Dayside Outer Magnetosphere ULF Waves Observed by Geotail

Abstract On the basis of our recent studies (Sakurai, 1999a, 1999b and 2001) the present paper is intended to review oscillation and propagation characteristics of Pc 3 and Pc 5 ULF waves observed near the dayside magnetopause by the Geotail satellite. These characteristics are studied on the basis of the simultaneous observations of the magnetic and electric fields, and low energy plasmas measured with the excellent instruments on board the satellite. The study reveals that the dominant ULF waves observed near the dayside magnetopause are Pc 3 and Pc 5 oscillations. The Pc 3 oscillations appear with a peak power around noon at the frequency of 25 mHz in the azimuthal component of the magnetic field. This frequency component shows clear resonant oscillations. In addition fast mode earthward propagation is recognized. The Poynting flux of Pc 3 waves is about 1-10 nW/m 2 on average, the strongest being along the magnetic field-line. Pc 5 oscillations are also dominant, and are observed mainly in the dawn and dusk-side flanks. They appear as clear oscillations in the radial component of the electric field, suggesting that resonant oscillations along the magnetic field-line are well established. The Poynting flux of Pc 5 can be estimated as 10 - 100 nW/m 2 in both directions across and along the magnetic field-line. This result suggests that Pc 5 wave energy is carried into both the ionosphere and the inner magnetosphere during an hour with the energy of 10 10 - 10 13 J, which is one or two orders of magnitude less than the substorm energy. By taking into account the continuous activation of ULF waves in the outer magnetosphere, these waves should play an important role in the energetics of the magnetosphere.