The Alfvén velocity in the magnetosphere and its relationship to ELF emissions

Summary. Ogo 5 measurements of the ion density and magnetic field strength near the equator in the dawn quadrant of the magnetosphere have been used to determine the Alfvén velocity. The Alfvén velocity in the outer magnetosphere ranges between 1000 and 2000 km/sec. At decreasing radial distances the Alfvén velocity increases to a maximum (average, 4800 km/sec) and drops to a minimum (average, 490 km/sec) just inside the plasmasphere. ELF emissions are well ordered by the Alfvén velocity profiles. ELF chorus from 100 to 1000 Hz exists in the outer magnetosphere only for Alfvén velocities below about 3000 km/sec. ELF hiss is found immediately inside the plasmapause. This relationship between ELF emissions and the Alfvén velocity is further support for unstable wave generation by Doppler-shifted cyclotron resonance.