We present an evidence of the coupling of a fast magnetospheric cavity mode to field line resonances. The power of filtered H and D components of Pi 2 pulsations during March-April in 1978 and 1979 from IGS chain in the United Kingdom (UK) shows that there are two enhancements located near L = 3.5 and 5.5 in the distribution profile. The irreversible hydromagnetic wave coupling is studied by using a box model for the magnetosphere. The cold linearized MHD equations is examined with realistic Alfven speed profile for the uniform and nonuniform ambient magnetic fields, respectively. With appropriate parameters to depict the magnetospheric environments during the aforementioned period, numerical results are acquired with the fourth order Runge-Kutta method and quite consistent with data analysis. We suggest that the coupling of a fast magnetospheric cavity mode driven by fast compressional waves owing to the impulsive source at the magnetotail to resonant field lines at low latitudes is a possible scenario of the propagation mechanism for low latitude Pi 2 pulsations.
