On the origin of kilometric continuum

The Radio Plasma Imager (RPI) and the extreme ultraviolet (EUV) imager instruments on the polar orbiting IMAGE satellite are providing new observations of the source region of kilometric continuum. Observations by the IMAGE instruments are now able to clarify some aspects of the source of this emission and put some previous observations into a new perspective. The IMAGE observations show that kilometric continuum is generated at the plasmapause, from sources in or very near the magnetic equator, within a bite-out region of the plasmasphere. It is not known if the bite-out region is a necessary condition for the generation of kilometric continuum. From ray-tracing calculations in a model plasmasphere having a bite-out feature, it is found that a kilometric continuum source can produce a relatively narrow emission cone that is largely confined to the longitudinal extent of the bite-out structure. Since the bite-out structure is observed to corotate with the plasmasphere, so would the beamed kilometric continuum. In addition, the observed narrow latitudinal extent of the emission cone does not appear to be a propagation effect but is perhaps produced by the emission mechanism, such as the linear or nonlinear mode-conversion processes at the upper hybrid resonance proposed by a number of investigators. The processes by which bite-out structures are produced in the plasmasphere are not completely understood at this time.