Low frequency periodic perturbations to the Earth's background magnetic field, known as ultra low frequency (ULF) waves, have been observed on the ground and in space for many decades. With a large scale length and low frequency, these waves can be effectively studied using magnetohydrodynamics (MHD). In this talk I'll discuss some of our recent numerical work on the resonant coupling of two MHD wave modes, namely fast-Alfven wave coupling, and put this in context in terms of waves in Earth's magnetosphere. I'll do my best to introduce all of the magnetospheric terminology and will also hopefully spark some interest about how the 3D nature of Alfven resonances could be important for waves in the solar corona.