Propagation of guided waves in moving media with application to the theory of small-scale electromagnetic waves in the solar wind plasma

We consider a specific case of Doppler Effect in moving homogeneous magnetized plasma. Medium velocity is constant but wave fronts vary in their direction because waves are guided by fluctuating magnetic field lines. As a result, an observer at rest sees frequency modulated emission. Such a mechanism might be implemented in the heliosphere. The quiet solar wind can be considered as nearly homogeneous moving plasma and the interplanetary magnetic field (IMF) is frozen in it. So if we have some electromagnetic waves with wave fronts orthogonal to the IMF lines we get a state described above. The theory presented in our work predicts the waves having this property really exist. They are self-excited due to small-scale electromagnetic ion cyclotron instability of interplanetary plasma. We investigated the corresponding dispersion equation and found its roots in the paraxial approximation. It was found that the wave amplification coefficient has a sharp maximum at propagation exactly along the external magnetic field, i.e., along the lines of IMF. IMF lines might be swayed by some lower frequency oscillations. We analyze a particular case of the mentioned modulation when the ion cyclotron waves excited in the solar wind experience a modulating exposure of Alfvén waves driven by photospheric motions with a characteristic period of five minutes. The lengths and periods of Alfvén waves are much greater than the lengths and periods of ion-cyclotron waves. The results show that a deep frequency modulation can be observed due to the considered effect. To test the theoretical predictions we have analyzed ground-based observations of the ULF geoelectromagnetic waves in order to find the quasi-monochromatic oscillations the carrier frequency of which varies with time within a wide range. The so-called “serpentine emission” (SE) turns to be the most suitable type of oscillations for our purposes. This emission is observed in the polar caps in the frequency band of 0.1-5 Hz. Clearly expressed 5-minute deep modulation of the carrier frequency is a particularly important feature of the SE. It is not excluded that SE demonstrates a non-trivial manifestation of the solar 5-min oscillations on the Earth.