Cross-modulation of whistler waves in a magnetized plasma

Abstract

Waves of the very low and extremely low frequency (VLF and ELF) ranges propagating in the near Earth plasma mainly in the whistler mode are of significant fundamental interest [1], are important for the organi zation and support of very long distance radio commu nication [2], and provide a powerful tool for the diag nostics of the ionosphere and magnetosphere [3]. Wave action on the ionosphere and magnetosphere in the VLF-ELF range is one of the urgent problems of modern plasma geophysics [4]. For the generation of waves with the intensity sufficient for their reliable detection after passage through long ionospheric and magnetospheric paths, ground based stations with powers up to several megawatts are used; the operation of these stations is inevitably accompanied by the development of nonlinear phenomena in the plasma, including the heating and redistribution of the iono spheric plasma [5][6][7], as well as the appearance of fluxes of accelerated charged particles [8,9]. As a result, in the case of the propagation of low frequency waves through common paths, nonlinear effects can result in the mutual enrichment of the frequency spec trum of these waves, in particular, cross modulation, i.e., the modulation of one wave by the law of change of the envelope of another wave [10].Mechanisms of the nonlinear interaction between waves that are responsible for such phenomena are often controversial [11]. Laboratory simulation of nonlinear plasma wave phenomena, as well as theo retical analysis and numerical experiments, is promis ing for the interpretation of data on the interaction of low frequency waves in the near Earth plasma. The advantage of this simulation over a natural experiment is the control of the parameters of the plasma and radi ation, as well as the possibility of multiple reproduc tion of the processes under study with purposeful vari ation of the conditions under which they occur. These experiments can be performed on a limited number of worldwide laboratory facilities which allow the forma tion of a highly uniform column of a low temperature magnetized plasma with dimensions much larger than the wavelength of radiation. The phenomenon of cross modulation of low frequency waves in the mag netized plasma is detected and studied for the first time under laboratory conditions close in similarity parameters to those in the Earth's ionosphere.The experiments were performed on the Krot plasma device in a column of the decaying plasma of an inductive discharge with a length of about 4 m and a diameter of 1.5 m at the electron density n e = 10 10 -10 12 cm -3 , electron temperature T e = 0.5-1 eV, and ambient magnetic field B 0 ~ 100 G (electron cyclotron frequency f ce = 300 MHz). The scaling parameters of the laboratory experiment-plasma density n e and ambient magnetic field strength B 0 -were similar to values in the outer ionosphere and magnetosphere of the Earth with the scaling factor γ = 300-3000 [12]. The collision frequency, which is a nonscaling param eter, is much smaller than the radiation frequ...