Huge loops of very hot, electrified gas rising above the Sun's surface vibrate with enormous energy at times of solar storms, like the strings of an immense guitar. This is the latest surprise from a flotilla of spacecraft -- the Solar and Heliospheric Observatory (SOHO), Ulysses, and the four Cluster satellites -- with which scientists are trying to make sense of how disturbances on the Sun affect the Earth. The vibrating loops are a new piece in the complex puzzle of solar storms, revealing intense, local, and short-lived activity of a kind that had escaped the scientists' notice.
Dr. Werner Curdt of Germany's Max-Planck-Institut für Aeronomie will report on the solar vibrations today, at a scientific meeting on the Greek island of Santorini. He is in charge of an instrument on SOHO called the Solar Ultraviolet Measurements of Emitted Radiation (SUMER), which can measure the speeds of electrified gas structures moving in the Sun's atmosphere.
SUMER has seen many hot loops, invisible to other instruments, which sway from side to side. After careful study, the scientists investigating them are now sure that the vibrating loops play a key role in the Sun's most violent activity.
"It's like twanging a guitar string, although one that's tuned to a very low bass note," Curdt says. "Nobody knew about these vibrations before. They occur only in extremely hot gas, which can be seen nice and clearly by the instrument as if it were designed for this purpose. But to be honest, when SUMER was built, we didn't expect anything as amazing as this."
SUMER observes the hot loops most plainly when they stand like enormous arches at the rim of the Sun, seen sideways-on by SOHO. The intense heat in the gas loops, between nine and twenty million degrees Kelvin (16.2 million and 36 million degrees Fahrenheit), removes electrons from iron atoms in the gas, causing the iron atoms (ions) to emit ultraviolet light, which is invisible to the human eye but detectable by SUMER. The "color", or wavelength, of the ultraviolet light changes slightly when the loops sway back and forth, allowing scientists to measure the loop's vibration speed with SUMER.
In a typical case, a hot loop 350,000 kilometers long (about 220,000 miles long) rocks to and fro every 20 minutes. The hot gas moves along the line of sight at speeds of up to 100 kilometers per second (approximately 62 miles/second). The gas quickly cools, and the motion dies down, after two or three oscillations.
The tension in the solar guitar string comes from an intense magnetic field that runs along the loop of gas. The "finger" that twangs it is probably a burst of very energetic particles coming from low in the Sun's atmosphere. When the gas in the loop is hit, the atoms lose almost all their electrons. That starts intense emission from hot iron ions and an oscillation of the entire loop.
When the vibrations die down they release energy into the Sun's outer atmosphere. The link to particle outbursts low in the solar atmosphere may help scientists to understand why the outbursts are sometimes so strong that they disrupt the loops and unleash a solar flare. Storms on the Sun can damage spacecraft and electric power systems. That's why NASA, ESA, and other space agencies put much effort into exploring the causes and effects of solar storms. Only by tracing connections between the different kinds of eruptions on the Sun can scientists expect to be able to issue reliable early warnings of solar outbursts affecting the Earth and its neighborhood.
Curdt's institute has organized the Santorini meeting jointly with the National Observatory of Athens. Its purpose is to draw together the results of observations of the Sun from the ground and from spacecraft, to see what's understood and what's not, in the behavior of the solar atmosphere. The Santorini meeting is looking ahead to future spacecraft, including ESA's Solar Orbiter due in about ten years' time. The scientists planning their space instruments will now want to make sure they are well tuned to the solar rock and roll found by SUMER.
The European Space Agency-NASA Ulysses spacecraft explores the heliosphere, the vast region around the Sun filled by the solar wind, where shocks can shake and squeeze the Earth's protective magnetic bubble, the magnetosphere. ESA's Cluster satellites investigate these solar effects near the Earth. SOHO itself uses many instruments to monitor the solar storms, including the huge explosions called flares, which are outbursts of light associated with energetic particles, and the billion-ton blasts of electrified gas called Coronal Mass Ejections.
SOHO is a project of international cooperation between ESA and NASA. The spacecraft was built in Europe for ESA and equipped with instruments by teams of scientists in Europe and the USA. NASA launched SOHO in December 1995, and in 1998 ESA and NASA decided to extend its highly successful operations until 2003.
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