Cusp ion structures and their relation to reconnection: Results from OpenGGCM simulation

Dispersed ion structures observed near the magnetosphere cusps have long been used to infer the location and properties of reconnection at the Earth's magnetopause. However, observations are often difficult to interpret because the spacecraft moves relative to the cusp ion structures, which creates a temporal/spatial ambiguity, and because one needs to employ empirical models to infer reconnection sites. In this study, we model the cusp ion structure with the OpenGGCM global MHD model and a particle tracing code to understand the relation between reconnection and a cusp ion structure. The OpenGGCM solves the resistive MHD equations and the particle tracing code integrates full Lorentz equation and equation of motion by using the 4th order Runge-Kutta method with electromagnetic fields obtained from the OpenGGCM. To model the cusp ion structure, we trace test-particles backward in time from satellite orbits to the magnetosheath, where the velocity distribution is known. By the Liouville's theorem, we calculate the phase-space densities of the test-particles, based on their velocity information at the magnetosheath. The calculated phase-space densities are displayed in the energy-time spectrogram. We first reconstruct the cusp ion structures during several cusp crossings of Cluster and Polar satellites, and compare the modeled results with the satellite observations. Then we study the reconnection processes on the dayside magnetopause by using the OpenGGCM simulation and discuss the relation between the reconnection and the cusp ion structures.