Distribution of equatorial Alfven velocity in the magnetosphere: a statistical analysis of THEMIS observations

It has been known that the Alfven velocity plays a significant role in generation and propagation of magnetohydrodynamic (MHD) waves. Until now, however, the global distribution of the Alfven velocity in the magnetosphere has not been reported. To determine the spatial distribution of the Alfven velocity, we have statistically examined the THEMIS magnetic field and electron density data obtained in the L (the equatorial geocentric distance to the field line measured in Earth's radii) range of4-12 and at all local times near the magnetic equator between -5 degrees and 5 degrees in magnetic latitude for 2008-2014. We observed a pronounced dawn-dusk asymmetry in the equatorial Alfven velocity calculated from the THEMIS magnetic field and density data. That is, the dawnside Alfven velocity is higher than the duskside Alfven velocity. This asymmetry is due to the duskside bulge in the plasmasphere. The radial profile of the Alfven velocity shows an increasing function of L between L=4 and 10 in the dusk sector, while a decreasing function in the dawn sector. By comparing these Alfven velocity distributions along the local time and radial distance, we discuss the occurrence distribution and propagation of MHD waves in the outer magnetosphere.