Using NASA's Polar spacecraft, researchers have found the first direct evidence that bursts of energy from the Sun can cause oxygen and other gases to gush from Earth's upper atmosphere. Space physicists have observed that the flow of "polar wind" increased substantially when a storm from the Sun smacked into Earth on September 24- 25, 1998. In effect, pressure from the solar ejection squeezed gas out of the ionosphere.
Scientists have known since the early 1980s that Earth's upper atmosphere leaks oxygen, helium, and hydrogen ions (atoms that have gained or lost an electron) into space from regions near the poles. But it was not until the Polar spacecraft flew through this fountain of ionized gas in September 1998 that scientists confirmed that the flow of ions is caused by solar activity.
"We now have direct, quantifiable evidence that disturbances in the solar wind produce changes in the flow of ions out of the ionosphere," said Dr. Thomas E. Moore of NASA's Goddard Space Flight Center, principal investigator for Polar's Thermal Ion Dynamics Experiment (TIDE). "This solar wind energy essentially cooks the atmosphere off of the Earth." Moore's observations were presented on December 8 in San Francisco during the Fall Meeting of the American Geophysical Union.
On September 22, 1998, the Sun ejected a mass of hot, ionized gas (plasma) toward Earth. This magnetic cloud of plasma (known as a coronal mass ejection) increased the density and pressure of the solar wind and produced a shock wave similar to a sonic boom. When that cloud arrived at Earth late on September 24, it rammed into and compressed Earth's magnetic shell in space, or magnetosphere. The shock and pressure excited the plasma trapped in Earth's ionosphere to a point where some ions gained enough energy to escape gravity and flow downwind of Earth.
The amount of oxygen and other gases lost from the ionosphere amounted to a few hundred tons, roughly equivalent to the mass of oxygen inside the Louisiana Superdome. "This is the supply of plasma that makes things interesting in space," said Moore. "Much of the gas ejected from the ionosphere is caught in Earth's wake. It then flows back toward the Earth while being heated and accelerated by the same processes that create auroral particles and the radiation belts."
The ionosphere is a series of invisible layers of ions and electrons that are suspended in Earth's atmosphere at about 50 to 400 kilometers (25 to 250 miles) in altitude. These particles are produced when the Sun's ultraviolet light ionizes the atoms and molecules in the upper atmosphere. The ionosphere makes long distance radio communication possible by reflecting radio waves back to Earth. It also is home to the aurora and the electrical currents that heat the atmosphere during magnetic storms.
"Our research shows that Earth"s own ionosphere is a major contributor to the growth of space storms," said Dr. Barbara Giles, a co-investigator on the TIDE team and researcher at NASA-Goddard. "These new observations will help us understand the conditions that enable space storms to form, thereby moving one step closer to the forecasting of the most damaging storms."
Prior to the launch of Polar, such observations of ions flowing out of the ionosphere were nearly impossible. But the TIDE instrument was specifically designed to neutralize the electrical charge that naturally builds up on the surface of spacecraft due to sunlight (about 40 to 50 volts). By squirting a small plume of Xenon ions and electrons, TIDE offsets the charge on the spacecraft and allows detectors to observe cold plasmas like the oxygen ions seen during the September event.