Soreq Nuclear Research Center, Yavne, Israel

Properties of the electromagnetic pulse are derived from the current moment. We present here (in red) the current moment generated by a nuclear explosion, as resulted from a simulation based on Longmire’s model [1]. This current moment is compared to that of a typical lightning discharge (blue) [2]. The NEMP is much shorter in time than the lightning discharge. Thus, its spectrum is dominated by higher frequencies content. These frequencies radiate and “broadcast” the information about the pulse origin over the earth-ionosphere waveguide. Broadcasting power is proportional to the product of the current moment spectrum by frequency, which is presented below (left panel). In the 10-40 kHz range the lightning spectrum decreases sharply with frequency whereas NEMP increases and then remains approximately flat. Multiplying the spectrum by the frequency yields different power spectrums for the two cases (right panel). This difference enables simple method for discrimination between NEMP and natural sources of EMP.

EMP sources

References [1] C.L. Longmire, On the electromagnetic pulse produced by nuclear explosions, IEEE

Trans. EMC 20, 3 (1978). [2] D. L. Jones, Electromagnetic radiation from multiple return strokes of lightning, J.

Atmos. Terr. Phys., 32, p. 1077 (1970). [3] J. A. Ferguson, Computer Programs for Assessment of Long-Wavelength Radio

Communications, Version 2.0. Space and Naval Warfare Systems Center, San Diego CA, May 1998. Technical Document 3030 (1998).

[4] R. A. Houghten and R. B. Harvey, Operation Plumbbob—project 6.4, Accuracy and reliability of short-baseline narol system, ITR-1438, (1958).

The capability of identifying the unique signature of the NEMP opens the way for a discrimination method between chemical and nuclear explosions. Once an infrasound indication for an explosion is detected, one should look for at the EM recordings from respective time and location. Absence of a signal implies a non-nuclear event, whereas an infrasound signal accompanied by a mutual EMP signature raises a significant evidence for a nuclear activity. As electromagnetic signals can be (and actually are) continuously monitored, this data fusion procedure can be easily automated. A routine comparison of infrasound alerts to EMP signals eliminates the need for a manual analysis of each and every infrasound signal. Furthermore, careful analysis of the received EMP signal may provide important information about its source. This holds true for any EMP, from either natural or man-made sources.

Results and Conclusions

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lightning NEMP

The propagation of the electromagnetic signal was simulated using the LWPC package [3]. Here we present the received signal 1000 km away from the source. As expected, the lightning signal exhibits different behavior than the NEMP. The NEMP spectrum is similar to that of measurements taken in US nuclear experiments [4].

NEMP lightning

Typical current moment 107 − 109𝐴 ∙ 𝑚 107 − 108𝐴 ∙ 𝑚

Typical width Few 10s 𝜇sec 100s 𝜇sec

Source localization 100s m Vertically distributed over several km

Signal shape Bipolar (changing sign) Unipolar (positive in most cases)

The Comprehensive Nuclear Test Ban Treaty (CTBT) monitoring technologies such as infrasound and seismic waves are capable of detecting aggressive events, including earthquakes, chemical and nuclear explosions. Further analysis is required in order to characterize the nature of each event and its relevance to the treaty. Here we propose a rapid and automated method for discriminating nuclear atmospheric explosions from other sources of infrasound signals. Gamma rays emitted by a nuclear explosion produce an electric current via emission of Compton electrons. This current is the source of a strong electromagnetic pulse (EMP) which can be detected large distances away. There are also natural sources of EMP, namely lightning. Here we show that the spectrum of nuclear EMP has different characteristics than that of lightning EMP, enabling reliable and simple discrimination between them. Non-nuclear sources of infrasound such as chemical explosions lack the unique signature of EMP. Thus, accompanying the infrasound monitoring by continuous measurements of electromagnetic signals may provide a powerful tool for highlighting the nuclear events among all infrasound events. Infrasound alerts which have no mutual nuclear EMP signal can be immediately classified as non-nuclear events, eliminating the need for further analysis.

The ionosphere is composed of nonhomogeneous plasma. Earth and the ionosphere form a waveguide which may carry signals over large distances. VLF signals emerge from both man made and natural sources. VLF received signals inherently contain information about the low ionosphere properties as well as about the nature of the signal source. Gamma rays emitted by a nuclear explosion produce an electric current via emission of Compton electrons. Due to asymmetry in ambient density and environmental conditions, this current radiates in the VLF range (3-30 kHz). The electromagnetic signal, known as EMP (electromagnetic pulse), propagates in the earth-ionosphere waveguide and can be remotely detected. Since chemical explosions do not emit such an EMP, one can use the detected EMP as an indication for a nuclear event. In order to use the EMP as a discriminator between chemical and nuclear explosions, nuclear EMP (NEMP) has to be distinguished from other EMP sources. The main natural source is lightning discharge. There are about 45 lightnings per second over the world (~1.4∙10⁹ per year). These are (almost) vertical currents of 10’s kA, with characteristic length of few km and time of tens microseconds. Lightning VLF waveforms (Sferics) are detectable thousands of km away. In the following we present the differences between the spectra of NEMP and that of lightning discharge, and show how these differences may serve as a tool for highlighting the NEMP among other EMP sources.

Background

Ionosphere

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