Characteristics of substorm-associated VLF/ELF emissions at medium latitudes
Substorm characteristics of VLF emissions are studied and we show that substorm-associated VLF emissions are an important and useful tool for investigating only the wave-particle interaction process, but also the particle in jection process during substorms. Special effects in VLF emissions with drifting center frequency are found to take place predominantly in the two local time sectors; dawn (LT---06) and premidnight (LT=20-24 h). First, the characteristics of dawnside VLF emissions
... with substorms will be reviewed based on our previous work. As a very interesting property, the substorm-associated VLF/ELF emissions on the dawnside exhibit a rise in their central frequency with local time. We have presented its interpretation in terms of the energy dispersion of the electrons drifting from the midnight particle injection region toward the generation region (LT---06). A few events of substorm-associated VLF hiss in the premidnight with a peculiar frequency drift were first discovered from our European VLF campaign and their characteristics are presented. The genera tion of hiss emissions just inside the plasmapause is interpreted due to a quasi linear cyclotron instability of the electrons injected from the plasma sheet. A new type of frequency drift in VLF hiss in the premidnight sector discussed here is such that the central frequency sharply increases first and followed by a gradual decrease during two successive substorms. This frequency drift is reasonably accounted for by a model combining an L-shell drift of energetic electrons in jected during the first substorm and a subsequent decrease under a large-scale convection electric field during the development of the event. The later decrease in frequency may be related to an additional injection of particles due to the de velopment of the second sunstorm. Finally, we will suggest future experimental work to be carried out to examine the validity of our proposed models.