Quasi-Periodic (QP) ELF-VLF Emissions Observed in High Latitudes

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The characteristics of quasi-periodic (QP) ELF-VLF emissions with periods of 3-150 s and their relationships to magnetic pulsations are studied by using data obtained from Syowa and Mizuho Stations in Antarctica and Husafell in Iceland which is located near the geomagnetic conjugate point of Syowa Station. From the relations of QP emissions to magnetic pulsations, QP emissions are classified into two types, Type 1 and 2 QP emissions, according to whether the emissions are clearly associated with magnetic pulsations or not. The typical characteristics of Type 1 QP emissions are as follows. The intensity and period of QP emissions change concurrently with variations in the intensity and period of magnetic pulsations. From the coherency analysis between QP emissions and Pc 3-4 magnetic pulsations it is found that the coherency between the D component of magnetic pulsations and the intensity fluctuations of QP emissions is much higher than that between the H component of magnetic pulsations and QPs. It is also found that the propagation time of magnetic pulsations (HM waves) from the interaction region between magnetic pulsations and QPs in the magnetosphere to the ground is 20-30 s. These properties are observed at conjugate-pair stations with good conjugacy. The results strongly suggest that Type 1 QP emissions are modulated by compressional mode Pc 3-4 magnetic pulsations near the equatorial plane in the outer magnetosphere. On the other hand, QP emissions are categorized as the Type 2 QP emissions when the period of concurrent pulsations is entirely different from that of QP emissions or the amplitude of pulsations is too small to determine the periodicity. The most striking feature of Type 2 QP emissions are their very regular periodicity as compared with that of Type 1 QP emissions, and the Q value of the spectral peak usually attains to more than 10. In most cases magnetic variations during Type 2 QP events, which occur under magnetically quiet condition, have no spectral peak corresponding to the peak in QP's spectrum. However, a small but significant peak in pulsation spectrum is occasionally noted, when the Type 2 QP event occurs in modelate disturbed condition. The peak value of magnetic spectrum, in such a case, is generally two order of magnitude less than the maximum power of background magnetic fluctuations. The amplitude of magnetic pulsations is in the 2 QP ELF-VLF Emissions Observed in High Latitudes order of 0.001 nT/s. The H component of magnetic pulsations tends to be more correlated with the VLF intensity variations than that the D component. These properties are observed at conjugate-pair stations with good conjugacy. It is suggested that weak magnetic pulsations, correlated with the Type 2 QP, are produced by wave induced particle precipitations. As to the periodicity of Type 2 QP emissions, either the period is controlled by compressional magnetic waves which are usually not detected on the ground, or the period of QPs is controlled by some other modulation mechanism which is unknown at present. The frequency-time (f-t) spectra of QP emissions are classified into five types. A phenomenological model to explain the f-t spectra is proposed, that is, the rising-tone type of QP emission is generated by compressional mode Pc 3 magnetic pulsations which propagate in radial direction toward the earth, while the non-dispersive type is generated by standing oscillations of the magnetic field which have effective compressional components.
国立極地研究所の論文

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Sato Natsuo
National Institude of Polar Research

Quasi-Periodic (QP) ELF-VLF Emissions Observed in High Latitudes

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