The effect of the earth's magnetic field on the penetration of E.L.F. radio waves into the daytime ionosphere

Computations hive been made of the attenuation constant of the earth-ionosphere waveguide for a range of vertical magnetic field strengths. The ionosphere profile used in this study is the ’ Aiken ’ daytime ionosphere described in the author's earlier work (Barr 1972). This profile has been terminated at various heights in the range 75 to 99 km by either free space or a homogeneous region characteristic of the ionosphere at the level of termination. From the effects of these terminations on the-attenuation constant of the earth-ionosphere waveguide it has been possible to make deductions of the penetration into and reflection from the ionosphere of E.L.F. and V.L.F. waves. It has been found that whereas V.L.F. waves (3 to 10 kHz) are reflected from the lower ionosphere and are not markedly affected by the presence of a magnetic field (< 50 µT), E.L.F. waves penetrate deeper and deeper into the ionosphere with increasing magnetic field and are reflected from any steep electron density gradients in their path, In particular it has been shown that the minimum in the E.L.F. attenuation constant reported by the author (Barr 1972) is due to reflection of E.L.F. waves from the region of rapidly changing electron density near 88 km. By considering the independent propagation of the ordinary and extraordinary waves in the upper ionosphere it has been found that it is predominantly the absorption of the extraordinary wave which is markedly reduced by the presence of a vertical magnetic field. This extraordinary wave is effectively liberated by the reduced absorption and is able to penetrate to great heights in the ionosphere. It is concluded that computations on E.L.F. propagation in the presence of a vertical magnetic field must take into account changes in the electron density profile well above 100 km and it is suggested that the presence of sporadic ionization in the E region may have a marked effect on E.L.F. wave propagation in the earth-ionosphere waveguide.