RESONANT AMPLIFICATION OF MAGNETOSHEATH MHD FLUCTUATIONS AT THE MAGNETOPAUSE

Broad-band ULF fluctuations are routinely observed upstream of the ter restrial magnetosphere: their amplitude, fairly large in the whole mag netosheath, peaks at the magnetopause, and decays to very small values in the magnetosphere. We test the idea that the source of all these f luctuations is in the magnetosheath and that the high magnetopause lev el is an amplification effect due to propagation. Starting from given stable magnetosonic waves in the magnetosheath, their behaviour is cal culated when, during their propagation, they encounter the magnetopaus e density gradient. For a certain category of incident waves, a ''reso nant amplification'', occurring inside the gradient layer, is found.