Resonance nature of the magnetosphere

A new approach toward an understanding of the nature of low frequency e.m. waves ( f⪡,<fB) ( fB is the ion gyrofrequency) observed in the magnetosphere is given in this essay. It is based on detailed studies of Fourier spectra of magnetic field records obtained at 14 observation points, with L=2.6–13, around the world: in Antarctica, Canada, Italy, Iceland, Russia, and the USA in the frequency band f≈1–250 mHz. It overlaps the frequency band of so-called Pc2–Pc5 micropulsations. Repetitive and non-repetitive single Pc1 micropulsations at f≈0.4–2.2 Hz are also briefly discussed here. Under different conditions they should be created, respectively, by interaction of a non-linear and/or only linear gyro-resonance instability mechanism. The oscillations considered exist at any time both in quiet (QC) and disturbed (DC) conditions. Their spectra possess a line structure. The background is composed of resonance oscillations, and weaker oscillations caused by noise. The resonance oscillations may occur in the entire magnetosphere or in its parts. In the frequency range examined 12, 20–25, and more resonance frequencies fs,res were accordingly found in the spectra of the NP and SP observation points (L≃13) and in the Tuckerton and Point Arena (L≃2.6) data. The higher-order resonance maxima are overlapped by the noise oscillations. All these oscillations can be set swinging, producing strong non-repetitive wave packets with durations τ0≈20–300 s and longer. The conditions for producing the swinging background can be impulse/shock excitations of the magnetospheric plasma, a gyro-resonance instability, etc. Thus, the well-known multiple manifestations of hydromagnetic wave packets observed in our research in the magnetosphere at f<0.25 Hz are considered to be the result of a single physical phenomenon: the fundamental resonance e.m. oscillatory nature of the background magnetospheric plasma environment. It is shown that the observed waves and the spectral resonance oscillations are temporally modulated by extra-low-frequency oscillations fmod≃1.8–2.0 mHz<and⪡fs,res. We believe that these oscillations are of Solar origin and were transferred to the Earth by the Solar wind. By analyzing calculated Fourier spectra of different kinds of wave packets – signals F(t) – we have found that any continuum of spectral maxima fs,max of these F(t) is characterized by a quasi-constant quantity – the spectral shape characteristic. It estimates the general image of the envelope of the signals and their duration.