National Aeronautics and Space Administration

Science

How do small-scale processes control large-scale phenomenology, such as magnetotail dynamics, plasma entry into the magnetosphere, and substorm initiation? Similar processes and questions arise in many other astrophysical plasma systems that are far beyond the reach of spacecraft.

Open Questions

• What are the processes that permit and control the reconnection of magnetic field lines across collisionless plasma boundaries?
• How do energy conversion processes accelerate particles at these boundaries, and what role do parallel electric fields play?
• How are electric currents, which connect distant regions of the magnetosphere, generated, controlled, and disrupted at boundaries?
• What is the importance and character of the coupling across scales (micro-to mesoscales) in all of these processes?

Mission Capabilities

• MMS will determine the small-scale basic plasma processes which transport, accelerate and energize plasmas in thin boundary and current layers and which control the structure and dynamics of the Earth’s magnetosphere.
• MMS will for the first time measure the 3D structure and dynamics of the key magnetospheric boundary regions, from the subsolar magnetopause to the distant tail.
• MMS will pave the way for future Constellation-type missions.

Mission Summary

• Broad regions of the magnetosphere are connected by fundamental processes operating in thin boundary layers. Processes of vastly different scale sizes can interact strongly.
• Understanding these fundamental processes requires multipoint measurements that uniquely separate temporal and three-dimensional spatial variations. Magnetospheric Multiscale gives us this essential capability.

Defining Terms

The MMS Mission exists to measure the speed and variability of an important process known as magnetic reconnection, and to relate them to boundary conditions and internal conditions within the electron diffusion region. The electron diffusion region (EDR) is small and fast moving, so MMS is specifically designed to maximize the time or spatial resolution available during its passages through the EDR. The EDR is three dimensional and must therefore be studied with a tetrahedral array of four identical spacecraft.

Magnetic fields serve as a “connective tissue” that binds plasmas together into cohesive cells sharing the same magnetic field lines. When different parts of a magnetized plasma cell move relative to each other, the magnetic field within it fights back and energy is stored in the stretched and deformed magnetic field. This energy is released when the plasma cell is divided by reconnection of the magnetic fields, disconnecting the magnetic linkage between the two regions in relative motion, and creating two distinct cells that are no longer linked, allowing the relative motion to proceed. In the reverse of that process, plasma cells that come into contact with each other may under the right conditions experience reconnection of their embedded magnetic fields such as to connect the cells together by common magnetic field lines.

Reconnection is the general term for magnetic field disconnection or connection, either of which may release energy stored in the magnetic fields. The rate of reconnection is measured by the amount of magnetic flux converted from closed to open for disconnection, or vice versa for connection. Large variations of the rate are observed or inferred, but without detailed observations of the EDR, it has been impossible to predict this rate or its variations. MMS will provide the needed observations for the first time.

+ MMS SMART Science Website