A predicted energetic ion signature of a neutral line in the geomagnetic tail

Under certain assumptions one can predict the formation of a magnetic neutral line on theoretical grounds, generally as a result of an unstable current sheet. Neutral points also seem to form spontaneously in a wide variety of plasma simulations, from MHD to fully kinetic. Hence such magnetic null regions are central to some models of magnetotail energization and magnetic substorms. Yet the observational evidence for such regions remains controversial. In this paper we present a new signature of a neutral line: the field‐aligned ion beam characteristic of current sheet acceleration becomes dispersed into a “ridge” in the velocity space distribution function as a neutral line is approached. The results were obtained using single‐particle simulations in a fixed magnetic field model. The origin of the ridge is shown to be due to initially earthward going ions which pass through the neutral line region. These ions come from a smaller part of the initially antiearthward flowing distribution, generating the depletion of the distribution function below the ridge.