Some properties of the ionosphere at low radio frequencies

Abstract

Low-frequency ionospheric polarization, relative virtual height, and relative reflection coefficient measurements were made at near vertical incidence during October 1952. These measurements were made at a frequency of 50 kc/s. A new, continuous-wave, phase-measuring technique was employed. The essential features of the system are (1) a horizontal transmitting antenna near the surface of the ground which is resonant at the operating frequency and which radiates no ground wave in the direction of the receiver and (2) a receiving station located within line-of-sight of the transmitter at which the phase of the low-frequency sky wave is directly compared with a reference phase transmitted over a very-high-frequency link. The transmitting antenna is considered in detail, and the electronics system is briefly described. Some of the general features of the earth's atmosphere and the ionosphere are presented. The magneto-ionic theory is developed, and the propagation of electromagnetic waves in an ionized atmosphere is discussed. This material is presented in order that the 50-kc/s ionospheric data may be properly interpreted. The applicability of the quasi-homogeneous, magneto-ionic theory to low-frequency-propagation problems is considered. The virtual height of the E-layer was found to change by approximately 7.5 km during the sunrise and sunset periods. Rapid fluctuations in the virtual height and reflection coefficients of the E-layer are attributed to (1) strong magneto-ionic splitting, (2) fluctuating ionization gradients in the lower nighttime E-region, (3) interference effects due to the presence of two or more partially reflecting layers (i.e. the fine structure of the E-layer), and (4) interference effects due to the interaction of the two magneto-ionic components. Not all of the polarization results are consistent with the predictions of the quasi-homogeneous, magneto-ionic theory. Low-frequency noise measurements were also made during October 1952. The 50-kc/s atmospheric noise field strength reached a maximum value during the middle of the night. The atmospheric noise received in the east-west direction reached a much higher maximum value than that received in the north-south direction.