The Magnetospheric Multiscale (MMS) mission is a Solar Terrestrial Probes Program mission within NASA’s Heliophysics Division. The MMS mission, consisting of four identically instrumented spacecraft, will use Earth's magnetosphere as a laboratory to study magnetic reconnection, a fundamental plasma-physical process that taps the energy stored in a magnetic field and converts it—typically explosivly—into heat and kinetic energy in the form of charged particle acceleration and large-scale flows of matter.
Magnetic reconnection occurs universally in plasmas, the electrically conducting mixes of positively and negatively charged particles that account for an estimated 99% of the observable universe. It is the ultimate driver of the phenomena we know as “space weather.” Eruptive solar flares, coronal mass ejections (CMEs), geomagnetic storms, and magnetospheric substorms all involve the release through reconnection of energy stored in magnetic fields.
In addition to its central role in solar-terrestrial relations, magnetic reconnection has been invoked in theoretical models of a variety of astrophysical phenomena including star-accretion disk interactions, pulsar wind acceleration, and the acceleration of ultra-high-energy cosmic rays in active galactic nuclei jets. Reconnection also occurs in man-made settings such as fusion machines (tokamaks spheromaks) and laboratory reconnection experiments.
The four MMS spacecraft will carry identical suites of plasma analyzers, energetic particle detectors, magnetometers, and electric field instruments as well as a device to prevent spacecraft changing from interfering with the highly sensitive measurements required in and around the diffusion regions. The plasma and fields instruments will measure the ion and electron distributions and the electric and magnetic fields with unprecedented high (millisecond) time resolution and accuracy. These measurements will enable MMS to locate and identify the small (10's of km) and rapidly moving (10-100 km/s) diffusion regions, to determine their size and structure, and to discover the mechanism(s) by which the plasma and the magnetic field become decoupled and the magnetic field is reconfigured. MMS will make the first unambiguous measurements of plasma composition at reconnection sites, while energetic particle detectors will remotely sense the regions where reconnection occurs and determine how reconnection processes produce large numbers of energetic particles.
The four satellites will be launched together on a single launch vehicle and inserted sequentially into Earth orbit. As they explore the dayside and nightside reconnection regions, the spacecraft will fly in a tetrehedral (pyramid) formation, allowing them to capture the three-dimentional structure of the reconnection sites they encounter. Onboard propulsion will be used to adjust the separation among the spacecraft, from hundreds of kilometers to as close as 10 kilometers to achieve the optimum interspacecraft separation for probing the diffusion region.
Much of what we know about the physics of magnetic reconnection comes from theoretical studies and computer models. True understanding requires that our theories and models be placed on the secure foundation of in situ observation. The reconnection mechanism cannot be studied in situ on the Sun or in remote astrophysical systems; nor can it be effectively studied in laboratory experiments where the scale sizes on which the critical processes operate are too small to be resolved. However, Earth’s magnetosphere, whose structure and dynamics are controlled by reconnection, is accessible to regular in situ measurement and provides the ideal natural laboratory in which to investigate magnetic reconnection as well as other plasma processes that occur throughout the cosmos.
Magnetic reconnection occurs in two main regions of the magnetosphere (red boxes); (1) the dayside magnetopause and (2) the magnetotail. MMS will employ a two-phase orbit strategy to explore each of these regions in turn. In Phase 1, MMS will probe reconnection sites at the dayside magnetopause. Here the interplanetary magnetic field (IMF), when oriented southward, merges with the geomagnetic field, transferring mass, momentum, and energy to the magnetosphere. The solar wind flow transports the merged IMF/geomagnetic field lines toward the nightside, causing a build up of magnetic flux in the magnetotail. In Phase 2, MMS will investigate reconnection sites in the magnetotail, where reconnection releases the stored magnetic energy in explosive events known as magnetospheric substorms and allows the magnetic flux stripped away from the dayside magnetopause by the solar wind/magetosphere interaction to return to the dayside.
Cosmic plasmas are threaded throughout with magnetic lines of force. The field lines and the plasma are tied to one another and move together in the flow of the plasma. If magnetic fields in adjacent regions have opposite or significantly different orientations, the field lines and plasma can become decoupled, with the field lines “breaking” and then “reconnecting” with those in the adjacent region. When this happens, the energy stored in the magnetic fields is released as kinetic energy and heat. The breaking and reconnection of the magnetic field lines takes place in a narrow boundary layer called the diffusion region.
The overarching goal of the MMS mission is to measure the plasma and the electric and magnetic fields inside the diffusion regions in Earth’s magnetosphere in order to answer the following fundamental questions: