Parting shots: just as the sun was calming down, it flared with a vengeance

Science News, July 31, 2004 by Sid Perkins

The large blob of charged particles flung from the sun during the flare accelerated into space along the sun's magnetic field lines. Called a coronal mass ejection because the material originated in the lower levels of the sun's atmosphere, or corona, this material traveled at more than 7 million km/hr. Nineteen hours after leaving the sun, many of those particles approached Earth, where they spiraled down Earth's magnetic field lines to pummel the upper atmosphere.

For several days, many aircraft scheduled for flight in polar regions were routed farther southward than normal--not only to retain radio contact but to avoid exposing passengers and crew to increased radiation. Even aircraft flying more southerly routes couldn't avoid encountering a boost in radiation. Quantas flight 107, a Boeing 747 en route from Los Angeles to New York on the afternoon of Oct. 29, is a ease in point. Ian L. Getley of the University of New South Wales in Sydney document that plane's experience in the May Space Weather.

During the middle portion of the plane's journey, while the aircraft flew between Utah and Ohio at a cruising altitude of about 11,300 meters, onboard instruments measured radiation doses normal for that latitude and altitude, about 3 microsieverts ([mu]Sv) per hour. Just as the aircraft began a climb to 11,900 m, however, a barrage of charged particles emitted by an X-10 solar flare earlier that day was arriving at Earth.

Radiation doses during the final hour of the Oct. 29 flight peaked at 4.65 [mu]Sv/hr, Getley says. Even so, total radiation exposure for passengers on the flight added up to only 12 [mu]Sv--about one-quarter of what patients typically receive during a chest X ray.

COLLATERAL DAMAGE The billion-ton burps of material from the sun last autumn didn't blast just Earth. On the basis of previous studies, NASA scientists calculated that shock waves from solar flares temporarily compressed the atmosphere on the sunward hemisphere of Mars, which has no planetwide magnetic field to protect it from such salvos.

As the flares swept past Jupiter, their interactions with that gas giant's intense magnetic field triggered low-frequency radio emissions for a day or so. At Saturn, similar emissions lasted for a week, says Thomas H. Zurbuchen of the University of Michigan in Ann Arbor.

In the next 10 months, when the shock waves from last fall's flares reach the heliopause--the boundary between the sun's solar wind and interstellar space--they'll push the surface of that bubble outward as much as 300 million miles, says Edward C. Stone of NASA'S Jet Propulsion Laboratory in Pasadena, Calif. Low-frequency radio emissions from that boundary, like those generated at Jupiter and Saturn, should alert scientists to the encounter, he notes.

The largest coronal mass ejection now hurtling toward the edge of the solar system thankfully missed Earth altogether. On Nov. 4, 2003, as Sunspot 486 was rotating out of view on the western edge of the sun, it launched a coronal mass ejection that overwhelmed space-based instruments viewing the event. For 12 minutes, their measurements of radiation in various wavelengths were off the charts. However, by analyzing data trends just before and after that gap, scientists estimate that the flare peaked at a record-setting strength: X-27.