Abstract
Medium-frequency waves in the ion-electron two-fluid plasma with frequencies between the electron gyrofrequency and the ion gyrofrequency are studied. The dispersion relations and the Friedrichs diagrams of the group velocities are plotted with color-coded electric-field information. The group-velocity distributions and the properties of the electric fields can help us explain the observed dynamic spectra of waves when the waves are generated by a localized broadband disturbance. Possible sources of such disturbances are explored. One of them is the magnetic trapped energetic electrons with pitch angles greater than 70°. The dynamic spectra of the waves generated by these energetic electrons are ray-angle dependent. For the ray angle greater than 2°, the dynamic spectra of the rising-tone quasielectromagnetic chorus waves are characterized by a frequency gap near half of the electron gyrofrequency. The dynamic spectra of the rising tone chorus waves without the frequency gap can only be found when either the ray angle is less than 2° or when the lower-band chorus waves are quasielectrostatic. Lightning from the neutral atmosphere provides another type of localized disturbance. Depending on the magnetic latitude, lightning can result in two types of falling-tone electromagnetic whistler waves. One of them is the low-k⊥ whistler waves with highly field-aligned phase velocity and group velocity. The other is the high-k⊥ whistler waves with highly field-aligned group velocity but with phase velocity almost perpendicular to the background magnetic field. The results of this study will provide a road map for future studies of the waves in this frequency range.
Highlights
Waves in a dense, warm, and magnetized plasma can be classified into four groups: (i) the high-frequency plasma waves with frequencies above the electron gyrofrequency, (ii) the medium-frequency plasma waves with frequencies between the electron gyrofrequency and the ion gyrofrequency, (iii) the low-frequency plasma waves with frequencies below the ion gyrofrequency but much higher than 0.001 ion gyrofrequency, and (iv) the very-low-frequency plasma waves with frequencies lower than 0.001 ion gyrofrequency
The results shown in panels (a) and (b) indicate that the waves are all quasielectromagnetic waves (R ! 1) when the wave frequency is less than the ion gyrofrequency Xi, which is about 0:00054 Xe
We examine the medium-frequency waves with wave frequencies between the electron gyrofrequency and the ion gyrofrequency in the ion-electron two-fluid plasma
Summary
Warm, and magnetized plasma can be classified into four groups: (i) the high-frequency plasma waves with frequencies above the electron gyrofrequency, (ii) the medium-frequency plasma waves with frequencies between the electron gyrofrequency and the ion gyrofrequency, (iii) the low-frequency plasma waves with frequencies below the ion gyrofrequency but much higher than 0.001 ion gyrofrequency, and (iv) the very-low-frequency plasma waves with frequencies lower than 0.001 ion gyrofrequency. When we study the high-frequency plasma waves, we can assume ions are at rest. When we study the low-frequency plasma waves, we can ignore the electrons’ inertial term in the momentum equation of the electron fluid and determine the perturbed electric field from this equation. When we study the MHD waves, we can assume that the electrons and ions move together at the same velocity V and the electric fields in the MHD waves satisfy the MHD Ohm’s law EþVÂB 1⁄4 0. When we study the medium-frequency plasma waves, we have to consider both the ion dynamics and the electron dynamics. The ion-electron two-fluid model becomes the simplest model to study the medium-frequency waves in a dense, warm, and magnetized plasma
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