Electron cyclotron harmonic waves observed by the Arase satellite: identification of their wavelengths and phase speeds

Electron cyclotron harmonic (ECH) waves are commonly observed in the low latitude region just outside the terrestrial plasmapause. They are purely electrostatic and their frequency spectra show harmonic structures. The observed ECH waves are classified into two types: diffuse type and enhanced type. The diffuse ECH waves are weak and they appear as continuous emissions that last for more than a few minutes. The enhanced ECH waves appear as sudden enhancements of diffuse ECH in shorter time periods.

Abrupt enhancements of the ECH imply strong wave-particle interactions. However, observed electron velocity distribution functions do not show any significant change in association with the enhancement of the ECH. Moreover it is difficult to explain the generation of enhanced ECH spectra using linear theories, because the feature of multiple harmonic structures from the fundamental to higher order branches cannot be explained by linear growth rates of the loss-cone instability. However there remain uncertainties in the results of the linear analysis because we assume the distribution of cold core electrons, which the low-energy particle experiments-electron analyzer (LEP-e) does not measure. The dispersion relation curves (omega-k diagram) of the ECH waves strongly depend on the temperature of this unseen cold electron population. Once we can identify wavelengths or phase velocities of the ECH waves, we can estimate the temperature of the unseen cold core electrons. In the present paper, we calculate the wavelengths and phase speeds of the observed enhanced-type ECH by referring to the linear dispersion relations and discuss the results. The results will be helpful for evaluating the interferometry observation of the ECH that has recently been carried out by the Arase satellite. The interferometry mode observation of Arase allows us to estimate phase speeds of the waves. We discuss the possibility of the interferometry mode applying to the ECH waves using the latest observation results.