VLF-LF Hiss Emissions Associated with Aurora

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It is shown that auroral hiss emissions observed in high latitudes can be categorized into two types on the basis of the power spectrum and its temporal variation. They are called the narrow-band continuous hiss and the wide-band impulsive hiss. The continuous hiss has a narrow frequency range with a center frequency of about 10 kHz and a band width of a few kHz and is characterized by a long time duration (longer than a few tens of minutes). The impulsive hiss, on the other hand, has a wide frequency range with a spectral peak frequency of about 10 kHz and a band width of a few tens of kHz and is of a short time duration (usually shorter than 10 minutes). On the basis of observations at the two Stations, Syowa and Mizuho and the ISIS 2 satellite, the continuous hiss emissions are found usually to occur associated with the steady auroral arc located near the poleward horizon of Syowa Station, and the impulsive hiss emissions, on the other hand, are found to occur accompanying the initial brightening of active aurora (corona, rayed-band, west-ward traveling surge) that appeared near the zenith. These relationships between two types of hiss and the global auroral activity are confirmed by the comparison between the global auroral data observed on the DMSP satellite and the ground data of hiss. A systematic local time variation of the arrival direction of the continuous hiss emissions is also found. It is shown that the arrival direction in early evening (18h-19h) is the east and it changes from east to west around 20h-21h. This local time variation of the arrival direction will be discussed in relation to the statistical distribution of the source region of the continuous hiss emissions. The arrival direction of the waves was next examined and it shows that the continuous hiss arrives from several hundreds of km equatorward of the related aurora, while the impulsive hiss sometimes arrives from the region nearer the auroral location. It also becomes clear that the distance between the exit (the arrival direction) of hiss and the auroral location depend on the magnetic dip angle of the station by comparing the results obtained at Churchill and Syowa Station. Namely the arrival direction nearly tends to coincide with the auroral active region at Church-2 VLF-LF Hiss Emissions Associated with Aurora ill with a large dip angle (Dip〜83°) and the wave exit is likely located hundreds of km equatorward of the aurora at Syowa Station with a small dip angle (Dip〜65°). Simultaneous observations of VLF emissions observed on ISIS 2 and on the ground support the conclusion that the continuous hiss does not propagate along the magnetic field line from the satellite altitude (h〜1400 km) to the ground but along a non-ducted path. The ray path of the VLF waves was calculated in comparison with the observational results mentioned above. It was found that the distance between the region where the wave normal angles lie within the transmission cone at the ionospheric level (h〜200 km) and the foot point of the field line along which the duct terminates at 3000 km in altitude, is about 300-400 km at Syowa Station. These results explain the fact that the arrival direction of continuous hiss is several hundreds of km equatorward of the related aurora, and further that the duct of auroral hiss terminates at about 3000 km in altitude. By contrast, the arrival direction of the impulsive hiss is found sometimes much nearer to the auroral activity. Likewise, the impulsive hiss seems to propagate along the field line lower than 3000 km in altitude on account of the strong field-aligned irregularities or the large-scale horizontal gradient of ionization. Assuming that particles and waves are coupled in the Cerenkov condition and waves are generated with an electron beam with energy of a few keV or less, and further assuming that the enhancement factor along the field line is smaller than 0.5, the generation region of auroral hiss observed on the ground is
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VLF-LF Hiss Emissions Associated with Aurora

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