Bremsstrahlung and Thermal Microwave Radiation from an Air Explosion

Abstract

reason, once again, it is important to develop methods to monitor the atmosphere and to detect and identify dangerous objects. One promising method is to detect electromagnetic radiation in the milli- and centimeter ranges. The mechanisms of generation and the structure of a microwave signal in the case of explosions in the stratosphere and the characteristics of the radiation from contact explosions and others have been studied in [3‐6]. In the present communication, we shall examine the characteristics of the generation of an incoherent signal by nuclear explosions in the lower atmosphere. It is known that at the initial stage in a time not exceeding 1 sec the cloud from a nuclear explosion is a symmetric fireball. At this stage, the upward motion of the fireball can be neglected. The region of hot plasma is surrounded by a cold region of partial ionization created by the emission of neutrons and γ-rays. The region of partial ionization is a source of microwave Bremsstrahlung in the first 1‐20 μsec [3, 4]. The duration of this part of the signal depends strongly on the energy and type of nuclear explosion. The remaining part of the incoherent radiation lasts for about 0.1‐1 sec and is due to the radiation from the front of an expanding shock wave [4, 5]. In the present work, the results were obtained by computer modeling of six-component plasma formed by a nuclear explosion in air. A package of modeling codes was used [7]. These codes made it possible to calculate the Bremsstrahlung from charged particles in the partial ionization region and the thermal radio radiation from a shock wave front. In this package, the Runge‐Kutta method is used to solve the kinetic equations for the six components of the plasma in the group approximation; the particle concentrations, the conductivity of the air plasma, and the absorption coefficients for the microwave radiation are calculated, the radiation transfer equation is solved, and the optical thickness and radio brightness temperature of the plasma along the line of sight are calculated. Let us examine the problem of an atmospheric nuclear explosion at the altitude H = 1 km above the Earth’s surface. The positions of the source (explosion) S and the detector (radiometer) Ra and the direction of the line of sight 1 are shown in Fig. 1. For simplicity, the source, radiometer, and line of sight are located in the same plane. A dashed line marks the boundary of the fireball. The radiometer can be placed in an airplane or satellite on altitude h > H. Let h = 10 km. The ray 1 passes at impact distance p from the center of the explosion. The form and parameters of the detected radiation depend not only on the conditions chosen for the explosion but also on the orientation of the axis of the directional pattern of the radiometer. We shall confine our attention to the case where the impact distance is less than the radius of the fireball p < r fr . Then, the line of sight intersects the region of high ionization. In this case, the radiation from the shock wave and the Bremsstrahlung from the charged particles in the region of partial ionization encompassing the fireball will be detected.