coincide with the occurrence of red sprites are due to the long impulse currents of positive return strokes. This slow field variation is rarely observed in connection with negative return strokes, Even when it is present, in the event of a negative return stroke, the amplitude and the duration of the tail are much less than those of its counterpart in positive return strokes. This explains why ionospheric lightning is predominantly associated with positive return strokes but not with negative return strokes. (C) 1998 Elsevier Science Ltd. All rights reserved.