Certain characteristics of trajectories of longitudinal waves in an ionospheric plasma near the resonance at the Langmuir frequency

Abstract

The refraction of longitudinal waves in an inhomogeneous magnetoactive plasma near the Langmuir frequency is investigated in connection with the topside sounding of the ionosphere. It is shown that in this case the propagation is possible only for forward waves whose wave vectors and group velocity form an acute angle. The possibility of appearance of loop-type trajectories is analyzed under the condition where the plasma frequency is greater than the electron gyrofrequency. The criteria for the existence of different types of trajectories are obtained as functions of the plasma parameters and the wave characteristics. The observation of resonance effects during topside sounding of the ionosphere led to a large number of theoretical studies on their interpretations (for example, see [1-8]). A number of studies had the objective of explaining the large durations of the signal observed at resonance frequencies Wp, u h, u H, and nu H (cop is the Langmuir frequency for the electrons, coh is the upper hybrid frequency, UH is the electron gyrofrequency, and n = 2, 3,...). The interpretation of these durations with the use of refraction of the packets of electrostatic waves in the homogeneous plasma has found extensive use. This interpretation was first proposed by McAfee in [2], where the propagation of plasma waves at u ~ Up was considered. In [2] the possibility of the group trajectories returning in the vicinity of the radiator after propagation in the in_homogeneous ionospheric plasma was analyzed for the case Up > w H. The computations [2] show that two types of group trajectories exist which can intersect the level of the radiator simultaneously. One of the trajectories has the form of a loop and, according to the authors, corresponds to the backward wave; the second trajectory has the usual form and corresponds to the direct wave. In another work of McAfee [3], the refraction of plasma waves is investigated for the case Up WH and Wp w H and in the case COp ~o H is confirmed. The criteria for the appearance of such trajectories are expressed in analytical form as functions of the plasma parameters and the wave characteristics.