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JOURNAL OF GEOPHYSICAL RESEARCH, VOL. 106, NO. A8, PAGES 15,599–15,607, 2001

The phase structure of very low latitude ULF waves across dawn

C. L. Waters

Cooperative Research Centre for Satellite Systems, Department of Physics, University of Newcastle, Newcastle, New South Wales, Australia


M. D. Sciffer

Cooperative Research Centre for Satellite Systems, Department of Physics, University of Newcastle, Newcastle, New South Wales, Australia


B. J. Fraser

Cooperative Research Centre for Satellite Systems, Department of Physics, University of Newcastle, Newcastle, New South Wales, Australia


K. Brand

Department of Physics, University of Oxford, Oxford, United Kingdom


K. Foulkes

Cooperative Research Centre for Satellite Systems, Department of Physics, University of Newcastle, Newcastle, New South Wales, Australia


F. W. Menk

Cooperative Research Centre for Satellite Systems, Department of Physics, University of Newcastle, Newcastle, New South Wales, Australia


O. Saka

Department of Physics, Kurume National College of Technology, Kurume, Japan


K. Yumoto

Department of Earth and Planetary Sciences, Kyushu University, Fukuoka, Japan


Abstract

Ultralow frequency (ULF) waves at the geomagnetic equator are studied by using a small magnetometer array and a one-dimensional electromagnetic wave model. Most ULF waves observed at low latitudes have been associated with shear Alfven mode hydromagnetic resonances in the plasmasphere. Near the equator it is difficult to excite these resonances, and the wave activity is attributed to fast mode waves. The phase difference data recorded at longitudinally spaced, equatorial magnetometers shows a marked change around dawn when one station is sunlit while the other is still in darkness. The phase structure with latitude also shows large phase shifts near the equator, in agreement with previous studies. A model of ULF wave propagation through the equatorial ionosphere is presented and used to compare with the observed amplitude and phase properties. It is shown that the observed frequencies are in the vicinity of the wave cutoff frequency where the phase structure becomes complicated. For frequencies above the cutoff, the phase structure across dawn is directly related to dawn-associated changes in electron density in the ionosphere.

Received 5 September 2000; accepted 7 November 2000.

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Citation: Waters, C. L., M. D. Sciffer, B. J. Fraser, K. Brand, K. Foulkes, F. W. Menk, O. Saka, and K. Yumoto (2001), The phase structure of very low latitude ULF waves across dawn, J. Geophys. Res., 106(A8), 15,599–15,607.