The Earth's immediate space environment, its "magnetosphere," is formed by the interaction of the high-speed magnetized plasma flowing out from the Sun (the solar wind) with the Earth's magnetic field and its plasmas. The global structure and dynamics of this complex system are controlled by small- scale plasma processes operating in thin boundary regions that separate vast and very different plasma regimes around the Earth. How some of these processes work is known on a conceptual level but not quantitatively. For other important processes there is basic theoretical disagreement. The Magnetospheric Multiscale mission is designed to provide quantitative answers to how this control is exerted on global plasma regimes. These same questions and processes are common to many other astrophysical plasma systems, far beyond the direct reach of exploring spacecraft.
Magnetospheric Multiscale is a mission both of exploration and understanding. Its primary thrust is to study on the meso- and microscale the heretofore inaccessible basic plasma processes that transport, accelerate, and energize plasmas in thin boundary and current layers--the processes which control the structure and dynamics of the Earth's magnetosphere. Magnetospheric Multiscale will for the first time measure 3D fields and particle distributions and their temporal variations and 3D spatial gradients, with high resolution, while dwelling in the key magnetospheric boundary regions, from the subsolar magnetopause to the distant tail. It will uniquely separate spatial and temporal variations over scale lengths appropriate to the processes being studied--down to the kinetic regime beyond the approximations of MHD. From the measured gradients and curls of the fields and particle distributions, spatial variations in currents, densities, velocities, pressures, and heat fluxes can be calculated. Magnetic reconnection will be directly observed in each of the regimes where it is thought to occur around the Earth. This process is vital for transferring energy from magnetic fields to plasmas, and for coupling different regimes. It is central to many astrophysical theories, yet its operation in collisionless space plasmas is very poorly understood. Concurrently Magnetospheric Multiscale will obtain stereo images of large-scale regions of the magnetosphere as they respond to the microscale phenomena.
Magnetospheric Multiscale consists of four identical spacecraft, flying in a tetrahedral configuration with spacing variable from 1 km to several Earth radii. Each spacecraft will contain an identical set of 3D instruments with high angular and temporal resolution (plasma electron and ion composition, energetic electron and ion composition, magnetometer, electric fields and waves). Interspacecraft ranging and communication will determine spacing and allow simultaneous high rate data recording on all spacecraft for maximum resolution of boundaries or events. Imaging by neutral atom imagers on two companion spacecraft in elliptical polar orbits will provide a global context for the in-situ measurements. With sensitive instrumentation, variable spacecraft spacing, and global imaging, Magnetospheric Multiscale will integrate for the first time observations and understanding over all geomagnetic scale sizes, from boundary layer processes that operate at the smallest scale lengths to the resulting global dynamics that engulf the Earth.