Fast mode magnetosonic waves have been discovered in the distant plasma sheet boundary layrs. The emissions have frequencies near the proton cyclotron frequency, are circularlty or elliptically polarized in a right-hand sense, and are propagating nearly along B↘(&thgr;kB<20¿). The wave compressional component can be as large as 30--90% of the wave amplitude. The emissions are substorm related. Several possible generation mechanisms are discussed, such as resonant interactions with ~1-2 keV streaming protons. In the firehose instability, energization of the protons would be caused by the adiabatic compression of plasma on freshly reconnected magnetic field lines. The role that the waves may play in the heating and expansion of the plasma sheet and as a component of internal pressure which keeps open the distant tail, are both briefly discussed.