A sudden northward rotation of the IMF observed by IMP 8 near the bow shock at 21:22 UT on January 10, 1997, triggered surface waves and fast mode waves at the flanks of the duskside magnetosphere, as observed by GEOTAIL instruments. The power spectral density of these waves peaked in the frequency range where field‐line resonances were observed by the POLAR satellite in the region L = 6.6–10.2. The oscillation periods of the observed field‐line resonances were L‐dependent with two distinct peaks which may be explained by increased heavy ion content at those field lines. The ULF waves also caused enhanced low‐energy ion fluxes from the ionosphere.
