Alfven velocity sudden increase as an indicator of the plasmapause

A radial distribution of ion mass density in the Earth's magnetosphere may be different from an electron one when local narrow enhancement of heavy ions flux is presented, e.g. oxygen torus. This difference may be important for ultra-low-frequency (ULF) waves research that includes Alfven waves whose eigenfrequency is dependent on Alfven velocity and thus on mass density. One of consequences is a shift of the plasmapause location that is thought as a resonator and generation area for ULF waves. In the present study we found an Alfven velocity increase by a factor of 1.65 or more within a radial distance of 0.5 RE to be a threshold that corresponds to the classic plasmapause definition that is the electron density drop by a factor of 5 or more within a radial distance of 0.5 RE. This result was obtained under all-proton plasma assumption to be confident that the Alfven velocity may be used to detect the plasmapause with the same accuracy as by electron density. We wish to use this threshold to reveal the plasmapause location when heavier ions will be included and direct comparison with electron density is not possible.