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

• Main Author: Hisao Yamagishi
• Format: Report
• Language: English
• Published: National Institute of Polar Research 1989
• Subjects: Antarc*; Antarctica; Memoirs of National Institute of Polar Research; Polar Research
• Online Access: https://nipr.repo.nii.ac.jp/?action=repository_uri&item_id=3511; http://id.nii.ac.jp/1291/00003511/; https://nipr.repo.nii.ac.jp/?action=repository_action_common_download&item_id=3511&item_no=1&attribute_id=18&file_no=1

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 ...