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Radio Propagation and Remote Sensing of the Environment - Chapter 9

Chương này trình bày một phương pháp khác để mô tả truyền sóng đó không phải là dựa trên phương trình Maxwell và các khái niệm sóng sau đó từ họ, mà là trên những xem xét năng lượng. Như đang trở nên rõ ràng, chẳng hạn mô tả là thích hợp nhất truyền sóng là hỗn loạn trong cả hai giá trị và chỉ đạo. Chúng tôi đã thiết lập các lĩnh vực xảy ra, đặc biệt, do tán xạ và bức xạ nhiệt. Đương nhiên, mô tả phát thải phải được dựa trên việc xác định các đặc. | 9 Transfer Equation of Radiation 9.1 RAY INTENSITY This chapter presents another method to describe wave propagation that is not based on Maxwell s equations and the wave concepts that followed from them but rather on energy considerations. As is becoming evident such a description is best suited for wave propagation that is chaotic in both value and direction. We have established that such fields occur in particular due to scattering and thermal radiation. Naturally the emission description must be based on the determination of the averaged field properties. Coherent components of the field at intensive scattering or upon thermal radiation are extremely scarce or do not exist at all. Squared characteristics are the primary values used to describe regularities of the radiation propagation. Poynting s vector S r s is found to be a random function of coordinates r and direction s. Its statistical properties are characterized by the probability density P1 S P1 S Q . This in particular defines the probability of having a Poynting s vector value in the interval S S dS and to be inside the solid angle dQ in the direction given by vector s. In the case being considered here Poynting s vector is not a very suitable energy characteristic because its main value might be equal to zero due to its vector character. This does not mean that the field power is also equal to zero. Simply due to the chaotic character of wave propagation Poynting s vector turns out to be on average equal to zero therefore it is more convenient to regard Poynting s vector characteristics in a particular direction. So we can define the concept of ray intensity as K5 I r s J SP1 S Q S2dS 9.1 0 which is the average value of Poynting s vector in a single interval of the solid angles in direction s. Let us develop this idea further. Because our discussion is about not only harmonic oscillations but also those for which the power is distributed in a spectral interval we will take the word power to mean .