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It has long been known th at dUTing sunrise VLF signals received over a lon g path show periodic variations in amplit ude. At sunset similar, though smaller , variations occur . Obser vations in recen t years [Pierce, 1957] of t he relative phase delay of such signals show t hat the amplitude vari,ttions are accompanied by phase v,triations. The original explanation [Yokoyama and T n.nimura, 1933] was that dW'ing the day the ionosphere behaves like ft m etallic reflector h ftv ing ft pse ucloBrewster angle near 90° (grazing incidence), while at night it behaves like ft dielectric wi th the pse udoBrewster angle n ear 30°. During sunset or s unrise the Brewster angle changes from 30° to 90° and at some instant passes through a va lue equal to the angle of incidence of the wave. The signal level then decreases at this time because of the reduced reflection coefficient. This drop in amplitude of the received signal would then be repeated each time the sunrise (SR ) or s unset (SS) line passed each " poin t" of reflection on a multihop path. This explanation now seems unlikely to be valid since calculations [Wait and Perry, 1957] of the reflection coefficient of the ionosphere for VLF waves show that the decrease in reflection coefficient at the pse udoBrewster minimum is small. More recently, Rieker [1963] has attempted to explain the periodic phase variations observed during sunrise and s unset at Neuchatel (Switzerland) on the 18 kc/s signals from NBA in the Canal Zone. His explanation , which is again based on a single ray geometrical optics approach, is that the five "steps" in t he sunrise phase variation are due to the SR line advancin g over each of the five reflection points of the five hop pftth. Rieker 's explanation does no t attempt to acco unt for the amplit ude minim a which accompany the phase "steps." Explanations of the sunset and sunrise phenomen a which are based on a ray-optics model such as those mentioned above and which invoke only one ray , seem. unlikely to b e able to account correctly for the obser vations on long pat.hs for two reasons . The

Periodic fading of VLF signals received over long paths during sunrise and sunset