NASA’s Magnetospheric Multiscale (MMS) mission is a constellation of 4 spacecraft that packs several new, game-changing capabilities to unlock the secrets of magnetic reconnection at an unprecedented level of detail.
Magnetic reconnection occurs where the Sun and the Earth’s magnetic fields “connect” with each other from opposite directions, resulting in the two canceling each other and the explosive conversion of the magnetic energy stored in the two fields into kinetic energy.
It is challenging to hit the “bullseye” of magnetic reconnection – the electron diffusion region (EDR) around an X-line where the plasma is believed to become diffusive and the magnetic field lines have an X-shape topology. This is because the EDR is extremely tiny compared to the size of the Earth’s magnetosphere – think of the tip of the foul pole at Yankee Stadium compared to the Bronx.
Previous satellites, notably NASA’s ISEE (International Sun Earth Explorer) and fleet of THEMIS (Time History of Events and Macroscale Interactions during Substorms) spacecraft, and ESA’s fleet of Cluster spacecraft, were able to study the many large-scale consequences of reconnection. However, unlike MMS they lack ultra-fast plasma and field instruments with sufficient measurement speed and capabilities to probe the kinetic processes that cause reconnection in and around the EDR.
By flying the four spacecraft in a well-controlled tetrahedral formation with an inter-spacecraft distance down to 10 km, MMS was able to capture over 3000 reconnection events in the dayside magnetopause (magnetosphere boundary), each lasting only a few seconds, over a 6-month period!
A special issue titled, First results from NASA’s Magnetospheric Multiscale (MMS) Mission, published in Geophysical Research Letters features a Frontier Article by Jim Burch and Tai Phan and some 50+ research articles, many of which provide a first glimpse of the several new discoveries in these encounters, including fascinating new features of reconnection on the electron scale and confirmations of important predictions from computer models.
A persistent theme that emerges from this special collection is the “babushka dolls” nature of magnetic reconnection. As one zooms in on the observation data at increasing spatial and temporal resolution, one uncovers increasingly smaller and fascinating features and structures of reconnection. Burch and Phan (2016) illustrates many of these reconnection babushka dolls and is decidedly a must read.
The gyro-radius (radius of gyration) of an ion is much larger than that of an electron of the same energy, due to its larger mass. Consequently, the laws of magnetohydrodynamics (MHD) do not apply below the scale of the ion gyro-radius, as the ions and electrons decouple their motions from each other and form an ion-decoupling region (IDR), setting up large electrical currents and (“Hall”) magnetic fields.
Likewise, cold ions originating from the ionosphere have gyro-radius that is much smaller than that of hot magnetospheric ions, due to their smaller velocity. Such cold ionospheric ions can reach the dayside magnetopause, and remain magnetized and form a sub-region surrounding the EDR inside the IDR.
In other words, the electron diffusion region (EDR) is embedded within a multi-layered ion decoupling region (IDR). The former is akin to the inner sanctum for the smaller babushka dolls; the latter to the inner and outer courtyards for the larger ones. The two are akin to the home of a wide-ranging variety of “babushka dolls” (manifestations) of reconnection: from crescent-shaped electron velocity distribution, which signifies electron demagnetization, to high-speed electron flows, highly filamentary current sheets, large-amplitude electric fields, and small magnetic holes, to name just a few.
—Andrew Yau, Associate Editor, Geophysical Research Letters; email: [email protected]