FIELDS 307 to be negligible. Correcting for the 12.8-hour activity of Cu 64 , and using the known total energy flux, and assuming that the absorption is strongly peaked about an average energy of 19.1 Mev, 4 it is found that ^r n (E)^E=(0.77±0.15)X10-24 cm 2 Mev for Cu 63 . In our opinion the above error is a fair estimate of uncertainties introduced by such factors as counting statistics, calibration of the counter, sensitive volume and input resistance of the chambers, assumed thin target spectrum, assumption of a resonance narrow with respect to 50 Mev, extrapolation to zero chamber thickness, Walker correction, etc.An error yet to be mentioned is the uncertainty in resonance energy, which is here assumed to be 19.1 Mev but has been reported by others as 17.6 Mev. 5,6 The calculated cross section is almost exactly inversely proportional to the assumed resonance energy.The quantum-mechanical sum rule 7,8 predicts for 4 L. Marshall, Phys. Rev. 82, 300(A) (1951); 83, 345 (1951). This issue. fi B. C. Diven and G. M. Almy, Phys.The stability of static magnetic fields in an electrically conducting liquid is investigated. The result of the study is applied to the stability of twisted cylindric magnetic fields. It is shown that instabilities may by caused by the twisting of a homogeneous field.

Characteristics of the radiation fields from (1) stepped leader, (2) return strokes, and (3) cloud flashes in the tropics are presented. The separation between the successive leader pulses immediately preceding the return strokes are distributed over the range of 4–40 μs with a mean value of 12 μs. The ratio of the amplitude of the last leader pulse to that of the return stroke is log‐normally distributed with mean and standard deviation of 0.1 and 0.03, respectively. A correlation between the amplitude of the last leader pulse and the return stroke radiation field peak is observed. Consequences of this correlation are interpreted using the transmission line return stroke model. The initial peak of the return stroke radiation field is followed by several subsequent peaks. The time intervals between the successive peaks and their amplitudes are studied and compared with those observed in Florida. The zero crossing time of the first stroke radiation fields are distributed over the range of 40–160 μs, with a mean and standard deviation of 95 μs and 30 μs, respectively. The corresponding values for the subsequent strokes are 15–75 μs, 42 μs, and 14 μs, respectively. The separation between the subsequent strokes are distributed log normally with a mean and standard deviation of 91 ms and 62 ms, respectively. The average number of strokes per flash is 3.2. Characteristics of a particular type of radiation field pulses from cloud flashes are presented. Initial polarity of these pulses is opposite to that of the return strokes. The amplitudes of these pulses are about 0.2–0.5 of the return stroke amplitudes. Total duration of these pulses is about 70 μs, with initial half‐cycle duration of about 10 μs.

Characteristics of some radiation field waveforms of lightning from thunderstorms Sweden are presented. The waveforms are distinctly different from previously published signatures from intracloud discharges. In general, they are similar to the radiation fields produced by return strokes in negative ground flashes except for the initial polarity, but several important differences are found in the detailed characteristics. The zero‐to‐peak rise times of these waveforms are found to be in the range 5–25 μs. The waveforms begin with an initial portion or front which rises slowly for 3–20 μs to about half of the field peak amplitude. The observed mean values of 13 μs and 9 μs of zero‐to‐peak rise time and front duration, respectively, of these waveforms are about twice the corresponding values observed for negative return strokes. The mean radiation field peak value, normalized to 100 km, for these waveforms is 2 times that for negative return strokes. Some waveforms were preceded by small‐amplitude pulses which are assumed to be produced by a leader process. The mean separation in time of these pulses is about 26 μs, which may be compared with 14 μs observed for negative return strokes. Another important feature is the presence of ‘slow tails’ in some of these waveforms, indicating the presence of long‐lasting currents in their sources. It is suggested that the sources of the observed waveforms are return strokes bringing down positive charge to earth.

small ions and their attachment to condensation nuclei, then, Model studies of density fluctuations of large and small ions in the atmosphere.

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The theoretical solution to the problem of radiation from a pulsed vertical dipole over a flat homogeneous earth is employed to study the changes in rise times and the attenuation of the initial peaks of the radiation fields from lightning constructed according to the most recent experimental data on lightning return stroke radiation fields. Expected changes in rise times and the attenuation of the initial peaks for different values of distances and conductivities are presented. It is shown that for a given initial rise time there is a spread in distant rise times due to the variability of the shapes of the radiation fields. The expected spread in distant rise times for different values of distances and conductivities is calculated. The results show the importance of taking into account the propagation effects in any attempt to estimate return stroke current parameters such as rise time, rate of rise, and peak current from radiation fields which propagated over land. The predictions are compared with the available experimental data, and a good agreement is found between them.

As a result of the accident in the nuclear power plant at Chernobyl, USSR, a considerable increase in radioactive background radiation was noted in some regions of Sweden. In areas with high radioactive fallout an increase in the amount of lightning flashes was observed during the 1986 thunderstorm season. A statistical test shows that there is a risk of less than 1% that the observed difference has occurred by mere chance.