Magnetic reconnection is a universal phenomenon enabling large scale transfer of magnetic energy to plasma kinetic energy and affecting its macroscopic transport by changing its magnetic connectivity. Among many systems in the universe, the Earth magnetopause, being close and “easily” targeted by spacecraft missions, is a fantastic laboratory to study the reconnection process in great details, besides being, by itself, an important space weather actor, as reconnection there critically couples the solar wind to our magnetosphere, leading to geomagnetic activity. The magnetopause is a three-dimensional collisionless asymmetric magnetic boundary separating the solar wind from the magnetospheric plasma, and through which the magnetic field is sheared between the interplanetary magnetic field (IMF) and the Earth dipole. Although we know magnetopause reconnection is overall greatly influenced by the IMF orientation, we do not understand how the magnetic shear affects the microphysics resulting in this large scale perspective. Besides the asymmetrical magnetopause configuration distinguishes it from the majority of reconnection models, mostly focused on magnetotail-like, symmetric current sheets, which guide our intuition although, strictly speaking, are quite singular. This three year project proposes to tackle the crucial issue of the impact of mesoscale environmental properties on collisionless magnetic reconnection, focusing on the impact of i) varying the magnetic shear angle in a fixed, asymmetric, reconnecting current sheet ii) the impact of a slowly varying degree of asymmetry of the current sheet on the reconnection process with a fixed magnetic shear and iii) the three-dimensional aspect of the problem. Such a systematic survey will lead to much clearer results than the simulation of a unique and arbitrary shear angle and high degree of asymmetry. We will use state-of-the-art 2D fully and hybrid kinetic simulations, later confronted to 3D kinetic and fluid simulations, and always in a close relationship with multi-mission space observations, at the magnetopause and in the solar wind, making an heavy use of innovative tools developed at the Institute for Research in Astrophysics and Planetology (IRAP), improving them and developing new ones. In october 2014, NASA will launch the major mission Magnetospheric MultiScale (MMS) to study the reconnection microphysics down to electron scales, and will explore the dayside magnetopause during its first phase. Our ambitious objectives are highly relevant to the MMS science priorities and very competitive. They will be reached by the gathering of the complementary strengths of the French space plasma community in theory, numerical modeling and space observations, together with the world leading experts in reconnection physics and its kinetic modeling, in a new and strong international collaboration, on a cross-disciplinary topic that is widely recognized as one of the most important and challenging one in experimental, spatial and astrophysical plasma communities. The project will be mainly performed at IRAP, but will also involve researchers from the Laboratory of Plasma Physics (LPP) and NASA Goddard Space Flight Center (GSFC). If the latter, building the MMS four satellites is obviously deeply engaged in the mission, IRAP and LPP are also strongly involved, as they both contribute to the spacecraft instrumentation. This project is at the heart of the IRAP Geophysical and Space Plasma group’s scientific activities, to which it will bring, as well as to the French astrophysical plasma community as a whole, a highly desirable expertise on numerical modeling of collisionless magnetic reconnection. It will be decomposed in five tasks, among which the coordination one and four scientific tasks based on a very strong theory/observation/simulation synergy and designed to deliver key results on reconnection physics, original databases, codes and innovative observational tools for the community.