Abstract.A theory of resonant conversion of fast magnetosonic (FMS) waves into slow magnetosonic (SMS) oscillations in a magnetosphere with dipole-like magnetic field has been constructed. Monochromatic FMS waves are shown to drive standing (along magnetic field lines) SMS oscillations, narrowly localized across magnetic shells. The longitudinal and transverse structures, as well as spectrum of resonant SMS waves are determined. Frequencies of fundamental harmonics of standing SMS waves lie in the range of 0.1-1 mHz, and are about two orders of magnitude lower than frequencies of similar Alfvén field line resonance harmonics. This difference makes an effective interaction between these MHD modes impossible. The amplitude of SMS oscillations rapidly decreases along the field lines from the magnetospheric equator towards the ionosphere. In this context, magnetospheric SMS oscillations cannot be observed on the ground, and the ionosphere does not play any role either in their generation or dissipation. The theory developed can be used to interpret the occurrence of compressional Pc5 waves in a quiet magnetosphere with a weak ring current.
A problem of coupling between fast and slow magnetosonic waves in Earth's magnetosphere (magnetosonic resonance) is examined. Propagation both slow magnetosonic wave and Alfven wave can easily be canalized along the magnetic field line direction. The main difference between the two is that slow magnetosonic waves dissipate strongly due to their interaction with the background plasma ions, whose temperature is above the electron temperature. In Earth's magnetosphere, however, there is a region where the dissipation of slow magnetosonic waves can be weak-the inner plasmasphere. The slow magnetosonic waves generated there can be registered directly. In other regions, with strong dissipation of slow magnetosonic waves, their signature may be detected through their impact on the Alfven resonance at frequencies for which the resonant Alfven and slow magnetosonic waves exist simultaneously in the magnetosphere. Owing to their strong coupling with the background plasma ions, resonant slow magnetosonic waves can transfer the energy and impulse from the solar wind to the magnetospheric plasma ions via fast magnetosonic waves penetrating into the tail lobes. A problem of resonant conversion of fast magnetosonic waves into slow magnetosonic oscillations in a magnetosphere with dipole-like magnetic field is also examined.
The boundary stability problem is solved for a dipole magnetosphere wrapped by an azimuthal solar wind flow. A simplified equation is obtained describing small‐scale fast magnetosonic waves in a dipole magnetosphere. The structure of surface fast magnetosonic waves along magnetic field lines is represented as an expansion into a set of standing (between the magnetoconjugate ionospheres) waves. Over the azimuthal coordinate, along which the plasma is homogeneous, the wave field is decomposed into a set of azimuthal harmonics. It is shown that, in solar wind flows with velocities actually observable near the Earth's magnetosphere, only surface waves corresponding to several first harmonics of these expansions are unstable. The frequency ranges of unstable oscillations do not overlap for harmonics with different wave numbers. As the plasma flow velocity increases, harmonics with higher and higher wave numbers become unstable. The characteristic frequencies of unstable surface waves cover the lowest‐frequency, Pc5–Pc6 range of geomagnetic pulsations observed in the magnetosphere. The frequency spectrum of these oscillations corresponds to geomagnetic pulsations at discrete “magic frequencies” occasionally observed in the Earth's magnetosphere.
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