Ken'Ichi Narita

Electrical characteristics of winter lightning

Abstract To make clear the characteristics of winter lightning flashes, the long term observation of winter lightning on the west coast of Japan has been continued using magnetic links, digital recording systems for the current oscillograms, field mills, still cameras and video camera systems for the images of lightning channels. Of the 66 magnetic links records exceeding 2 kA, 73% were negative in polarity, and 27% were positive. Median peak values for winter flashes were 17 kA for negative flashes and 45 kA for positive flashes, respectively. 145 current waveforms for the winter lightning flashes, which have current amplitudes exceeding 1 kA, have been obtained by the shunt systems and/or the coils system. They show that winter lightning flashes often have a very long duration or continuing current, and sometimes have a very large amplitude exceeding 200 kA in positive flashes. As one example flash on 9 January 1987, the maximum current amplitude was +280 kA, the maximum current derivative 1.0 × 10 10 As −1 , the total charge +400 C and the action integral 1.5 × 10 7 A 2 s. The winter lightning current waveforms are classified into three types: single stroke flashes, monopolar multiple stroke flashes and bipolar flashes. Moreover, each flash is categorized as positive or negative, single peak or multiple peak, and with or without a continuing current.

Wavelet analysis of meteorological variables under winter thunderclouds over the Japan Sea

To investigate a winter thundercloud structure, we have been acquiring data on the electrostatic field and the meteorological variables (atmospheric pressure and wind speed) over a full winter season. The obtained data were investigated by a wavelet transform method that is a mathematical technique introduced recently for analyzing seismic and acoustic signals. Microscopic variations in time series of the pressure and the wind speed are extracted from nonprocessed ones using a moving average method, then analyzed by a wavelet method. For each variable, wavelet transform coefficients (modulus) are displayed using gray scales as its grade. These figures provide a clearly interpretable visual representation of time series signals. Namely, it is easily understood what significant scales exist at the concerned time. Mutual correlation coefficients of modulus between the pressure and the wind speed variation are also calculated to clarify how these two variables are related to each other. A scale parameter and a phase shift in time are adopted as two-dimensional coordinates in the same manner as the wavelet transform. This figure shows whether there is a strong correlation between the two variables or not, and if so, its time and/or position. When thunderclouds pass near the observation station, characteristic oscillations are detected in a pressure variation. A strong correlation between the pressure and the wind speed has been proved by a method of a wavelet transform method. On the other hand, when the climate is quiet, there is no correlation. A correlation between the pressure variation and the electrostatic field at the ground has also been clarified.

Observations of earthquake waves by the vertical earth potential difference method

Abstract It is still not clear what relationship exists between earthquake phenomena and electromagnetic ones. The earth self-potential difference has been observed with high time resolution for continuously monitoring earthquakes at Sendai, where artificial leakage current sources always exist. A simple method to reduce the unnecessary signals induced by artificial and natural earth current sources is proposed. With this method the potentil difference is measured through two electrodes buried at different vertical positions under the ground. For all earthquakes with seismic intensities of more than 1 at Sendai we have observed clear variations in the potential difference signal. The observed electric P and S wave arrival times agree exactly with seismic data obtained by the Japan Meteorological Agency (JMA).

Characteristics of earth potential difference generated by seismic waves

To clarify the relationship between earthquakes and electrical phenomena, the earth potential difference has been observed continuously at Aobayama in Sendai and at Tsukidate, 60 km north of Senday.

Measurement of small variations of atmospheric electrical field signals in fair weather

The variations in atmospheric electrical field signals in fair weather were observed both at Aobayama in Sendai and at Tsukidate in the Miyagi Prefecture. The measurements were made using a slow antenna type electrical field meter composed of a metal plate electrode and a CR integrator. They were recorded using a portable pen-recorder. A field meter of this type is suitable for measuring relatively rapid variations. The observations were made in a period of a few minutes in fair breezy weather. The data collected using a bare metal electrode were compared with those using an insulated electrode in order to detect an air-earth current. Examination of the dependence of the amplitude of the variation to the electrode size and height showed that the atmospheric electrical field is affected by these parameters. Observation of signals at two different positions gave the time delay between positions. This indicated that the electrical field perturbation was about several hundred meters. The well known atmospheric circulation with a period of a few minutes suggests that the observed variations in the electrical field signals were caused by electrical charges moving together with the atmospheric circulation.

Forecast of Winter Thundercloud by Frequency Analysis of Athmospheric Pressure

The electric power facilities such as large nuclear power plants and ultra high voltage principal transmission lines have recently increased so much along the coast of the Japan Sea. Particularly in winter seasons, these facilities have been frequently suffering from serious lightning troubles. Brook et al.1 have pointed out that winter lightning in this district has different characteristic features compared with summer ones. The exact forecast of thunderstorms and the detailed knowledge of weather circumstances that cause thunderstorms are urgently needed to protect these facilities and execute effective system control of power facilities against winter thunderstorms.