Things that Go Bump in the Light
Jul 06, 2010

The northern aurora contains regions that sometimes emit energetic bursts of light.

"It seems to be a natural consequence of our points of view to assume that the whole of space is filled with electrons and flying electric ions of all kinds."
--- Kristian Birkeland

The curtains of blue, green, and red that hang down from the sky like wind-wavering veils of color have both puzzled and entranced observers for countless millennia. In 1621, Pierre Gassendi called the shimmering curtains of light seen in the Arctic the Aurora Borealis after Aurora, the Roman goddess of dawn, and Boreas, the Greek name for the north wind.

Earth possesses a complex electrically active structure called a magnetotail (or plasma tail) that extends for millions of kilometers, always pointed away from the Sun. Streams of charged particles ejected from the Sun, conventionally called the solar wind, are captured by our planet's magnetosphere, and along with ions generated by Earth itself, collect in a plasma sheet within the magnetotail, where they are held together by Earth's magnetic field.

Solar ions follow Earth's magnetic field down into the poles, causing atmospheric molecules to emit light: red from oxygen at high altitudes, then green from oxygen lower down, along with blue from nitrogen.

According to a press release from NASA's Time History of Events and Macroscale Interactions during Substorms (THEMIS) mission, large areas of the Aurora Borealis are sometimes seen to create intense bursts of light called substorms. Such bursts have long been a puzzle to astrophysicists, but now team members think they finally have a solution: "plasma bullet collisions."

THEMIS scientists propose that small, fast-moving aurorae "crash into" the large-scale auroral complex that moves slowly. An auroral "knot" of plasma speeds in from the north and the two formations collide, releasing an abrupt flash.

How is Earth's magnetotail involved in these "collisions?"

Larry Lyons of UCLA, one of the team's leading researchers, believes that the knots move in conjunction with a plasma jet traveling through the magnetotail. Plasma waves and instabilities are generated when the jet reaches the magnetotail's inner boundary. Radar stations in Alaska and Greenland have detected return signals from streams of plasma in Earth's upper atmosphere just before the aurorae collide.

The five THEMIS spacecraft repeatedly fly through the magnetotail and have also confirmed the existence of plasma streams speeding toward Earth. It is those packets of field aligned charged particles that are called plasma bullets.

Strong electromagnetic disturbances are observed when a bright aurora is seen. In 1903, Kristian Birkeland's Arctic expedition discovered that electric currents from the Aurora Borealis flowed parallel to the auroral formation. Since electric currents move in a circuit, and since the auroral glow seemed to be caused by events in space, he proposed that the currents flowed down from space at one end of the auroral arc and back out to space at the other.

In 1973, the magnetometer aboard the U.S. Navy satellite Triad found two gigantic electric current sheets carrying a million amperes or more. One plasma sheet descended from the aurora's morning side and the other ascended from the evening side. Since Birkeland's research predicted those currents linking Earth with space, they are called Birkeland currents.

Today, Birkeland's polar electric currents are known as auroral electrojets; they are connected to electric currents that follow the geomagnetic field into and away from the Arctic region. They are the "new" discovery made by NASA scientists who continue to ignore Birkeland's work. Although the words "plasma" and "currents" are used in the press release, they are coupled with "impacts," "collisions," and "streams."

The light bursts from colliding aurorae are said to be the result of "magnetic reconnection" events. As the theory states, the solar wind "stretches" our magnetic field like a rubber band. When it "snaps back" the over-stretched magnetic field lines initiate a powerful explosion when some of the "magnetic energy" is converted to heat and light.

As was discussed in a previous Picture of the Day, how the energy is released, as well as what starts the process, are still controversial subjects. Energy in nature cannot be destroyed, as the conservation of energy law states. When electricity powers a motor, it is converted to kinetic energy. When friction stops motion, its kinetic energy converts to heat. Magnetic energy is also thought to reappear in different forms. Some becomes heat, increasing the velocity of plasma ions and electrons. Some changes to visible light.

Again it seems that space scientists are reversing cause (electric currents) and effect (magnetic fields). They downplay or deny the importance of the electrical processes and ignore the work of plasma pioneers like Kristian Birkeland. New discoveries are really confirming the experiments done by others more than 100 years ago.

Stephen Smith