Diagnostics of the plasmasphere and plasmapause using ULF waves F. W. Menk (1) , M. A. Clilverd (2) , B. R. Sandel (3) , D. L. Carpenter (4) , Z. C. Dent (5) , I. R. Mann (5) (1) School of Mathematical and Physical Sciences and Cooperative Research Centre for Satellite Systems, University of Newcastle, Callaghan, NSW 2308 Australia; fred.menk@physics.org (2) British Antarctic Survey, High Cross, Cambridge, CB3 0ET, U.K.; MACL@bas.ac.uk (3) Lunar and Planetary Laboratory, University of Arizona, Tucson, AZ 85721, USA. (4) Space, Telecommunications and Radioscience Laboratory, Stanford University, Stanford, CA (5) Department of Physics, University of York, Heslington, York, YO10 5DD, U.K. Ultra-low frequency (ULF; f~1-100 mHz) plasma waves distribute energy of solar wind origin throughout the Earth's magnetosphere and down to the ground, where they are recorded as pulsations of the geomagnetic field. The waves propagate through the magnetosphere in the fast magnetosonic mode and may couple to shear transverse mode field line eigenoscillations. The frequency of these field line resonances (FLRs) is determined by the field geometry and the plasma mass density near the equatorial plane. The FLRs occur during the local daytime and can often be observed for several hours. Ground-based measurements of the resonant frequency can therefore provide information on the plasma mass density near the field line apex in the dayside magnetosphere. An array of latitudinally-separated ground magnetometers allows the plasma density profile to be monitored throughout the magnetosphere. This may be compared with ground-based VLF electron density measurements, and satellite-borne particle and imager observations. _______________ Presented at the 27th General Assembly of International Union of Radio Science (URSI), Maastricht, The Netherlands, August 17-24, 2002.