Photos of Solar Flares Alter Perception of Sun
BOSTON — Images taken from two satellites orbiting above Earth have revealed our closest star - the sun - in greater detail than ever before. The results are so striking and unexpected that scientists have begun revising their theories about the sun.
"We're having a renaissance driven by the observations. There's never been a time when there have been so many rapid advances about the sun," says Richard Fisher of the
Goddard Space Flight Center, in Greenbelt, Md.
In one study, using images taken from the Solar and Heliospheric Observatory (SOHO), a satellite orbiting 1 million miles above Earth, scientists report that they have seen a sunquake, similar to an earthquake, of magnitude 11.3. By comparison, the 1994 earthquake in Los Angeles measured only 6.7.
A second study, using images from the Transition Region and Coronal Explorer (TRACE) satellite, 400 miles above Earth, demonstrates that the sun has more activity and variation than previously thought. Time-lapse videos show small explosions in the sun's atmosphere constantly erupting and sending out streams of light. Glowing particles flow along magnetic field lines. The images also display cool and hot materials mixing together, instead of the expected smooth temperature continuum.
The recent results could help in making predictions about activities on the sun that influence Earth. Large explosions in the sun's atmosphere eject charged particles that can damage electronic parts in satellites, cause surges in power lines, and expose astronauts to high levels of radiation.
In 1989, a solar flare disabled the entire power system of Quebec, proving the power of these solar ejections.
"Remarkable," is how Ernie Hildner, director of the Space and Environment Center at the National Oceanic and Atmospheric Administration in Boulder, Colo., describes the sunquake. "To my mind, it was controversial about whether there would be a quake."
Resulting from a moderately sized solar flare - an explosion named for its radiating arc of light - Alexander Kosovichev of Stanford University in Palo Alto, Calif., and Valentina Zharkova of Glasgow University in Scotland saw seismic waves propagating across the sun's surface "like ripples from a pebble thrown into a pond."
The quake traveled at an initial velocity of 22,000 miles an hour and accelerated to reach a final velocity of 250,000 miles an hour.
Dr. Kosovichev and Dr. Zharkova reported their findings in the May 27 issue of the journal Nature.
Prior to their discovery, the two scientists had predicted the existence of sunquakes. They had speculated that a shock wave, created by energized electrons traveling faster than the speed of sound, would hit the sun's surface and cause a compression or dent.
This compression, they anticipated, would cause the sun to vibrate like a plucked guitar string.
Nonetheless, Dr. Hildner says, because solar flares typically occur within strong magnetic fields, scientists didn't know whether the magnetic fields would act as a barrier and prevent the seismic waves from spreading.
The sunquake was approximately 10 times larger than Kosovichev's and Zharkova's expectations, suggesting that factors beside the shock wave must influence the depth of the initial compression.
In 1986, scientists had a hint that solar flares might cause sunquakes. Researchers in New Mexico noticed an increase in the normal oscillation signals coming from the sun. The scientists attributed the signals to a possible quake resulting from a large solar flare.
But Deborah Haber, a scientist at the University of Colorado in Boulder and a graduate student at the time, says, "We never saw the wave. We only saw the signal. The data they have is much cleaner and nicer."
High-resolution images from TRACE are also helping scientists understand solar explosions and other processes on the sun.
"The TRACE images showed us just how enormous our ignorance was," says Joan Schmelz, a solar physicist at the University of Memphis in Memphis, Tenn.
In findings presented at the meeting of the American Geophysical Union in Boston last month, scientists could see magnetic fields rearranging in the sun's atmosphere or corona.
Scientists say these magnetic field rearrangements cause solar flares and another type of solar explosion - the coronal mass ejection.
The pictures, taken approximately every minute since April 20, may help explain how solar explosions get ignited, a problem presently baffling solar physicists.
"We're able to see the visual cues of the two magnetic structures interacting and releasing heat," says Dr. Fisher of the Goddard Space Flight Center.
The TRACE images may also give scientists a clue as to how the corona gets heated. Scientists have known that the corona is much hotter than the surface of the sun, but the heating mechanism is a mystery.
"We know it's hot, but we don't know how it gets that way," Dr. Schmelz says.
According to Leon Golub, an astronomer at the Harvard-Smithsonian Center for Astrophysics in Cambridge, Mass., who helped make part of the TRACE telescope, the TRACE images show looping threads rising into the corona.
"What used to be thought of as a single loop, now looks as though it's composed of many threads of hot material," Dr. Golub says. Like electric burners, these threads with electric current may heat the corona. The next step in solving the puzzle, he says, is to "bring in the knowledgeable theorists."
The sun may inhabit space 94 million miles away, but the problems examined by solar physicists will play an important role in discerning how the sun interacts with Earth.
"We don't understand the critical physics of the sun," Fisher says. And as technology increases its dependence on satellites for communication, improving that understanding may play a crucial role in keeping our lives running smoothly.