Resonant amplification of magnetosheath MHD fluctuations at the magnetopause

Broad‐band ULF fluctuations are routinely observed upstream of the terrestrial magnetosphere: their amplitude, fairly large in the whole magnetosheath, peaks at the magnetopause, and decays to very small values in the magnetosphere. We test the idea that the source of all these fluctuations is in the magnetosheath and that the high magnetopause level is an amplification effect due to propagation. Starting from given stable magnetosonic waves in the magnetosheath, their behaviour is calculated when, during their propagation, they encounter the magnetopause density gradient. For a certain category of incident waves, a “resonant amplification”, occurring inside the gradient layer, is found.