LATITUDE VARIATION IN VLF MODAL INTERFERENCE

Sunrise modal interference patterns are analyzed to reveal the variation with latitude of VLF propagation parameters. For VLF transmissions from NLK/NPG and WWVL received in New Zealand, the sunrise interference observations confirm Lynn's result that anomalies in the phase and. magnitude of mode conversion occur when the sunrise line is near the geomagnetic equator. A smaller anomalous effect is observed during sunset. Analyzing data for many paths in the frequency range 18-20 kHz, it is found that D (the interference wavelength of the first and second modes in the nighttime waveguide) differs little from the theoretically expected value (about 2 Mm) between geomagnetic latitudes 30 and 60. At smaller geomagnetic latituds, however, values of D up to 3 Mm are observed for westward propagation. This large increase in D explains anomalous observations of nighttime amplitude. © Copyright 1968 by the American Geophysical Union.