EARTH’S DYNAMIC MAGNETIC SHIELD
The solar wind does not impact Earth directly but is deflected—like a stream diverted around a boulder—by our planet’s magnetic field, which forms a tear-drop-shaped cavity in the solar wind flow. This cavity—the magnetosphere—contains highly dilute plasmas whose behavior is controlled by Earth’s magnetic field and its interaction with the solar wind. Its size depends on the velocity and density of the solar wind and on the IMF. The magnetosphere typically extends to around 10 times the radius of Earth in the sunward or “upstream” direction, and hundreds of Earth radii in the direction away from the Sun, or “downstream.” (1 Earth radius = 6,378 kilometers.)
How the magnetosphere responds to the continuous buffeting by the solar wind depends strongly on the direction and strength of the IMF. Put simply, like poles repel and opposites attract. When the magnetic orientation of the IMF is opposite that of Earth’s magnetic field on the sunward face of the magnetosphere, the two magnetic fields slam together and can cancel each other out. This annihilation process is called magnetic reconnection. Reconnection is the primary process by which Earth’s magnetic shield is breached and energy transferred from the solar wind to the magnetosphere. This transfer of energy in turn drives the flow of plasma within the magnetosphere and leads to the buildup of magnetic energy in the magnetotail—the downstream portion of the magnetosphere, which looks a little like the tail of a comet. The energy stored in the tail is then explosively released in events known as magnetospheric substorms. During substorms, the magnetic field lines in the magnetotail behave like elastic bands that are stretched and then abruptly released, snapping back to their original configuration. The reconnection of stretched and stressed magnetic field lines in the tail is an important element of this process. Some of the energy released in substorms drives powerful electrical currents that inject several billion watts of power into the upper atmosphere and produce spectacular displays of the aurora borealis and aurora australis—the northern and southern lights.
While the importance of reconnection is widely recognized and its effects well-known, scientists still do not fully understand how oppositely directed magnetic field lines cancel themselves out at reconnection sites. Achieving this understanding requires in situ measurement of charged particles, and of electric and magnetic fields at the reconnection site. Such measurements are challenging, owing to the small size of the sites, and must be made simultaneously from multiple vantage points. The exploration of these localized regions surrounding reconnection sites is one of the principal objectives of the Magnetospheric Multiscale (MMS) mission, a four-spacecraft Solar Terrestrial Probe designed to study microphysical processes in key boundary regions of Earth’s magnetosphere, from the subsolar magnetopause to the high-latitude magnetopause, and from the near-Earth magnetotail to its most remote regions. In addition to reconnection, MMS will study the roles of particle acceleration and turbulence in the transfer of energy within and across magnetospheric boundaries. MMS is the Survey Committee’s highest-priority initiative in the moderate-cost category.
The circulation of energy through plasmas occurs throughout the universe, in solar flares, in planetary magnetospheres, and in protostars, as well as in Earth’s magnetosphere. In Earth’s magnetosphere, researchers can measure the flow of energy directly, but the system is so vast and dynamic that localized measurements from individual satellites are of little use in capturing the overall dynamic behavior and structure of the magnetosphere. The Magnetospheric Constellation (MagCon) mission will solve this problem by deploying dozens of small, autonomous microsatellites to perform continuous measurements of the magnetospheric plasma and magnetic fields. These measurements will provide the “pixels” from which the “big picture” of the entire magnetosphere can be composed. Computer modeling and theory will play a critical role in integrating the data pixels into a coherent depiction of magnetospheric dynamics.