Dayside electron cyclotron harmonic emissions

Abstract

Observations of waves exhibiting an often complex frequency banding related to the electron gyrofrequency were first reported in 1970¹. Later observations^2–5 showed that the spectral content of these electrostatic electron cyclotron harmonic waves is highly variable^6,7, including single frequency emissions between the electron gyrofrequency Ω_e and its first harmonic, multiple bands between consecutive harmonics of Ω_e, and emissions in a single band far above the electron gyrofrequency^8–10. These observations have led theorists to investigate both the linear instability which generates the waves^11–15 and various wave–particle^14,16 and wave–wave^17–18 interactions in which they may be involved. The consensus seems to be that at least two plasma components with different temperatures are required to destablilize electron cyclotron harmonic waves. We adopt this view here and test our present knowledge of electron cyclotron harmonic instabilities against the observed reality. To do this we use simultaneous observations of wave spectra and electron distribution functions, available in the GEOS 1 data from 25 August 1977 (see ref. 15). From these we present the wave event and then use the measured particle fluxes to derive a model distribution function, providing the input to a computer program which solves the plasma dispersion relation. From the computed temporal growth rates and group velocities, the total amplification of waves which are unstable within a limited volume of space is estimated.