Characteristics of ionospheric thermal radiation

Abstract

A theoretical observation of the characteristics of ionospheric thermal radiation is made using a linear theory, based on a macroscopic concept with the aid of fluctuating electromagnetic field theory. The thermal noise power generated in the ionosphere per unit volume, per unit frequency bandwidth, and the available thermal noise power at a receiving antenna per unit frequency bandwidth are calculated. The spectral distributions of ionospheric thermal radiation are obtained and discussed in detail. A study of the calculated power level of the thermal noise generated in the ionosphere shows that it is exceedingly low and decreases rather rapidly with an increase of frequency ƒ. A large portion of the noise signal generated appears to be in the frequency range of ƒ < 10 7 c/s , with the microwave noise signal being negligible, and appears to come mainly from the region between 60 km and 100 km of the ionosphere. Furthermore, the study reveals that the available noise power at a receiving antenna depends upon the geographical location of the antenna in general and that the power level is higher in the equatorial zone than in the polar cap zone. For a noise signal frequency of less than 10 8 c/s, the power level increases monotonically with an increase of polar angle θ from θ = 0 and reaches its maximum value at θ = 90°, where it is at least a hundred times greater than at θ = 0. For a noise signal with a frequency of 10 9 c/s, the available noise power has its maximum at θ ⋍ 50°, where the power level is comparable to that of a noise signal with a frequency of 10 6 c/s. For a noise signal of a frequency above 5 × 10 9 c/ s, the angular dependence disappears.