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Coupling of Global Magnetospheric MHD Eigenmodes to Field Line Resonances



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JOURNAL OF GEOPHYSICAL RESEARCH VOL 91 NO A4 PAGES 4345 4351 APRIL 1 1986 Coupling of Global MagnetosphericMHD Eigenmodes to Field Line Resonances MARGARET G KIVELSON Instituteof Geophysics andPlanetaryPhysicsandDepartmentof Earth andSpaceSciences Universityof California Los An ieles DAVID J SOUTHWOOD Institute of Geophysicsand Planetary Physics University of California Los Angeles Department of Physics Imperial College of Scienceand Technology London United Kingdom Many features of the magnetospherichydromagnetic wave spectrum are fully understood but a remainingpuzzleis why selectedfield line resonances appearto be excitedby a broadbandsource In this paper we propose that the preferential resonancescorrespondto global eigenmodesof the magnetosphericcavity In order to understandthe behavior of ultralow frequencywavesin the terrestrial magnetosphere geomagneticpulsations we describethe structureof global eigenmodesin the hydromagneticbox model of an inhomogeneousplasma Global modes are large scalemodes whose frequenciesmatch field line resonancefrequenciessomewherein the system The couplingbetween localizedfield line resonanceeffectsand the large scalemode leadsto dampingof the latter In the box modelthe eigenmodeequationyieldsa spectrumof discreteeigenfrequencies For eacheigenfrequency the equation contains singular points where the field line resonanceoccurs We base much of our discussion on earlierworks whichhave usedthe sameequationin other contexts We describesolutions and then usethe resultsto outlinefeaturesof the modesthat may be observable 1 INTRODUCTION Magnetohydrodynamicwave phenomena in the terrestrial magnetospherecan be monitored by spacecraftinstruments ground magnetometerarrays ground radars Cummingset al 1969 Samson 1972 Walker et al 1982 and in other ways One can use the measurements as diagnostics of the magnetosphericsystem but also one may examine the physicsof the waves themselves The propagation anisotropiesof the wave modes and the



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