Observations of Wave Mode Conversion at the Magnetopause

We present observations of possible wave mode conversion (WMC) at the magnetopause during two magnetopause crossings by Geotail in flank regions. The polarization of ULF waves changed near the magnetopause with strong transverse waves observed in the magnetopause boundary layer and strong compressional oscillations in the magnetosheath adjacent to the magnetopause, which may indicate the occurrence of WMC. One crossing occurred when magnetic field shear was low (about 5^o). In this crossing, the Poynting flux In the magnetosheath side is basically along the plasma flow direction, while in the magnetopause boundary layer, the wave energy flux is largely field aligned, but with a significant earthward normal component. The second crossing occurred with larger magnetic field shear angle (about 70^o). During this event several clear bipolar signatures in the normal component B_N which indicated the passages of twisted flux tubes, were observed in the magnetopause boundary layer. It may be the signatures of nonlinear wave mode conversion process. The Poynting flux during this event is enhanced in the period of enhanced transverse fluctuations, flowing in the southward and tailward directions. The observations will be compared to the linear and nonlinear mechanisms of WMC.