Excitation of low-frequency waves in the topside ionosphere in heating experiments at the leading edge of accelerated suprathermal electron beams

Abstract

We consider the excitation of low-frequency waves in the upper ionosphere at the leading edge of suprathermal electron beams produced in the reflection region of a powerful radio wave. The integral growth rate Γof electron-beam instability is found for a wave packet intersecting an instability domain. The growth rate Γis strongly dependent on the ratio R=(∂ω/∂κ _∥ ) /(ω/κ _∥ )of the group velocity of the wave packet along the geomagnetic field to its phase velocity, which is equal to the velocity of the resonant particles.

It is shown that in the upper ionosphere (the electron gyrofrequency f _Be ≃ 1MHz and the ratio of ion to electron masses M/m=18364the growth rate Γof longitudinal lower-hybrid oscillations is maximal at the frequency f ≃ 200kHz when R ≈ 1and the phase velocity ω/κ _∥ begins to grow due to the influence of ions. The excitation of longitudinal oscillations with f < f _Bi increases with decreasing f, since the ratio R approaches unity in this case. The enhancement of ion cyclotron oscillations is most effective in the vicinity of the second ion cyclotron harmonic f<2f _Bi ≃ 1MHz.

Magnetohydrodynamic (MHD) waves interact with higher-energy particles having velocities on the order of the Alfvén velocity V _A ∽~ 10⁸ cm/sec. This interaction is damped in the small parameter (m/M)=1/1836.However, the integral growth rate Γof low-frequency Alfvén waves becomes considerable in the case of their quasiorthogonal (with respect to the magnetic field) propagation (κ_∥ ≪ κ_⊥)in a strongly magnetized ionospheric plasma when the Alfvén velocity is much greater than the thermal velocity of the electrons. The interaction of a fast magnetosound (FMS) wave with resonant electrons increases considerably for f ≫ f _Bi when R=1and the wave packet moves along the magnetic field with resonant particle velocity (this condition is similar to the well-known R. Gendrin condition obtained earlier for high-frequency whistlers excited by higher-energy electrons). The magnitude and direction of the vector of the FMS wave as a function of its frequency f are found for R=1.

The possibility of using these processes for explanation of the same features of low-frequency waves excited in heating experiments is discussed.