Magnetospheric Multiscale (MMS), a NASA four-spacecraft constellation mission launched on March 12, 2015, will investigate magnetic reconnection in the boundary regions of the Earth's magnetosphere, particularly along its dayside boundary with the solar wind and the neutral sheet in the magnetic tail. The most important goal of MMS is to conduct a definitive experiment to determine what causes magnetic field lines to reconnect in a collisionless plasma. The significance of the MMS results will extend far beyond the Earth's magnetosphere because reconnection is known to occur in interplanetary space and in the solar corona where it is responsible for solar flares and the disconnection events known as coronal mass ejections. Active research is also being conducted on reconnection in the laboratory and specifically in magnetic-confinement fusion devices in which it is a limiting factor in achieving and maintaining electron temperatures high enough to initiate fusion. Finally, reconnection is proposed as the cause of numerous phenomena throughout the universe such as comet-tail disconnection events, magnetar flares, supernova ejections, and dynamics of neutron-star accretion disks. The MMS mission design is focused on answering specific questions about reconnection at the Earth's magnetosphere. The prime focus of the mission is on determining the kinetic processes occurring in the electron diffusion region that are responsible for reconnection and that determine how it is initiated; but the mission will also place that physics into the context of the broad spectrum of physical processes associated with reconnection. Connections to other disciplines such as solar physics, astrophysics, and laboratory plasma physics are expected to be made through theory and modeling as informed by the MMS results.

The Earth's radiation belts--also known as the Van Allen belts--contain high-energy electrons trapped on magnetic field lines. The centre of the outer belt is usually 20,000-25,000 km from Earth. The region between the belts is normally devoid of particles, and is accordingly favoured as a location for spacecraft operation because of the benign environment. Here we report that the outer Van Allen belt was compressed dramatically by a solar storm known as the 'Hallowe'en storm' of 2003. From 1 to 10 November, the outer belt had its centre only approximately 10,000 km from Earth's equatorial surface, and the plasmasphere was similarly displaced inwards. The region between the belts became the location of high particle radiation intensity. This remarkable deformation of the entire magnetosphere implies surprisingly powerful acceleration and loss processes deep within the magnetosphere.

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The Magnetospheric Multiscale (MMS) mission and operations are designed to provide the maximum reconnection science. The mission phases are chosen to investigate reconnection at the dayside magnetopause and in the magnetotail. At the dayside, the MMS orbits are chosen to maximize encounters with the magnetopause in regions where the probability of encountering the reconnection diffusion region is high. In the magnetotail, the orbits are chosen to maximize encounters with the neutral sheet, where reconnection is known to occur episodically. Although this targeting is limited by engineering constraints such as total available fuel, high science return orbits exist for launch dates over most of the year. The tetrahedral spacecraft formation has variable spacing to determine the optimum separations for the reconnection regions at the magnetopause and in the magnetotail. In the specific science regions of interest, the spacecraft are operated in a fast survey mode with continuous acquisition of burst mode data. Later, burst mode triggers and a ground-based scientist in the loop are used to determine the highest quality data to downlink for analysis. This operations scheme maximizes the science return for the mission.

[1] We present an observational study of the global dynamics of the plasmasphere, aurora, ring current, and subauroral ionosphere on 17 April 2002 during a substorm. Global observations by the Imager for Magnetopause-to-Aurora Global Exploration (IMAGE) and in situ observations by DMSP F13 provide a comprehensive view of the coupled response of the inner magnetosphere and ionosphere. At 1900 UT a substorm onset initiated a sunward convective impulse, which caused a ring current injection. The motion of this impulse past the plasmasphere caused ripples to propagate along the plasmapause, eastward and westward from premidnight magnetic local time (MLT). The motion of the ripples agrees exceptionally well with the motion of the aurora and the ring current, implying strong coupling. The westward moving ripple (on the duskside) participated in a two-phase plasmapause undulation effect. In the first phase (1915 UT to 1936 UT), a mild 0.4-0.5 R E bulge formed near 2000 MLT, probably caused by an E-field induced by local reduction of the magnetic field by the ring current pressure increase. In the second phase (1936 UT to 2037 UT) this mild bulge was removed by a subauroral polarization stream (SAPS) westward flow that stripped away the outer 1 R E of the duskside plasmasphere. The SAPS effect was observed in the ionosphere by DMSP between about 1930 UT and 2000 UT and is evident in vector E-fields inferred from plasmapause motion. All the observations of this event suggest strong coupling among the plasma populations of the magnetosphere-ionosphere system. This event represents the first identification of the directly observed global plasmaspheric effects of a substorm-driven impulse, the SAPS flow channel, and the ring-current magnetic field reduction.