Coupling of the directional and isotropic modes in the Mercury's magnetosphere

Details Coupling of the directional and isotropic modes in the Mercury's magnetosphere Coupling of the directional and isotropic modes in the Mercury's magnetosphere

Apr 2003

adsabs.harvard.edu/cgi-bin/nph-data_query?bibcode=2003eaeja.......84m&link_type=abstract

EGS - AGU - EUG Joint Assembly, Abstracts from the meeting held in Nice, France, 6 - 11 April 2003, abstract #84

Computer Science

Sound

Scientific paper

We investigate ULF waves in the Mercury's magnetosphere. Within the MHD approximation we obtain a system of partial differential equations, describing the coupled directional and isotropic modes (analogs of the Alfvén and fast magnetosonic modes in the Earth's magnetosphere). The derivation of these equations took into account the plasma inhomogeneity and multicomponent composition, and the curvature of magnetic field lines. Unlike the terrestrial magnetosphere, the directional anisotropic modes in the Mercury's magnetosphere remain coupled even at azimuthal wave numbers m=0. In the terrestrial magnetosphere, this coupling is important only for Pc1 pulsations, i.e. for running wave packets along field lines, whereas standing waves (Pc3-5) have too low frequencies in order for the coupling to have a marked influence on them. In the Mercury's magnetosphere, the ratio of the wave frequency to the proton gyrofrequency is only slightly less than unity even for low standing harmonic numbers along field lines of the oscillations. Furthermore, a considerable input of sodium ions, for which the gyrofrequency can be even lower than the wave frequency, renders the coupling between the directional and isotropic modes, perhaps, more important than does the coupling caused by plasma inhomogeneity. The coupling between the directional and isotropic modes is also not small at large m as well. This is in marked contrast to the situation in the Earth's magnetosphere for which it was established that at low frequencies the region of localization of magnetosound lies at the peripheral part of the magnetosphere, and, with increasing m, it is increasingly more strongly pressed against the magnetopause; as a consequence, when m>>1 the magnetosonic wave exerts a negligibly small influence on the Alfvén mode. In addition, in the case of the field-line resonance in the Earth's magnetosphere the back influence of the Alfvén mode on magnetosound is negligibly small. For the Mercury's magnetosphere, one would expect this influence to be far more significant, and this gives rise to relatively large values of the longitudinal components of the wave magnetic field in the region of localization of the directional mode. This is confirmed by visual inspection of plots of the ULF wave as observed by Mariner-10.

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