Sunrise effects on VLF signals propagating over a long north-south path

We present a detailed study of the times of amplitude minima observed on th e 12-Mm path from NAA (24 kHz, 1 MW, Cutler, Maine) to Faraday, Antarctica, during the period 1990-1995. (NAA is a naval transmitter call sign.) This study represents the first account of the effect of the sunrise terminator when it is parallel to a propagation path at some times of the year. Since the NAA-Faraday path is within 3 degrees of the north-south meridian, paral lel orientation happens close to the equinoxes, while the maximum angle of incidence occurs during the solstices. During the solstices the terminator takes a significant length of time to cross the entire propagation path, so modal conversion effects are observed over a range of hours. During the eq uinoxes, however, the leading edge of the night-day transition region cross es the whole propagation path within 20 min. The interpretation of the timi ng of minima is consistent with modal conversion taking place as the sunris e terminator crosses the NAA-Faraday transmission path at specific, consist ent locations. The timing of minima is remarkably consistent from year to y ear. Long wave propagation modeling is used to show that the location of ni ghtside minima at an altitude of 45-75 km in the subionospheric waveguide r epresents the location of the sunrise terminator on the great circle path w hen dayside minima occur.