The relation between ionospheric profiles and ELF propagation in the earth-ionosphere transmission line

An approximate method, based on a wave-solution, is used to sketch the dependence of ELF propagation in the earth-ionosphere transmission line on time of day, latitude, season of the year, epoch in the sunspot cycle, and occurrence of sudden ionospheric disturbances and polar cap absorption events. As the frequency descends through the ELF band, penetration of the D region occurs in succession for the O and X waves, leading to reflexion from the E region at the Schumann resonant frequency. Under quiet day-time ionospheric conditions the penetration frequency-band is around 20-60 hz in middle and high latitudes, but around 75-150 hz near the equator. At a frequency low enough to be reflected primarily from the E region under quiet ionospheric conditions, an increase in D region ionization that is just sufficient to transfer primary reflexion from the E region to the D region results in an increase in the rate of attenuation. On the other hand, when once reflexion is firmly established at the lower level, further increase of ionization in the D region causes a reduction in the rate of attenuation. Similar effects are expected to occur at night in association with a sub-E region ledge of ionization. Small variations in the ionization profile of such a ledge are the likely cause of night-time fluctuations of transmission at 45 and 75 hz.