Triaxial search coil measurements of ELF waves in the plasmasphere: Initial results from EXOS‐D

Abstract

Triaxial search coil measurements of ELF waves in the frequency range below 50 Hz have been made with the polar orbiting satellite, EXOS‐D [AKEBONO], in the plasmasphere. Single component electric field measurements were also obtained along with measurements of the magnetic field. A survey of realtime telemetry data received for the period of March – October, 1989 reveals the existence of variety of plasma waves in the frequency range around ion gyrofrequencies near the equatorial region of the plasmasphere. Electromagnetic noises with a multi‐band structure were observed mostly above the local helium gyro‐frequency (fHe+), in the region of L values of 1.5 ∼ 2.5. Multi‐band ELF emissions are common in the daytime and were observed in 49 out of 250 orbits examined here. Another type of electromagnetic ELF waves with a broad‐band spectrum are also observed in the frequency range above the helium gyrofrequency in 10 orbits. Both types of wave spectra extend above the local proton gyrofrequency, but no absorption effect is observed at the local proton gyrofrequency. The electric field measurements clearly show spectral cut‐off for both of these two types of waves near f = (1.3 ∼ 2.5) fHe+. On the other hand, several spectral bands sometimes appear between the helium and oxygen gyrofrequencies in magnetic field components of multi‐band emissions.