The generation of a microwave pulse by the plasma created by a stratospheric source of ionizing radiation is examined. The structure, the mechanisms for generating the signal, and the information contained in the signal observed at a satellite positioned directly above the explosion are discussed. It is shown that the microwave pulse is generated by two mechanisms: radiation from the shock front lasting up to one second and radiation from a partially ionized plasma lasting tens of microseconds. A package of programs developed by the authors is used for model calculations of microwave pulses. The parameters of the signal are such that they can be picked up by modern pulse radiometers.Microwave monitoring of the atmosphere is extensively used nowadays for detecting explosions and the consequences of engineering accidents. This type of monitoring is convenient because the microwave technique works in all weather conditions and is accurate, while the air is transparent to microwave radiation. In addition, compact pulsed radiometers are now available, with integration times of 0.1-1 µsec and a sensitivity of hundreds of degrees Kelvin, that can be mounted on the earth's surface or aboard any airborne platform [1].We examine the influence of the mechanisms for generation of incoherent microwave radiation on the structure of the radio-frequency pulse, as well as possible information in the signal which may make it possible to determine the properties of the microwave source. Let us assume that the explosion is accompanied by the release of radiation with a long mean free path, e.g., in the form of neutrons or x-rays and γ rays. The positions O of the point source (explosion) and the radiometer Ra are illustrated in Fig. 1. An explosion takes place at an altitude ranging from H = 10 km to H = 40 km above the earth's surface. The signal is picked up by a radiometer located on a satellite immediately above the explosion at an altitude h = 200 km.Two different mechanisms participate in the formation of a microwave pulse. One is associated with the development of gas dynamic processes in the immediate environment of the explosion and the thermal radio frequency emission from the front of the shock wave. A thermal wave, behind whose front a shock wave develops, initially propagates from the source at the time of the explosion. The shock front moves through the heated medium and, in time, catches up with the front of the thermal wave. The heated layer between the fronts emits radiation. The radio-frequency brightness temperature increases at first, and then falls off sharply as the shock front emerges into the thermal wave front. After a sharp drop, the temperature passes through a minimum; it then rises again owing to radiation from the front of the shock wave and reaches a maximum. At the time when the second maximum is passing, the heated region radiates as an absolute black body. The mechanism for the visible radiation from the shock front was first examined by Zel'dovich and Raizer [2]. Calculations of the microwave

The use of microwave monitoring of the atmo sphere for the detection and identification of radiation sources is attractive for several reasons. The microwave method successfully combines advantages of the radio technical and optical methods, all weatherness of the former and high accuracy of the latter. A great advan tage of the microwave over radio technical method is the possibility of recording microwave pulses from sat ellites. In addition, the level of noise, largely arising from lightning discharge radiation, is low over the microwave range.Let us consider various mechanisms of microwave radiation generation by an atmospheric explosion and their influence on the structure of radio pulses. We assume that an explosion is accompanied by the emer gence of long range radiation (in the form of neutrons, X ray, and gamma quanta). All estimates were made for radiation wavelength intervals in one of air transparency windows, close to λ ~ 2.3 mm (2.0-2.5 mm).The structure of a radio pulse is caused by three mechanisms of radiation generation.1. Radiation created by a flux of Compton elec trons knocked out by gamma quanta when they are scattered in air. The current of Compton electrons is responsible for two signal parts.Coherent radiation. One signal part is a coherent pulse caused by asymmetry of instantaneous gamma radiation and, accordingly, asymmetry of Compton currents. Asymmetry of the escape of gamma quanta results in the generation of a short pulse 0.01-1 μs wide. Coherent radiation arises as a result of collective plasma particle motions, when the effective frequency of their collisions is much smaller than the electro magnetic wave frequency. Pulse parameter calcula tions were reported in [1]. According to [2], the num ber of gamma quanta emitted in an explosion decreases as time passes following the lawwhere α = 10 8 -10 7 s -1 and N is the total yield of gamma quanta. For an air explosion with TNT equiv alent q and weak gamma quantum escape asymmetry, spectral radiation power obtained in [1] can be reduced to P ν ≈ 0.8 ⋅ 10 24 q 2 /ν 2 ,where q is in kt, ν is the radiation frequency in Hz, and P ν is in W/Hz. It follows from (2) that energy is proportional to the square of the TNT equivalent and inversely propor tional to the square of frequency. A coherent pulse is exceedingly short, its width is determined by the time dependence of intensity of gamma quanta (1); it is on the order of 10 -6 s or less. The spectral power can be estimated as P ν ≈ 20 kW/Hz for air transparency band near λ ~2.3 mm (ν ≈ 1.3 ⋅ 10 11 Hz). Incoherent radiation. Medium ionization occurs under the action of fast Compton recoil electrons, and ionization at a given point lags behind the moment of gamma quantum front passage by the time of fast elec tron deceleration (~10 ns). The number of secondary electrons that appear when the instantaneous pulse of fast electrons with a ~1 MeV energy is decelerated lin early increases with time at the initial time moment. The secondary electrons formed are thermalized and stick to oxygen molecu...

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