Part II: Focusing on Magnetic Reconnection
CMEs and other solar phenomena ride the solar wind that flows throughout the solar system. Earth is protected by its magnetosphere, a comet-shaped cocoon of magnetic fields connected to the planet. As described earlier, CMEs can distort the magnetosphere, extending its magnetotail. This extension ratchets up magnetic processes that are already a significant source for energetic particles harmful to space- and ground-based assets. Known as magnetic reconnection, these processes are the research subject of Alex Klimas, scientist in GSFC's Heliophysics Science Division.

From the Sun's interior to Earth's magnetosphere, magnetic reconnection can occur anywhere close-lying magnetic structures move towards one another, which brings nonparallel magnetic field lines into contact. Instabilities cause opposing field lines to merge and pinch off, forming entirely new configurations. Reconnection is small-scale (tens to hundreds of kilometers), making current observations extremely difficult, so computation is the best way to probe its intricacies. Klimas focuses on reconnection within a specific part of the magnetotail, a relatively thin plasma called the plasma sheet (see Figure 4).

This turbulent, highly dense plasma contains many reconnection sites that can interact and evolve into major events. Of particular interest are substorms. These explosive reconnection events accelerate energetic particles and other disturbances towards "the near-Earth region, where they produce nasty effects," Klimas said. Effects become magnified when substorms occur with solar magnetic storms.

Harnessing the NCCS' Explore supercomputer, "we're trying to understand how an avalanche of reconnection sites can occur," Klimas said. His unique approach allows the modeled plasma sheet to self-organize. The simulation continuously loads plasma and magnetic fields from the solar wind into the magnetotail and also lets them exit on their own. Over time, a plasma sheet forms and within it a current sheet separating magnetic fields in opposing directions. Next, reconnection starts in isolated spots, and the reconnection drives turbulence, which fuels more reconnection, and so on (see Figure 5).

"It is very exciting that we have gotten to this stage," Klimas said. "That is really our primary accomplishment so far, to make a code run long enough so that the model becomes stable enough for us to study the properties of reconnection and turbulence in the magnetotail."

Figure 5: A simulation using the 3D Driven Current Sheet Model traces magnetic reconnection occurring in the top half of a plasma sheet within Earth's magnetotail. The redder the coloring, the more active the reconnection. Simulation by Alex Klimas and Vadim Uritsky.

Not surprisingly, a self-organizing simulation takes time. A single simulation with Klimas' 3D Driven Current Sheet Model using 16 processors needs months to come into proper equilibrium. The sheer scale of the plasma sheet—50 Earth radii wide by hundreds of Earth radii long—adds to the challenge. His 3D studies would have been impossible without an NCCS User Services programmer helping to speed up the code by a factor of 20. "Bless him; it was wonderful," Klimas said.

An earlier, 2D code remarkably matched aurora signals in the ionosphere observed by Polar and other spacecraft. Now poised to study the physical content with his 3D model, Klimas will be using data from several NASA spacecraft for statistical analysis.

Especially helpful are spacecraft that fly in the magnetotail, including Geotail, Cluster, and Time History of Events and Macroscale Interactions During Substorms (THEMIS). Guiding observation mission planning, results from Klimas' simulations are providing background support for the Magnetospheric MultiScale Mission (MMS). Scheduled for launch in 2015, MMS will have four spacecraft with detectors sensitive and fast enough to image magnetic reconnection sites in far more detail.