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

It has been known that the Alfvén velocity plays a significant role in generation and propagation of magnetohydrodynamic (MHD) waves. Until now, however, the global distribution of the Alfvén velocity in the magnetosphere has not been reported. To determine the spatial distribution of the Alfvén 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 of ~ 4–12 and at all local times near the magnetic equator between − 5° and 5° in magnetic latitude for 2008–2014. We observed a pronounced dawn–dusk asymmetry in the equatorial Alfvén velocity calculated from the THEMIS magnetic field and density data. That is, the dawnside Alfvén velocity is higher than the duskside Alfvén velocity. This asymmetry is due to the duskside bulge in the plasmasphere. The radial profile of the Alfvén 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 Alfvén velocity distributions along the local time and radial distance, we discuss the occurrence distribution and propagation of MHD waves in the outer magnetosphere.