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Topic Name: UNH scientists report first findings problem on plasma astrophysics and space physics

Category: STAR (Space, Telecommunications & Radioscience)

Research persons: physicist Li-Jen Chen

Location: University of New Hampshire, United States

Details

In a paper published recently in the journal Nature Physics, an international team of space scientists led by researchers from the University of New Hampshire present findings on the first experimental evidence that points in a new direction toward the solution of a longstanding, central problem of plasma astrophysics and space physics.

The mystery involves electron acceleration during magnetic explosions that occur, for example, in solar flares and "substorms" in the Earth's magnetosphere - the comet-shaped protective sheath that surrounds the planet and where brilliant auroras occur.

During solar flares, accelerated electrons take away up to 50 percent of the total released flare energy. How so many electrons are accelerated to such high energies during these explosive events in our local part of the universe has remained unexplained.

A mainstream theory holds that the mysterious, fast-moving electrons are primarily accelerated at the magnetic explosion site - called the reconnection layer - where the magnetic fields are annihilated and the magnetic energy is rapidly released. However, physicist Li-Jen Chen of the Space Science Center within the UNH Institute for the Study of Earth, Oceans, and Space discovered that the most powerful electron acceleration occurs in the regions between adjacent reconnection layers, in structures called magnetic islands.

When Chen analyzed 2001 data from the four-spacecraft Cluster satellite mission, which has been studying various aspects of Earth's magnetosphere, she found a series of reconnection layers and islands that were formed due to magnetic reconnection.

"Our research demonstrates for the first time that energetic electrons are found most abundantly at sites of compressed density within islands," reports Chen.

Another recent theory, published in the journal Nature, has suggested that "contracting magnetic islands" provide a mechanism for electron acceleration. While the theory appears relevant, it needs to be developed further and tested by computer simulations and experiments, according to the UNH authors.

Until the UNH discovery there had been no evidence showing any association between energetic electrons and magnetic islands. This lack of data is likely due to the fact that encounters of spacecraft with active magnetic explosion sites are rare and, if they do occur, there is insufficient time resolution of the data to resolve island structures.

In the Nature Physics paper, entitled "Observation of energetic electrons within magnetic islands," lead author Chen reports the first experimental evidence for the one-to-one correspondence between multiple magnetic islands and energetic electron bursts during reconnection in the Earth's magnetosphere.

"Our study is an important step towards solving the mystery of electron acceleration during magnetic reconnection and points out a clear path for future progress to be made," says Chen. UNH collaborators on the paper include Amitava Bhattacharjee, Pamela Puhl-Quinn, Hong-ang Yang, and Naoki Bessho.

Note for Magnetic reconnection

Magnetic reconnection is the process whereby magnetic field lines from different magnetic domains are spliced to one another, changing their patterns of connectivity with respect to the sources. It is a violation of an approximate conservation law in plasma physics, and can concentrate mechanical or magnetic energy in both space and time. Solar flares, the largest explosions in the solar system, may involve the reconnection of large systems of magnetic flux on the Sun, releasing in minutes energy that is stored in the magnetic field over a period of hours to days. Magnetic reconnection in Earth's magnetosphere is one of the mechanisms responsible for the aurora, and it is important to the science of controlled nuclear fusion because it is one mechanism preventing magnetic confinement of the fusion fuel.

The most common type of magnetic reconnection is separator reconnection, in which four separate magnetic domains exchange magnetic field lines. Domains in a magnetic plasma are separated by separatrix surfaces: curved surfaces in space that divide different bundles of flux. A separatrix surface may be compared to the fascia that separate muscles in an organism: field lines on one side of the separatrix all terminate at a particular magnetic pole, while field lines on the other side all terminate at a different pole of similar sign.

Note for Astrophysical plasma

An astrophysical plasma is a plasma (an ionized gas) found in astronomy whose physical properties are studied in the science of astrophysics. Much of the baryonic matter of the universe is thought to consist of plasma, a state of matter in which atoms and molecules are so hot, that they have ionized by breaking up into their constituent parts, negatively charged electrons and positively charged ions. Because the particles are charged, they are strongly influenced by electromagnetic forces, that is, by magnetic and electric fields.

All known astrophysical plasmas are influenced by magnetic fields. Since plasmas contain equal numbers of electrons and ions, they are electrically neutral overall and thus electric fields play a lesser dynamical role. Because plasmas are highly conductive, any charge imbalances are readily neutralised.

Note for Space physics

Space physics, also known as space plasma physics, is the study of plasmas as they occur naturally in the universe. As such, it encompasses a far-ranging number of topics, including the sun, solar wind, planetary magnetospheres and ionospheres, auroras, cosmic rays, and synchrotron radiation. Space physics is a fundamental part of the study of space weather and has important implications not only to understanding the universe, but also to practical every-day life, including the operation of communications and weather satellites.

About Researcher

Li-Jen Chen
Space Science Center, UNH Institute for the Study of Earth, Oceans, and Space.
EDUCATION
Ph.D. in Physics, University of Washington, 2002
Dissertation: Bernstein-Greene-Kruskal Electron Solitary Waves in Collisionless Plasmas
Advisor: George K. Parks
M.S. in Physics, University of Washington, 1997
B.S. in Physics with distinction, National Taiwan University, 1993