mercredi 10 décembre 2014

The Science of Magnetic Reconnection

NASA - Magnetospheric Multiscale (MMS) mission logo.

December 10, 2014

Understanding vast systems in space requires understanding what's happening on widely different scales. Giant events can turn out to have tiny drivers -- take, for example, what rocked near-Earth space in October 2003. On Oct. 28, 2003, and again on Oct. 29, massive solar flares erupted on the sun, sending X-rays zooming through the solar system. Along with the flares, the sun expelled giant clouds of solar material, called coronal mass ejections, or CMEs. The CMEs slammed into Earth's magnetic field pushed material and energy in toward Earth. This created what's called a geomagnetic storm.

MMS Science Overview: The Many Mysteries of MMS

Video above: In March 2015, NASA will launch the Magnetospheric Multiscale mission, or MMS, to learn more about a mysterious process that drives giant explosions in space via a process called magnetic reconnection. Image Credit: NASA's Goddard Space Flight Center/Duberstein.

The Halloween Storms, as they have come to be called, triggered brilliant aurora that could be seen over much of North America -- reaching as far south as Texas. But they also interfered with GPS signals and radio communications, and caused the Federal Aviation Administration to issue their first ever warning to airlines to avoid excess radiation by flying at low altitudes.

Every step leading to these intense storms -- the flare, the CME, the transfer of energy from the CME to Earth's magnetosphere – was ultimately driven by the catalyst of magnetic reconnection. This little understood process can occur in thin layers just miles thick. Yet it can accelerate particles up to nearly the speed of light and can initiate giant eruptions from the sun many times the size of Earth. The effects of reconnection have been observed in space, but the actual reconnection process has only been observed in the laboratory.

In March 2015, NASA will launch a new mission to study magnetic reconnection. The Magnetospheric Multiscale, or MMS, mission will be the first ever mission dedicated to studying this universal process by orbiting Earth to pass directly through nearby magnetic reconnection regions and to observe the minute details of such events.

Reconnection occurs wherever charged gases, called plasma, are present. It's rare on Earth, but plasma makes up 99% of the visible universe. Plasma fuels stars and fills the near vacuum of space. Plasmas behave unlike what we regularly experience on Earth because they travel with their own set of magnetic fields entrapped in the material. Changing magnetic fields affect the way charged particles move and vice versa, so the net effect is a complex, constantly-adjusting system that is sensitive to minute variations.

Under normal conditions, the magnetic field lines inside plasmas don't break or merge with other field lines. But sometimes, as field lines get close to each other, the entire pattern changes and everything realign into a new configuration. The amount of energy released can be formidable. Magnetic reconnection taps into the stored energy of the magnetic field, converting it into heat and kinetic energy that sends particles streaming out along the field lines.

Magnetospheric Multiscale mission, or MMS spacecrafts constellation. Image Credits: NASA

Scientists want to know exactly what conditions, what tipping points, trigger magnetic reconnection events. Much of what we currently know about the small-scale physics of magnetic reconnection comes from theoretical studies, computer models, and laboratory experiments. True understanding, however, requires observing magnetic reconnection up close – so MMS will take its measurements in Earth's own magnetosphere, an ideal natural laboratory in which reconnection can be observed under a wide range of conditions.

Orbiting Earth, MMS will pass through known areas of magnetic reconnection. During its first phase it will travel through reconnection sites on the sun side of Earth. Here the interplanetary magnetic field connects with Earth's magnetic field, transferring particles, momentum and energy to the magnetosphere via magnetic reconnection. During the second phase of its mission, MMS will observe reconnection on the night side of Earth, where that connected field flows around both sides of Earth to a second reconnection point in what's known as the magnetotail, where they then disconnect.

These reconnection sites are so thin, that MMS will fly through them in under a second -- but the MMS sensors have been built to be fast, operating at unprecedented speed. As the spacecraft fly through such a site, they will measure the magnetic and electric fields present as well as the movement of particles.

Armed with this data, scientists will have their first chance to watch magnetic reconnection from the inside, right as it's occurring. By focusing on the small-scale process, scientists open the door to understanding what happens on larger scales throughout the universe.  Determining how reconnection occurs nearby will improve our understanding of how this fundamental process works on the sun, on other stars, throughout space -- and, of course, it will teach us more about giant geomagnetic storms like the Halloween storms, thus helping us safeguard our home planet Earth.

For more on MMS:

Image (mentioned), Video (mentioned), Text, Credits: NASA's Goddard Space Flight Center/Karen C. Fox.


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