Low latitude boundary of polar hiss in the topside ionosphere

Tadanori Ondoh, ondoh@mbp.ocn.ne.jp Space Earth Environment Laboratory, Tokorozawa, Japan Latitudinal variation of VLF hiss observed in geomagnetic quiet and disturbed periods are in-vestigated by using VLF electric field (50 Hz -30 kHz) data of ISIS-2 (circular polar orbit at 1400 km height) received at Syowa station, Antarctica. The VLF hiss observed are classified of the broad-band polar hiss and narrow-band plasmapause hiss. The wide band polar hiss is the whistler-mode Cerenkov emissions in the polar magnetosphere generated by energetic electrons (100 eV -40 keV) precipitating from the plasmasheet boundary layer. The plasmapause hiss is narrow-band whistler mode waves excited by the cyclotron instability of energetic electrons convected from the magnetotail. The low latitude boundary of polar hiss which depends on geomagnetic local time lies at a geomagnetic invariant latitude above 70 degrees in a geomag-netic quiet period, but it comes down to below 65 degrees in a disturbed period. We have not yet discussed on physical meaning of the low latitude boundary of polar hiss, that is, a lati-tudinal boundary of precipitating energetic electrons generating the polar hiss. Is this simply mean an inner boundary of precipitating energetic electrons generating the polar hiss or not ? This solution will give us some suggestion why wide-band polar hiss is not observed relatively near outside of the plasmapause, although the mid-latitude narrow-band VLF hiss is normally observed inside the plasmapause.