Latitudinal changes of polar hiss and plasmapause hiss associated with magnetospheric processes Original Research Article

The magnetospheric VLF (very low frequency, 3–30 kHz) hiss observed by a low altitude polar orbiter represents integrated spectra of whistler mode VLF hiss waves propagating along geomagnetic field lines from wide-altitude sources in the magnetosphere. We investigate latitudinal changes of the magnetospheric VLF hiss associated with magnetospheric processes in geomagnetic quiet and disturbed periods by using VLF hiss intensity data at six frequency bands processed from VLF electric field (50 Hz–30 kHz) data of ISIS-2 (circular polar orbit at 1400 km in altitude). The magnetospheric VLF hiss observed by the ISIS-2 mainly consists of a broad-band polar hiss and narrow-band plasmapause hiss. The polar hiss is the whistler mode Cerenkov emissions generated in the polar magnetosphere by inverted-V electrons (100 eV–40 keV) precipitating from the plasmasheet boundary layer. The plasmapause hiss is also whistler-mode waves excited by the cyclotron instability of energetic electrons convected inward from the magnetotail. In a geomagnetically very quiet period, only a plasmaspheric ELF (extremely low frequency, 0.3–3 kHz) hiss and diffused whistlers appear at invariant latitudes below 73°. The broad-band polar hiss and narrow-band plasmapause hiss occur, respectively, at night-side invariant latitudes from the middle of the auroral zone to the polar cap and for nighttime invariant latitudes of 50–64° in a quiet or weakly disturbed period. In a substorm period, the polar hiss region shifts to lower latitudes due to an inward movement of the plasmasheet inner edge associated with an intensified substorm westward electric field in the magnetotail. Also, an intensity hump of the plasmapause hiss appears between regions of the polar hiss and plasmapause hiss on the night side in a lingering substorm period. This seems to be generated in a plasmaspheric plume outside the night-time plasmapause. As the substorm develops in the expansion phase, the polar hiss region finally joins to the plasmapause hiss region. These show that the invariant latitudinal changes of magnetospheric VLF hiss generated by the micro-scale mechanism are greatly affected by the macro-scale dynamical processes in the magnetosphere.