Field-line resonances (FLRs) are standing modes of the Earth's dipole magnetic field lines, and their polarization properties depend on local time and L shell [Samson et al., 1971; Hughes, 1994]. The FLRs are usually quite narrow in the radial direction while a large part of an L shell can oscillate azimuthally. This is a toroidal resonance mode with an azimuthal magnetic field component and a radial electric field component, commonly observed at geosynchronous distances [Laakso and Schmidt, 1989].
A well-known driver of an FLR is a fast mode wave which is coupled to a shear Alfven wave at a certain resonant L shell [Southwood, 1974; Singer et al., 1982]. There is a variety of energy sources that can produce fast mode waves in the magnetosphere. It has been suggested that a major source is the Kelvin-Helmholtz (K-H) instability which can produce surface waves along the magnetopause and emit fast mode waves into the magnetosphere [Pu and Kivelson, 1983]. The orientation of the magnetic field with respect to the surface can play an important role in switching on the K-H instability [Miura, 1995b]. As a consequence of this instability, large-scale vortices are formed in the flanks of the magnetosphere [Hones et al., 1981; Miura, 1995a].
On the other hand, it has been found that wide-band sources, like sudden impulses in the solar wind, produce field-line resonances only at a few L shells, implying that fast modes of certain frequencies can only exist in the magnetosphere. Therefore it has been suggested that only those fast modes that can get trapped within the magnetospheric cavity between the magnetopause and the plasmapause, can be coupled to shear Alfven waves, producing field-line resonances [Allan et al., 1986].
In this paper we investigate field-line resonances observed on January 10, 1997. At 21:22 UT, the magnetometer on the IMP 8 satellite located near the bow shock observed a sudden northward turning of the IMF. Then GEOTAIL instruments began to observe fast mode waves and magnetic field vortices in the LLBL/plasma sheet region for several hours. The fast mode waves can trigger field-line resonances on dipole field lines. In fact, POLAR, while moving from the northern polar cap onto dipole field lines, observed field-line resonances at L = 10.2-6.6.