Low-Latitude VLF Hiss Associated with the Severe Magnetic Storm on October 19-21, 1989.

Abstract

Low-latitude VLF hiss was observed at dawn time at Moshiri (L = 1.6) and Kagoshima (L = 1.2), Japan during 20hOOm UT to 23h30m UT on October 20, associated with the severe magnetic storm (Kp = 8) during October 19-21,1989. The VLF hiss with a band-limited spectrum exhibited an increase of frequency (fmax) of the maximum energy with local time. Based on the quasi-linear model for hisstype mid-latitude VLF emissions developed by SAZHIN (1984) and using electron density data by EXOS-D satellite, it is estimated that injected electrons of about 5 keV associated with the maximum depression of the magnetic H-component around 13h30m UT on October 20 contribute to the generation of the VLF hiss at dawn time around L = 4.8 of the magnetospheric equatorial region. The plasmapause has been disrupted due to the strong magnetospheric disturbances when the VLF hiss was observed. The increase of fmax may be due to a combined effect of the energy dependences of different drift starting longitudes of the hot electrons injected in the evening sector and of the drift velocities of the electrons encircling the Earth, and a temporal variation of the cold electron density in the generation region at dawn sector. Furthermore it is implied that the VLF hiss has penetrated through the ionosphere at the higher latitude than that of Moshiri and then propagated toward the low-latitude stations in the Earth-ionosphere waveguide mode.