A typical Pi3 pulsation is examined by magnetic field measurements
   from multiple satellites and ground stations. Low-latitude ground
   observations with a wide longitudinal span indicate that the amplitude
   of the Pi3 pulsation peaks on the dayside and is gradually decreasing
   toward the nightside. This effect and the fact that the wave phase on
   the dayside leads that on the nightside, imply that the source of the
   Pi3 lies on the dayside. Variations in solar wind dynamic pressure
   observed by the GEOTAIL satellite (just outside of the magnetosphere)
   are highly correlated with these ground magnetic field variations. We
   argue in this case that the global Pi3 pulsation is directly driven by
   impulsive variations in the solar wind dynamic pressure. The Pi3
   pulsation observed along the latitudinal magnetometer chain at 0930LT
   shows significant equatorial enhancement and additional observations
   along a latitudinal chain at 1630LT show that the phase of the Pi3
   pulsation at high latitudes lags behind that at low latitudes. The
   low-altitude polar orbiting satellite Oersted also observed this
   pulsation in the dayside inner magnetosphere. The B-parallel to
   (northward) component at Oersted is strictly out of phase with the X
   component observed at the dip equator below the spacecraft path, which
   indicates that the Pi3 pulsation at the dip equator is caused by
   oscillation of an ionospheric current. We propose that the Pi3
   pulsations at different latitudes are generated by different mechanisms.

We study the onset and development of an ultra low frequency (ULF)
   pulsation excited by a storm sudden commencement. On 30 August 2001,
   14: 10 UT, the Cluster spacecraft are located in the dayside
   magnetosphere and observe the excitation of a ULF pulsation by a
   threefold enhancement in the solar wind dynamic pressure. Two different
   harmonics are observed by Cluster, one at 6.8 mHz and another at 27
   mHz. We observe a compressional wave and the development of a toroidal
   and poloidal standing wave mode. The toroidal mode is observed over a
   narrow range of L-shells whereas the poloidal mode is observed to have
   a much larger radial extent. By looking at the phase difference between
   the electric and magnetic fields we see that for the first two wave
   periods both the poloidal and toroidal mode are travelling waves and
   then suddenly change into standing waves. We estimate the azimuthal
   wave number for the 6.8 mHz to be m = 10 +/- 3. For the 27 mHz wave, m
   seems to be several times larger and we discuss the implications of
   this. We conclude that the enhancement in solar wind pressure excites
   eigenmodes of the geomagnetic cavity/waveguide that propagate tailward
   and that these eigenmodes in turn couple to toroidal and poloidal mode
   waves. Thus our observations give firm support to the magnetospheric
   waveguide theory.