ELF Emissions in High Latitudes-Ray Path Calculation and Ground-Satellite Observations

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This paper deals with ELF emissions observed in the dayside magnetosphere, called ELF hiss and polar chorus. These waves are generated in the magnetic equatorial region through Doppler-shifted cyclotron wave-particle interaction by high- energy electrons. According to the satellite observations, the features of ELF emissions such as power spectrum and wave normal direction in low-altitude polar region are significantly different from magnetic equatorial plane after their propagation in the magnetosphere. Especially in ELF range, magnetospheric ions play a fundamental role in low-altitude propagation, for example in LHR reflection and multi-ion cutoff. Previous work on ELF wave ray tracing from the high-altitude magnetic equatorial plane to low altitudes revealed that most of the rays undergo LHR reflection before they reach the usual level of polar orbiting satellites. So, it seems that ELF emissions observed by polar orbiting satellites and on the ground have propagated along field-aligned ducts. However, there is abundant evidence from satellite ELF wave observations that the wave normal of ELF emission makes a large angle with the earth's magnetic field at low altitudes, and this feature can be explained only by nonducted propagation. Therefore, it is important to find out the condition required for the nonducted ray path between the magnetic equatorial plane and low altitudes without suffering from LHR reflection. This kind of ray path is of special interest in the present study, and it is referred to as "penetrating ray path" in this paper. This paper is devoted to the study of the above two kinds of propagation from the magnetic equatorial plane to the polar region with the observations of ELF/VLF waves by ISIS satellites over Antarctica, ELF waves at Syowa Sation, Antarctica and Bremsstrahlung X-rays by balloons. Characteristics of ELF emissions observed at the high-altitude equatorial region and the low-altitude polar region are briefly reviewed first. Conditions for the existence of penetrating ray paths are obtained from a 2-dimensional ray tracing. Wave normal directions and arriving latitudes at low altitudes are also calculated. These calculated characteristics of ELF wave normals are compared with those from ISIS satellite observation in the altitude range of 1450-3500 km by means of spin-modulated characteristics and multi-ion cutoff characteristics of received ELF signals. Seasonal variation of ELF emission intensity at the ISIS satellite altitude and on the ground is attributable to a north-south asymmetry in the penetrating ray paths depending on a field-aligned electron density gradient in the both hemispheres. Another mode of ELF wave propagation, i.e., ducted propagation outside the plasmapause, is investigated for periodic emissions. This type of emission is recorded on the ground at regular intervals of several seconds, alternately in the opposite hemispheres. Hence the emission is thought to be caused by an echoing of wave packet between the two hemispheres along a field-aligned duct. Periodic emissions with the same repetition interval are often observed over an area of more than 2000 km wide at the ISIS satellite altitude and on the ground. Such a wide spatial extent of emission reception is explained in this chapter from a viewpoint that the ELF duct terminates at a certain altitude, below which the waves propagate downward widely in nonducted mode. Finally, ELF emissions observed on the ground are compared with Bremsstrahlung X-rays observed by balloons. As the electrons producing such X-rays precipitate along magnetic field lines, a pattern of electron precipitation observed on the ground is a projection onto the polar ionosphere of the wave-particle interaction region near the magnetic equator. Hence the electron precipitation associated with ELF emission activity affords useful information on the source location of ELF emissions. With this background in mind, three balloon experiments were carrie
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ELF Emissions in High Latitudes-Ray Path Calculation and Ground-Satellite Observations

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