LOWER-HYBRID WAVE CAVITIES DETECTED BY INSTRUMENTED SPACECRAFTS

Localized electrostatic wave packets in the frequency region of lower- hybrid waves have been detected by instrumented rocket payloads and re cently also by the instruments on the FREJA satellite. These waves are often associated with local density depletions, indicating that the s tructures can be interpreted as wave-filled cavities which are strongl y elongated along the magnetic field. The basic features of the observ ations are discussed. Based on simple statistical arguments, we attemp t to present some characteristics which have to be accommodated within an ultimate theory describing the observed wave phenomena.