Analyzing data obtained by four identical spacecraft that passed directly through a magnetic reconnection site on the boundary between the solar wind and Earth's magnetosphere, researchers have observed how this explosive physical process converts stored magnetic energy into kinetic energy and heat. Magnetic reconnection is a process responsible for explosive solar events, such as solar flares and coronal mass ejections, and geomagnetic storms.
On October 16, 2015, NASA's Magnetospheric Multiscale (MMS) mission, made of 4 identical satellites flying in a precise formation roughly 10 km (6.2 miles) apart, flew through the heart of a magnetic reconnection region, a SwRI-led team reported on May 12. In only a few seconds, MMS collected hundreds of observations of the way the magnetic fields and particles were moving.
"We hit the jackpot," says Dr. Roy Torbert, MMS deputy principal investigator, and director of SwRI's Earth, Oceans, and Space office at the University of New Hampshire. "The spacecraft passed directly through the electron dissipation region, and we were able to perform the first-ever physics experiment in this environment."
Examining the data from the encounter, the MMS team saw a drop in the magnetic field to near zero, oppositely directed ion flows, accelerated electrons, an enhanced electric field, and a strong electrical current - all indications that the spacecraft had entered the dissipation region. The tell-tale signature of reconnection, however, was a spike observed in the electric power generated by the electrons. This was the 'smoking gun' for reconnection. It was theoretically predicted but never seen until MMS.
Another feature observed for the first time by MMS as it traversed the dissipation region was a rapid change in the electrons as they streamed into the dissipation region and were accelerated outward along field lines opened during reconnection. This observation was the first definitive measurement of the interconnection of the solar and terrestrial magnetic fields.
"We've studied it theoretically, and we've simulated it with supercomputers. But up to now we haven't known what controls the conversion of magnetic energy into particle energy," said Dr. James L. Burch, vice president of SwRI's Space Science and Engineering Division and MMS principal investigator.
"We designed the MMS mission to use Earth's magnetosphere as a giant laboratory to perform the definitive experiment on reconnection."
The solar wind and Earth's magnetospheric plasmas are both magnetized. For reconnection to occur, the plasmas become "demagnetized" - that is, the plasma and the magnetic field become decoupled. The critical and final stage in this process occurs in a relatively small region in space known as the "electron dissipation region." As the electrons become demagnetized, the magnetic fields of the Sun and the Earth interconnect and the solar wind and magnetospheric plasmas mix.
"One of the mysteries of magnetic reconnection is why it’s explosive in some cases, steady in others, and in some cases, magnetic reconnection doesn’t occur at all," said Tom Moore, the mission scientist for MMS at NASA's Goddard Space Flight Center in Greenbelt, Maryland.
Whether explosive or steady, the local particles are caught up in the event, hurled off to areas far away, crossing magnetic boundaries they could never have crossed otherwise. At the edges of Earth's magnetic environment, the magnetosphere, such events allow solar radiation to enter near-Earth space.
The magnetic reconnection data obtained by MMS shows that magnetic reconnection is dominated by the physics of electrons — thus providing crucial information about what powers this fundamental process in nature.
- Electron-scale measurements of magnetic reconnection in space - Burch et. al. - Science - May 12, 2016 - DOI: 10.1126/science.aaf2939
- NASA’s Magnetospheric Multiscale (MMS) mission puts magnetic reconnection under the microscope - Article published by SwRI on May 12, 2016
Featured image credit: NASA/Goddard