PLASMA-WAVES OBSERVED INSIDE PLASMA BUBBLES IN THE EQUATORIAL F-REGION

Plasma waves have been detected within and around density depletions i n the topside equatorial F region by the electric and magnetic held se nsors of the Extremely Low Frequency Wave Analyzer (ELFWA) instrument which is part of the Low-Altitude Satellite Studies of Ionospheric Irr egularities experiment on the Combined Release and Radiation Effects S atellite. The plasma waves include both electrostatic waves that have a small magnetic field component and electromagnetic waves propagating in the extraordinary mode. Thus they are not simply zero-frequency ir regularities as generally assumed by previous investigators who were w orking without the benefit of high-sensitivity ac magnetic field measu rements. The waves exhibit no resonances or cutoffs at characteristic frequencies of the plasma within the range of the ELFWA which is 2-125 Hz. They occur from late evening to early morning primarily at altitu des around 400-500 km. An example has been observed as high as 1500 km . The waves are associated with plasma depletions from a factor of 2 t o a factor of 80 less than the surrounding density. The observations i ndicate that the waves are related to equatorial spread F.