Magnetosheath lion roars

The characteristics of lion roars, which are intense packets of electromagnetic waves characteristically found in the magnetosheath, are studied. The average frequency of the emissions is 120 Hz, with over 90% occurring between 90 and 160 Hz (which is near one-half the local electron gyrofrequency); over 70% of all emissions last a mere 2 sec or less; the maximum amplitude of lion roars has an average value of 85 milligamma, over 80% being between 40 and 160 milligamma. Occurrence of lion roars is related to the level of geomagnetic activity, measured by Kp. The probability of occurrence ranges from 10% during magnetically quiet intervals to 75% during disturbed periods. Polarization and wave normal direction of lion roars, determined by variance analysis of triaxial wave forms, are righ-handed circularly polarized, with propagation essentially along the ambient magnetic field.