> Larger image
They come out at night over the equator - giant bubbles of plasma, a gas of electrically charged particles, silently rise in the upper atmosphere. While invisible to human eyes, they can disrupt crucial radio communication and navigation signals, like the Global Positioning System (GPS). NASA is collaborating with the Air Force on a unique investigation that will study how these bubbles form by conducting the Coupled Ion Neutral Dynamic Investigation (CINDI) as part of the payload for the Air Force Communication/Navigation Outage Forecast System satellite.
"Understanding when and where plasma bubbles occur, how severe they will be and how long they will last is vitally important since interference from plasma bubbles affects GPS signals and other radio signals that can travel around the globe by reflection from layers in Earth's upper atmosphere, called the thermosphere and the ionosphere," said CINDI Principal Investigator Prof. Rod Heelis of the University of Texas at Dallas. "These signals are used for communication and navigation by a wide variety of commercial and government entities including the Federal Aviation Administration and search and rescue operations. Most of us are directly or indirectly dependent on the proper function of these space-based systems and it is imperative that we attempt to predict the times when such systems may not be reliable."
Plasma bubbles form at night because the thermosphere and ionosphere have a mix of plasma and electrically neutral gas which becomes unstable after sunset. During the daytime, radiation from the sun creates plasma by tearing electrons from atoms and molecules in the thermosphere and ionosphere. The solar radiation maintains relatively constant levels of plasma in these regions, so they are quite smooth and well behaved. But during the nighttime, there is no solar radiation to prevent the charged particles from recombining back into electrically neutral atoms or molecules again.
The recombination happens faster at lower altitudes, because there are more heavy charged particles (molecular ions) there, and they recombine more quickly than charged particles made from single atoms. More rapid recombination makes the plasma less dense at lower altitudes. The region then becomes unstable because the less dense plasma below, which is trapped in the neutral gas, wants to rise above the higher density plasma above it.
This nighttime instability actually happens at all latitudes, but the equatorial regions become especially turbulent because the plasma bubbles are suspended on Earth's magnetic field, which is horizontal over the equator.
When the overturning starts, the low-density plasma rises to the top of the region, much like air bubbles in water. Scientists use the term equatorial plasma bubbles to describe these regions of low-density charged particles. The boundaries of these equatorial plasma bubbles are where the communication and navigation signals are interrupted. However, at the present time we do not know when these plasma bubbles will appear or how large a region they will occupy.
Scientists aren't sure exactly what triggers the rise of the plasma bubbles. One theory is that winds in the upper atmosphere play a role. CINDI is designed to fly through these regions and determine the conditions that exist just prior to the onset of plasma bubbles and how their evolution is related to these conditions.
The CINDI mission will simultaneously explore the motions of the charged and neutral gases for the first time, and will discover the differences in their behavior when plasma bubbles form and when they do not. This information will help explain the fundamental relationships between charged and neutral particles, allowing scientists to build a better forecast model for plasma bubbles for use in the Earth's environment and in other planetary environments as well.
The CINDI investigation is a critical part of the science objectives of the Communication/Navigation Outage Forecast System (C/NOFS) satellite undertaken by the Air Force Research Laboratory and the Space and Missile Command Test and Evaluation Directorate. CINDI consists of two instruments on-board the satellite, the Ion Velocity Meter (IVM) and the Neutral Wind Meter (NWM), which separately measure the ionized (electrically charged) and neutral particles that exist in the ionosphere.
CINDI and the C/NOFS satellite will be launched April 16, 2008, on a Pegasus XL rocket carried aboard Orbital Science Corporation's L-1011 "Stargazer" jet. The Pegasus starts its mission secured to the belly of the L-1011, where it's carried to the planned launch altitude. Using the Stargazer again and again saves money by eliminating the need for a first stage motor to lift each Pegasus off the ground. The C/NOFS spacecraft was launched into an equatorial orbit that ranges from 250 to 530 miles (400 to 850 kilometers) in altitude, and it is scheduled to collect data for two years.
CINDI is a NASA sponsored Mission of Opportunity conducted by the University of Texas at Dallas (UTD). NASA's Explorer Program at Goddard Space Flight Center, Greenbelt, Md., manages the CINDI mission. The Explorer's Program provides frequent flight opportunities for world-class scientific investigations from space within heliophysics and astrophysics.