Emissions of plasma waves from VLF to LF ranges in the magnetic polar regions

The data of natural plasma waves obtained from the initial three month observations of the planetary plasma sounder system are investigated, focusing on VLF to LF range phenomena in the polar ionosphere. It is disclosed that the dynamic spectrum of the auroral VLF hiss phenomena consists of the main portion and wings. The main portion whose lowest frequency is less than 1 kHz is associated with the leaked AKR phenomena, suggesting that the main portion is generated at the acceleration region of the auroral particles. The wings that have a cut-off frequency at several kHz, higher than the main portion, are formed as the result of the cut-off at the lower hybrid resonance frequency in side area of the source region. These features suggest that the auroral VLF hiss emissions are generated through the Landau type interactions with the auroral particles in the large wave number domain of the whisler mode which is of the nature of a electrostatic plasma wave. The detailed features of the LF hiss emissions are derived for the first time from the data with high time resolution in time and frequency. The auroral LF hiss emissions in a frequency range from 100 to 450 kHz have weak and relatively steady dynamic spectra showing a clear contrast with the leaked AKR phenomena which shows the very intense and highly variable feature. It is found that the generation of the LF hiss is sometimes taking place at the level of 500 km altitude. The upper hybrid mode wave are also generated in the polar ionosphere by the electron beams streaming down from the acceleration region, and the waves are observed in the form of locally trapped Z-mode waves that are converted while propagation through the inhomogenous media. (Kubozono, M.)