Abstract
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.
Highlights
In studies of magnetospheric MHD oscillations, one of the most productive ideas was the theory of field line resonance
The interaction between these modes is confined in a narrow range of magnetic shells, where a local frequency of Alfven eigenoscillations is close to fast magnetosonic (FMS) wave frequency
We introduce a curvilinear orthogonal coordinate system (x1, x2, x3) of the magnetic field lines, in which the coordinate x3 is oriented along the field line, x1 is directed across the magnetic shells, and the azimuthal x2 coordinate completes the righthand coordinate system
Summary
In studies of magnetospheric MHD oscillations, one of the most productive ideas was the theory of field line resonance. Having appeared for the first time in Tamao (1965), this theory was later developed by Chen and Hasegawa (1974), Radoski (1974), and Southwood (1974). This theory describes resonant driving of Alfven field line oscillations by a monochromatic fast magnetosonic (FMS, or compressional) wave. The field line resonance theory was first developed for a one-dimensional inhomogeneous model of the magnetosphere, where the magnetic field lines are assumed to be straight, and plasma inhomogeneity is directed across the field lines. This theory was later advanced for magnetosphere models with plasma inhomogeneous, both along magnetic field lines and across magnetic shells with either straight magnetic field lines (Kivelson and Southwood, 1986; Southwood and Kivelson, 1986) or a dipole-like magnetic field (Lifshitz and Fedorov, 1986; Leonovich and Mazur 1989; Chen and Cowley, 1989; Wright, 1992)
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