On the Role of Electric Field Changes when Calculating Thunderstorm Currents Yu.V. Shlugaev, V.V. Klimenko, E.A. Mareev Institute of Applied Physics RAS, Russia Workshop on Coupling of Thunderstorms and Lightning Discharges to Near- Earth Space June 2008, University of Corsica, Corte, France

Introduction The measurements have been performed during convective seasons of in Gorodets [56041´ N; 43026´ E] 60 km from Nizhny Novgorod, Russia. The simultaneous observations of quasistatic electric field and thunderstorm radioemission in the range kHz with high time resolution show that many electric field changes of big magnitude do not accompanied by the intensive sferics. Central problem is how to estimate an average contribution of a thunderstorms into the current of the global atmospheric electric circuit. Usually used Lightning detection networks operates in VLF band and can obtain only relatively fast currents.

Statistics Fig.3. Thunderstorm activity in the second part of summer, 2006 from Jul 15 to Aug 30. Fig.3 shows the distribution of thunder-storm activity near Gorodets station in summer 2006 from July 15 to August 30. The data of ELF/VLF observations have been used to derive the “index of lightning activity”. Note that after July, 18 a cold weather came (the night from 21 to 22 July was the most cold for the full period of meteo observations), and as a result this July became the most cold for last 10 years in this region.

Meteoradar Fig.4. Meteo radar data, synoptic activity 200 km around Nizhny Novgorod. July 17, Altitude: 3±0.5 km. Radar data allowed us to observe the dynamics of precipitation particles determining the reflection of radio signal from the cloud. as illustrated by Fig.4 where the radar data are shown in the evening time of July 17, when active storms over Upper Volga region were observed.

Temporal evolution of discharge characteristics Figure 1. The results of simultaneous recording of quasi-static electric field and magnetic fields of lightning discharges – sferics (thunderstorm of 16 July, 2006). Simultaneous observation of quasi-static and electromagnetic fields from thunderstorm discharges allows one make some conclusions on the development of a thunderstorm cell and its structure. On Fig.1 the results of there is an interval of positive-polarity field, and primarily negative discharge occur. Then the mean-field direction and discharge polarity change, which means that the complicated multi-charge structure of a thundercloud forms with the high level of intra-cloud flash activity. The mature stage of this thunderstorm is characterized by the positive field direction and positive polarity of discharges. The intensity of intra-cloud discharges decreases.

Spherics & E-field Figure 2. Figure 3. Several discharge recordings with higher temporal resolution are presented in Figs. 2,3 (positive and negative CG flashes).

“Special-type” discharges Fig.4. An example of simultaneous recording of quasi- static field (1) and electromagnetic radiation of 10 kHz frequency (2) during lightning discharges. Records of For some special-type discharges (Fig.4) a monotonic change of the quasi-static field during several hundred of milliseconds connected obviously with sufficiently long-term continuing current. Mean values of the current duration are of order of 300 ms, electric field jump – of order of 30 kV/m, which for the distance of 5 km from the discharge corresponds to the charge momentum change about 20 C  km. Note that the electric field change starts before the fast currents leading to the radiation of sferics typical for the return stroke stage. The total time of the slow change is about 1 second and is accompanied by weak sferics. It is obvious that the discharges transfer the great amount of the charge but do not provide intensive sferics. Because the lightning detection systems use just electromagnetic radiation (sferics) in their operation, the registration of similar powerful discharges which give substantial contribution to the global circuit, may be “lost”, but they can be detected with the quasi-static field measurements.

CONCLUSIONS First, the simultaneous observations of quasistatic electric field and thunderstorm radioemission in the range kHz with high time resolution show that many electric field changes of big magnitude do not accompanied by the intensive sferics. It was found that the duration of pulse fronts for the quasistatic field often many times exceeds 25 ms (the time constant of our fluxmeter) and clearly exceeds typical durations of sferics. Relations between the magnitudes of electromagnetic pulses are rather arbitrary and practically not-correlated. Second, analysis of data allows us to suggest the presence of relatively slow (compared to the durations of leader and return stroke stages) and intensive currents both inside the thundercloud and in the Clod-to-ground space. It allowed us to give more rigorous estimation of the thunderstorm currents, and to conclude on the important contribution of these currents into the global atmospheric electric circuit. These observations are important also in terms of improvement of operation of existing lightning detection systems. Third, the analysis of observations of several isolated thunderstorm cells allowed us to characterize the dynamics of electrical activity of a typical cell at mid-latitudes in a particular region, and to estimate an average contribution of such a small thunderstorm into the current of the global atmospheric electric circuit.

Bipolar pulses associated with runway breakdown

Gama bursts

Simultaneous observation gamma&radio