The phase structure of very low latitude ULF waves across dawn

Ultralow frequency (ULF) waves at the geomagnetic equator are studied by us ing a small magnetometer array and a one-dimensional electromagnetic wave m odel. Most ULF waves observed at low latitudes have been associated with sh ear Alfven mode hydromagnetic resonances in the plasmasphere, Near the equa tor it is difficult to excite these resonances, and the wave activity is at tributed to fast mode waves. The phase difference data recorded at longitud inally spaced, equatorial magnetometers shows a marked change around dawn w hen one station is sunlit while the other is still in darkness. The phase s tructure with latitude also shows large phase shifts near the equator, in a greement with previous studies. A model of ULF wave propagation through the equatorial ionosphere is presented and used to compare with the observed a mplitude and phase properties. It is shown that the observed frequencies ar e in the vicinity of the wave cutoff frequency where the phase structure be comes complicated. For frequencies above the cutoff, the phase structure ac ross dawn is directly related to dawn-associated changes in electron densit y in the ionosphere.