Radiation from sounder-accelerated electrons

Abstract

Quasi-electrostatic Z -mode waves observed in the two-point OEDIPUS-C (OC) transmission experiment have been interpreted as incoherent radiation by sounder-accelerated electrons (SAE). A consistent interpretation of slow Z -mode waves created by SAE, based on wave and particle observations, has been built around the theory of incoherent radiation. The question therefore arises as to whether some transmitter-induced waves observed by monostatic sounder receivers in the same frequency domain, from the greater of the electron plasma and gyro frequencies to the upper-hybrid-resonance frequency, can also be explained as caused by SAE. Two candidate signal types routinely observed in the ISIS-II sounder receiver have been examined: (a) a diffuse resonance ’spike’ lasting a few milliseconds and (b) highly elongated pulses distributed smoothly throughout the entire frequency range observed when the ambient ionospheric plasma exhibits density irregularities. An examination of Z -mode wave phase and group velocities, combined with consideration of wave and spacecraft kinematics, indicate that the hypothesis of plane slow Z waves does not suffice. The particle detector located on the same payload as the OC transmitter measures SAE pulses lasting milliseconds. Consideration of the role of the transmitter payload body appears to be necessary to account for the retention around the payload of SAE at various pitch angles and energies up to about 100 eV.