April 19, 1997

From sun to Earth: Tracking a new storm

by R. Cowen

Space scientists who gathered last week at NASA's Goddard Space Flight Center in Greenbelt, Md., had but one goal: to hone their understanding of a solar eruption that sent a huge cloud of gas hurtling toward Earth last January. In addition to grappling with the earlier data, however, the scientists found themselves dealing with a new event.

Birth of a storm: On April 7, SOHO recorded an outburst in the sun's lower corona. (credit: NASA)

On April 7, the day before the meeting began, another large solar outburst blazed forth, blasting Earthward a blob of gas and magnetic energy at a speed of 1.6 million kilometers an hour. Researchers called the resulting storm, which didn't reach Earth's vicinity until 3 days later, relatively mild. There were no reports of problems with spacecraft or electric power outages on Earth. Ground-based detectors, however, measured sizable increases in the energy of electrons in Earth's ionosphere, and skywatchers as far south as Boston were treated to a dazzling auroral display.

As they had for the January eruption, the researchers relied on an armada of spacecraft to track the disturbance, known as a coronal mass ejection because it originates in the sun's outer atmosphere, or corona (SN: 2/1/97, p. 68). This time, an ultraviolet camera aboard the SOHO spacecraft, which continuously monitors the sun, was in operation, and researchers could view the disturbance deeper in the corona than ever before.

"If the last event was [captured] from cradle to grave, this one was from birth to grave," says Nicola Fox of Goddard.

Like the earlier outburst, which researchers classify as a magnetic cloud, the blob of material ejected on April 7 forced the interplanetary magnetic field around it to point south, allowing it to dump energy efficiently into the oppositely oriented magnetic domain of Earth.

Despite similarities, the April event was much more complex, says Charles C. Goodrich of the University of Maryland at College Park. Researchers predicted, from January's records, that the material would strike Earth's vicinity late on April 9. That night came and went, however, with no sign of the blob's arrival.

Scientists worried that they had miscalculated. In fact, says Fox, the material had plowed into an unusually slow solar wind, the stream of charged particles blowing out from the sun. The expanding blob "had to go through molasses to get to us," Fox says.

In addition, says Goodrich, this material does not qualify as a magnetic cloud because it abruptly switched the direction of its magnetic field and did not show a drop in temperature.

Curiously, he notes, the Wind craft, situated so that it detects solar disturbances about an hour before they get to Earth, recorded two large increases in the density of ionized gas. One increase occurred about 3 p.m. eastern daylight time on April 10 and the other 10 hours later. The researchers are currently examining the ground-based data for evidence of a double signal. Goodrich speculates that the solar outburst may have separated into two independent blobs of gas or that, like an erupting volcano, the sun actually spewed two separate blasts of gas.

X-ray image of Earth's aurora, taken April 10 by the Polar satellite, shows that the fireworks extended below the Canadian-U.S. border. Red denotes highest intensity, blue the lowest. (Credit: NASA)

Further Readings:

Cowen, R. 1997. Solar cloud hits Earth's magnetosphere. Science News 151(Feb. 1):68.

Additional information on the eruption, as well as images, are at http://www-spof.gsfc.nasa.gov/istp/event_apr97/.

Sources:

Charles C. Goodrich
Space Science
University of Maryland
College Park, MD 20742

Sources - all articles