Observational Study of Ion Diffusion Region tailward of the Cusp: Polar and Cluster Observations in 1998-2008

Abstract:

Asymmetries in plasma density and the presence of a guide field significantly alter the structure of the ion diffusion region (IDR) in symmetric, collisionless reconnection. These features have been shown by numerical simulations under moderate density asymmetries (~10), and theoretical analyses. However, very few studies have addressed these issues with in-situ observations. We have compiled a collection of Cluster and Polar crossings of the high-latitude magnetopause poleward of the cusp under northward interplanetary magnetic field in the years 1998-2008 when signatures of reconnection inside the IDR are observed. They encompass a wide range of density asymmetries (~10 to 1000), magnetic field asymmetries (~0.2 to 0.9), and guide fields (~10 to ~60 %). In this dedicated observational study, we target the following topics: (1) The alteration of the structure of the IDR -- i.e., its width, the non-colocation of stagnation and X-lines, jet outflow speed, and biasing of the reconnection outflow jet toward the magnetosphere -- as a function of increasing density asymmetry, and (2) the diamagnetic drift of the X-line. Further, focusing on IDR crossings during plasma flow reversals and/or near-simultaneous crossings on either side of the X-line by two spacecraft under steady ambient conditions, we report on the contrast in the Hall fields and the plasma behavior on the sunward versus the tailward sides of the X-line in its dependence on the strength of the guide field.