BERKELEY-- An invisible aurora exists as a companion to the familiar visible aurora, also known as the aurora borealis, or northern lights, in the Northern Hemisphere and the aurora australis (southern lights) in the Southern Hemisphere. Researchers using NASA's Fast Auroral Snapshot (FAST) spacecraft established the presence of the invisible aurora by consistently detecting upward flows of electrons interspersed with the downward flowing electrons that produce the visible aurora.
The discovery provides the first detailed picture of how the aurora and its inverted companion function together to complete a huge electric circuit in the magnetosphere, which is that region of space where electrically charged particles are controlled by the Earth's intrinsic magnetic field.
The spacecraft's passages through the electrified auroral regions also provide the best explanation yet as to how they turn the Earth into a giant radio transmitter capable of broadcasting billions of Watts of high frequency radio waves into space. The theory grants new insight into how natural radio emissions are generated throughout the solar system and the cosmos.
"Researchers had some previous indicators that the invisible aurora existed, but earlier spacecraft provided only isolated observations of its presence," said Dr. Robert Pfaff, FAST Project Scientist at NASA's Goddard Space Flight Center, Greenbelt, Md. "The FAST results establish such upward-moving electrons as consistent, regular features of the aurora."
Visible auroras occur when electrons are accelerated by electric fields in the Earth's magnetosphere down along the nearly vertical magnetic field lines of the polar regions, into the upper atmosphere. Auroral light is emitted when the energetic electrons collide with the neutral atmosphere, about 60-180 miles (100-300 kilometers) above the Earth's surface. They create haunting, multicolored displays as they crash into atmospheric particles, often appearing as shifting curtains of light from the ground.
The invisible magnetic field lines converge as they approach the Earth's upper atmosphere at the polar regions, forming a funnel or nozzle shape. FAST data have established that electric fields oriented parallel to these magnetic field lines accelerate the auroral electrons, in much the same way as water is accelerated when passing through a nozzle.
The idea of parallel electric fields was proposed over 50 years ago by Nobel laureate, Hans Alfven of Sweden. Although ridiculed at the time as electric fields directed this way were believed to "short out" when oriented along the highly conducting magnetic field lines, observations gathered in space, such as those from the FAST satellite, as well as recent theoretical advances, have clearly shown that such processes produce the aurora and may indeed be widespread in nature.
Furthermore, the FAST data show that the direction of such electric fields are reversed in cases where they accelerate electrons up out of the ionosphere to produce the newly discovered invisible aurora. This new type of aurora, also known as the inverse or black aurora, is invisible because the accelerated electrons are above most of the faint, upper atmosphere by the time they reach speeds that are sufficient to allow the atmospheric atoms and molecules to glow via collisions with the electrons. In this fashion, the upward-moving electrons complete the electrical circuit returning current to the distant magnetosphere.
The controversial parallel electric fields are also directly linked to intense radio emission from the auroral regions, called Auroral Kilometric Radiation, according to new observations from FAST. The parallel electric fields power a process that generates the radio emission, called the Electron Cyclotron Maser, which is like a natural laser except at radio frequencies. This powerful radio emission, generally at several hundred kilohertz to a few megahertz, is not detected on the ground because it is blocked by the Earth's upper atmosphere (ionosphere). Several decades of observations by other spacecraft, as well as from radio telescopes, have detected similar emissions emanating from other planets such as Jupiter and Saturn, as well as from electrically charged gas (plasma) throughout the Universe.
"This radio emission is observed throughout the Universe by radio astronomers," said Dr. Robert Ergun, an astrophysics professor at the University of Colorado. "It's a fundamental process that occurs in many stars and planets, so understanding how these radio waves are generated is important to understanding how the Universe works. Many theories have been proposed to explain it, but so far, observations have been inadequate to resolve the issues. The new data from FAST is changing that."
Key to the discoveries are the new, high-resolution electrostatic detectors aboard FAST called "Top Hats". Designed and built by Dr. Charles Carlson at the University of California, Berkeley, such top hat detectors can sample incoming electrons over a 360-degree field of view with unprecedented time resolution of just a few milliseconds for a complete distribution. Unlike earlier detectors with a limited field of view, the FAST energetic particle detectors can continuously sample rapidly moving electrically charged particles in all directions, independent of the satellite's spin. Dr. Carlson is also the Principal Investigator for the FAST spacecraft.
Note: More information and images can be found on the Internet at: http://pao.gsfc.nasa.gov/gsfc/Spacesci/sunearth/sunearth.htm#fast.
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