Structure of electron and ion distribution functions near a reconnection line

We examine the transport of electrons and ions in the vicinity of an X line in the magnetotail, using single‐particle trajectory codes. It is shown that the particle distribution functions downstream of the reconnection region exhibit two distinct density domains separated by a narrow channel of enhanced densities. This narrow structure, which was first reported for ions by Martin and Speiser [1988] and referred to as a ridge, is due to particles which are essentially E × B convected from the lobes and experience large energy gains upon interaction with the neutral line. This structure weakly depends upon the characteristics of the inflowing population. As for the density domains on either side, they result from mixing of the particle populations during the reconnection process, high and low densities corresponding to particles originating earthward and tailward from the neutral line, respectively. The simulations also reveal fine low‐density structures embedded in the overall pattern. It is shown that these fine structures result from prominent gradient drift effects upon approach of the field reversal. It is demonstrated that, at a given observation point downstream of the reconnection region, the computed distribution functions strongly depend upon the azimuthal direction of view. An analytical estimate is proposed for the ridge location, which provides information on the neutral line characteristics.