We investigate the propagation, growth, and decay of fast magnetosonic waves in the Earth's magnetosphere which are believed to contribute to proton heating up to energies of a few hundred eV near the magnetic equator. We construct a model of the proton and electron distribution functions from spacecraft data and use the HOTRAY code to calculate the path-integrated growth and decay of the waves over a range of L shells from L=2 to L=7. Instability calculations show that the waves are excited at very large angles of propagation with respect to the magnetic field, &psgr;≈89¿, at the harmonics of the proton gyrofrequency &OHgr;H+ up to the lower hybrid resonance frequency &ohgr;LHR by a proton ring distribution at energies of the order of 10 keV. As a rule of thumb, we find that growth is possible for &ohgr;>30&OHgr;H+ when the ring velocity exceeds the Alfv¿n speed vR>vA, and for &ohgr;2vA. For propagation in the meridian plane, waves generated just outside the plasmapause grow with large amplification as they propagate away from the Earth but eventually lose energy to plasma sheet electrons at energies of a few keV by Landau damping. The waves grow to large amplification at frequencies just below &ohgr;LHR. For inward propagation we find that waves generated just outside the plasmapause can propagate to L≈2 with very little attenuation, suggesting that waves observed well inside the plasmasphere could originate from a source region just outside the plasmapause. Strong wave growth only occurs for large angles of propagation, and thus the waves are confined to within a few degrees of the magnetic equator. Waves generated near geostationary orbit and which propagate toward the Earth are absorbed by Doppler-shifted cyclotron resonance when they propagate into a region where vRA. Cyclotron resonant absorption causes pitch angle scattering and heating transverse to the ambient magnetic field. The amount of absorption, and hence transverse proton heating, increases significantly as the thermal proton temperature is increased up to 100 eV, suggesting a feedback process. Ray tracing shows that transverse heating of the thermal proton distribution is most likely to occur just outside the plasmapause where vA is large. Since proton ring distributions are formed during magnetic storms at ring current energies, we suggest that fast magnetosonic waves provide an additional energy loss process for ring current decay. ¿ 2000 American Geophysical Union