Absorption of radio waves reflected at vertical incidence as a function of the sun's zenith angle

Eloise W. Taylor
1948 Journal of research of the National Bureau of Standards  
The diurnal variation of ionospheric absorption is related to the sun's zenith angle. The absorption values for this study were obtained from the continuous automatic fieldintensity recordings made at the Central Radio Propagation Laboratory on two frequencies. Because of proximity of the receiving station to the transmitting station, reflections are obtained at nearly normal incidence. An analysis of data covering a period of 3 years indicates that little error is introduced by assuming a
more » ... r dependence of absorption upon the cosine of the sun's zenith angle. Maximum ionospheric absorption of radio waves per unit length of virtual path occurs at that height where the product of the electron density and the frequency of collisions with gas molecules is a maximum. This may be shown to be the lower part of the ionosphere for certain radio frequencies. Here the ionization is produced mainly by ultraviolet radiation from the sun. Various theories of the formation of the i?-layer indicate that the absorption should be proportional to some power of cos X, where X is the sun's zenith angle. The effect of this absorption is to diminish the field intensity of the reflected wave relative to what it ,would be in absence of absorption. Thus, if E o is the intensity of the wave without loss of energy through absorption, and E is the intensity of the wave as actually received, then #=#010-(1) wiiere A is the index of absorption. The purpose of this report is to express the diurnal variation of this so-called nondeviative absorption, the absorption without appreciable refraction, as X, (2) in which X is the zenith angle of the sun and A Q is the subsolar absorption on the frequency investigated. In 1944 the Central Radio Propagation Laboratory began a program of recording continuous emissions at vertical incidence. For this purpose four frequencies, namely, 2,061, 4,272, 5,892, and 6,992 kc, were emitted from the radio-transmitting station at Beltsville, Md., and received at the laboratory at Sterling, Va. The base-line distance, about 60 km, is sufficiently short that the reflection can be considered to be at vertical incidence. The recordings for 3 years, from March 1945 through February 1948, of the 2,061-and 4,272kc frequencies were analyzed, and the results are presented in this paper. The two higher frequencies were above or near the critical frequency of the .F2-layer for a considerable part of the time and have, therefore, been excluded from this study. During the night when the absorption is negligible, the field intensity was assumed to be diminished only by the inverse distance attenuation. This field intensity is referred to as the unabsorbed field intensity. The absorption for any time can be found from the logarithm of the ratio of the received field intensity at that time to the unabsorbed field intensity. The recorded values are in logarithms of microvolts input to the receiver, thus the absorption for any value of cos X is the difference between unabsorbed field intensity and the field intensity received at the time for which that value of cos X obtains. The median values of received field intensity in logarithms of microvolts input to receiver for each hour of the day were determined from the _ recordings. These data were grouped by seasons; summer
doi:10.6028/jres.041.044 fatcat:lkl7fap6izbqlczvo4f5mepxri