Polarizations and directions of propagation of ELF magnetospheric emissions

Search coil magnetometers on orbiting geophysical observatories have detected within the magnetosphere emissions at frequencies below approximately 1000 Hz. Data are available from polar-orbiting spacecraft (OGO 4 and 6) in which ELF hiss have a sharp, low-frequency cutoff is frequently present at high latitudes. The altitudes of the two space-craft vary between 400 and 1000 km, thereby monitoring a region of the lower magnetosphere in which there are large variations in ion composition. Data are also available from OGO 3 and 5 which have highly eccentric orbits with apogees exceeding 20 earth radii. Emissions have been detected on these spacecraft both inside and outside the plasmasphere. The principal signals inside the plasmapause are hiss, similar to that seen at low altitudes by the polar-orbiting spacecraft, which between the plasmapause and magnetopause, chorus is commonly observed. Since the search coil magnetometers are triaxial instruments, it has been possible to investigate the polarization and direction of propagation of the several signals in the various regions, Preliminary results of this study will be presented.