NEWS RELEASE, 12/14/99
Fast spacecraft discovers invisible aurora
By Bill Steigerwald
BERKELEY-- An invisible aurora exists as a companion to the familiar visibleaurora, also known as the aurora borealis, or northern lights, in theNorthern Hemisphere and the aurora australis (southern lights) in theSouthern Hemisphere. Researchers using NASA's Fast Auroral Snapshot(FAST) spacecraft established the presence of the invisible aurora byconsistently detecting upward flows of electrons interspersed with thedownward flowing electrons that produce the visible aurora.
The discovery provides the first detailed picture of how the aurora andits inverted companion function together to complete a huge electriccircuit in the magnetosphere, which is that region of space whereelectrically charged particles are controlled by the Earth's intrinsicmagnetic field.
The spacecraft's passages through the electrified auroral regions alsoprovide the best explanation yet as to how they turn the Earth into agiant radio transmitter capable of broadcasting billions of Watts ofhigh frequency radio waves into space. The theory grants new insightinto how natural radio emissions are generated throughout the solarsystem and the cosmos.
"Researchers had some previous indicators that the invisible auroraexisted, but earlier spacecraft provided only isolated observations ofits presence," said Dr. Robert Pfaff, FAST Project Scientist at NASA'sGoddard Space Flight Center, Greenbelt, Md. "The FAST results establishsuch upward-moving electrons as consistent, regular features of theaurora."
Visible auroras occur when electrons are accelerated by electric fieldsin the Earth's magnetosphere down along the nearly vertical magneticfield lines of the polar regions, into-More-FAST 2-2-2the upper atmosphere. Auroral light is emitted when the energeticelectrons 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, oftenappearing as shifting curtains of light from the ground.
The invisible magnetic field lines converge as they approach theEarth's upper atmosphere at the polar regions, forming a funnel ornozzle shape. FAST data have established that electric fields orientedparallel to these magnetic field lines accelerate the auroralelectrons, in much the same way as water is accelerated when passingthrough a nozzle.
The idea of parallel electric fields was proposed over 50 years ago byNobel laureate, Hans Alfven of Sweden. Although ridiculed at the timeas electric fields directed this way were believed to "short out" whenoriented along the highly conducting magnetic field lines, observationsgathered in space, such as those from the FAST satellite, as well asrecent theoretical advances, have clearly shown that such processesproduce the aurora and may indeed be widespread in nature.
Furthermore, the FAST data show that the direction of such electricfields are reversed in cases where they accelerate electrons up out ofthe ionosphere to produce the newly discovered invisible aurora. Thisnew type of aurora, also known as the inverse or black aurora, isinvisible because the accelerated electrons are above most of thefaint, upper atmosphere by the time they reach speeds that aresufficient to allow the atmospheric atoms and molecules to glow viacollisions with the electrons. In this fashion, the upward-movingelectrons complete the electrical circuit returning current to thedistant magnetosphere.
The controversial parallel electric fields are also directly linked tointense radio emission from the auroral regions, called AuroralKilometric Radiation, according to new observations from FAST. Theparallel electric fields power a process that generates the radioemission, called the Electron Cyclotron Maser, which is like a naturallaser except at radio frequencies. This powerful radio emission,generally at several hundred kilohertz to a few megahertz, is notdetected on the ground because it is blocked by the Earth's upperatmosphere (ionosphere). Several decades of observations by otherspacecraft, as well as from radio telescopes, have detected similaremissions emanating from other planets such as Jupiter and Saturn, aswell as from electrically charged gas (plasma) throughout theUniverse.
"This radio emission is observed throughout the Universe by radioastronomers," said Dr. Robert Ergun, an astrophysics professor at theUniversity of Colorado. "It's a fundamental process that occurs in manystars and planets, so understanding how these radio waves are generatedis important to understanding how the Universe works. Many theorieshave been proposed to explain it, but so far, observations have beeninadequate to resolve the issues. The new data from FAST is changing that."Key to the discoveries are the new, high-resolution electrostaticdetectors aboard FAST called "Top Hats". Designed and built by Dr.Charles Carlson at the University of California, Berkeley, such top hatdetectors can sample incoming electrons over a 360-degree field of viewwith unprecedented time resolution of just a few milliseconds for acomplete distribution. Unlike earlier detectors with a limited fieldof view, the FAST energetic particle detectors can continuously samplerapidly moving electrically charged particles in all directions,independent of the satellite's spin. Dr. Carlson is also the PrincipalInvestigator for the FAST spacecraft.
Note: More information and images can be found on the internetat:http://pao.gsfc.nasa.gov/gsfc/Spacesci/sunearth/sunearth.htm#fast.
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