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On 24 August 2005, an impulse of solar wind dynamic pressure ( P sw) hit the magnetosphere. Using the high resolution geomagnetic field data from 15 ground stations and the data from Geotail and TC-1, we studied the geomagnetic pulsations at auroral latitudes driven by the sharp decrease of P sw at the trailing edge of the impulse. The results show that the sharp decrease of P sw can excite a global pulsation in the frequency range 4.3–11.6 mHz. The pulsation has a reversal of polarization between two auroral latitude stations, a larger power spectral density (PSD) close to resonant latitude and increasing frequency with decreasing latitude. All these features indicate that the pulsations are associated with field line resonance (FLR). The fundamental resonant frequency (the peak frequency of PSD between 4.3 and 5.8 mHz) is dependent on magnetic local time and is largest around magnetic local noon. This feature is due to the fact that the size of magnetospheric cavity is dependent on local time and smallest at noon. A second harmonic wave at about 10 mHz is also observed, which is strongest in the daytime sector, and becomes heavily attenuated in the night sector. The comparison of the PSDs of the pulsations driven by sharp increase and sharp decrease of P sw shows that the frequency of pulsations is negatively proportional to the size of magnetopause. Since the FLR is excited by compressional cavity/waveguide waves, the above results indicate that the resonant frequency in the magnetospheric cavity/waveguide is controlled not only by solar wind parameters but also by magnetic local time of observation point.
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