NASA Satellites Solve A Mystery About Earth’s Magnetic Field
In the first major results of NASA’s Magnetospheric Multiscale (MMS) mission, a new study contains a groundbreaking look at the interaction between the magnetic fields of Earth and the sun.
The Earth’s magnetic field presents an invisible but critical barrier which protects Earth from the sun’s magnetic field. That force drives a stream of charged particles known as the solar wind outward from the sun’s outer layers.
The interaction between the two magnetic fields can result in explosive storms in the space near Earth, knocking out satellites and causing problems here on Earth’s surface, despite the protection offered by Earth’s magnetic field.
The new study, co-authored by University of Maryland physicists, details the first direct observation of a phenomenon known as magnetic reconnection, which occurs when two opposing magnetic field lines break and reconnect with each other, releasing massive amounts of energy. The research team was led by Southwest Research Institute (SwRI).
Dr. James L. Burch, vice president of SwRI’s Space Science and Engineering Division and MMS principal investigator, said:
“Just as in astronomy a new telescope like Hubble opens a new window on the universe, with MMS a new ‘microscope’ has opened a new window to clearly see reconnection. The four MMS satellites hone in on the role of electrons in magnetic reconnection, which allows two magnetic fields to interconnect. MMS measures the process 100 times faster than previously possible to clearly visualize the rapidly evolving process.”
Evidence supports reconnection as a major driving force behind events such as solar flares, coronal mass ejections, magnetic storms, and the auroras observed at both the North and South poles of Earth. Although researchers have tried to study reconnection in the lab and in space for nearly half a century, the MMS mission is the first to directly observe how reconnection happens.
Energy Slingshot Jackpot
Co-author James Drake, a professor of physics at UMD, said:
“Imagine two trains traveling toward each other on separate tracks, but the trains are switched to the same track at the last minute. Each track represents a magnetic field line from one of the two interacting magnetic fields, while the track switch represents a reconnection event. The resulting crash sends energy out from the reconnection point like a slingshot.”
In late summer 2015, the four identically instrumented MMS spacecraft began to survey the magnetopause. In this boundary between the solar wind and Earth’s magnetosphere, the scientists are searching for locations where the solar wind magnetic field and the terrestrial magnetic field reconnect.
The four MMS spacecraft fly in a tightly controlled tetrahedral (pyramid) shape that can be re-scaled by changing the distances between each spacecraft. MMS can image the behavior of electrons within this tetrahedron once every 30 milliseconds — providing 100X greater resolution than previous efforts. Credit: NASA GSFC
In half of the over 4,000 magnetopause encounters, MMS has seen evidence of reconnection, but most often the spacecraft did not pass through the reconnection sites themselves. Then on October 16, 2015, MMS flew through the heart of a reconnection region.
“We hit the jackpot. The spacecraft passed directly through the electron dissipation region, and we were able to perform the first-ever physics experiment in this environment,”
says Dr. Roy Torbert, MMS deputy principal investigator and director of SwRI’s Earth, Oceans, and Space office at the University of New Hampshire.
MMS measures plasmas, hot ionized gases consisting of approximately equal numbers of positively charged ions and negatively charged electrons. 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.
Just observing the reconnection in detail is an important milestone. But a major goal of the MMS mission is to determine how magnetic field lines briefly break, enabling reconnection and energy release to happen.
Measuring the behavior of electrons in a reconnection event will enable a more accurate description of how reconnection works; in particular, whether it occurs in a neat and orderly process, or in a turbulent, stormlike swirl of energy and particles.
“Just looking at the data from MMS is extraordinary. The level of detail allows us to see things that were previously a blur,”
explained Drake, who served on the MMS science team and also advised the engineering team on the requirements for MMS instrumentation.
“With a time interval of three seconds, seeing reconnection with Cluster II was impossible. But the quality of the MMS data is absolutely inspiring. It’s not clear that there will ever be another mission quite like this one.”
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,” explains Burch. “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.
The discoveries have significant implications for space and solar physics, astrophysics, and fundamental plasma physics.