Lion roars, which are intense packets of electromagnetic waves characteristically found in the magneosheath, have been studied. The average frequency of the emissions is 120 Hz with over 90% occurring between 90 and 160 Hz; these results imply that the observed frequencies are near one-half the local electron gyrofrequency. The duration of the emissions is relatively short, over 70% of all emissions lasting less than 2 s. The maximum amplitude of the lion roars is found to have an average value of 85 m&ggr;, over 80% of the amplitudes being between 40 and 160 m&ggr;. The occurrence of lion roars is related to the level of geomagnetic activity as measured by Kp. The probability of occurrence ranges from 10% in magnetically quiet intervals to 75% during disturbed periods. The polarization and wave normal direction of lion roars, determined by variance analyses of the triaxial wave forms, are found to be right-hand circularly polarized with propagation essentially along the ambient magnetic field. More than 75% of all events analyzed were found to propagate at angles of less than 30¿ to the magnetic field. A correlation between decreases in the magnetic field magnitude and the occurrence of lion roars has been observed. Typically, the lion roars stars as the field magnitude decreases and end as the magnitude recovers. On the basis of these observations, several possible wave generation mechanisms are examined. Landau resonance is considered to be an unlikely source because this mechanism requires a substantial component of the wave electric field paralle to B, and the observation that the waves propagate along the ambient field is contrary to this requirement. It is not obvious that electron cyclotron resonance is responsible, because the field magnitude decreases should cause T∥/T⊥ to increase, and this rise could lead to wave damping rather than wave growth. A model which is consistent with all the observations of this study is a proton cyclotron overstability involving 10-keV protons streaming through the magnetosheath. It appears possible that the streaming protons could produce both the waves and the field decreases and that all three would be coincident.