Solar prominence heads back to the sun.

Solar prominence heads back to the sun

Many physicists have believed that when solar prominences -- masses of gas suspended over the sun's surface -- leap into space, the ones stretching out by more than about half the sun's 700,000-mile radius either detach and keep going, or else simply disappear. The only solar prominences scientists observe consistently falling back again extend little more than a tenth of a solar radius. But on June 20, an instrument aboard the Solar Maximum Mission satellite took a series of photos showing a prominence that rose nearly 2 million miles, about 2.5 solar radii from the sun's edge, and then headed back down.

Until that day, none of the several hundred extended prominences photographed over nine years by Solar Max's coronagraph/polarimeter had been seen returning, says the instrument's chief experimenter, Arthur J. Hundhausen of the National Center for Atmospheric Research in Boulder, Colo.

"None of the people I've talked to can remember seeing a prominence that has erupted more than about half a solar radius do this before," says Solar Max project scientist Joseph B. Gurman of NASA's Goddard Space Flight Center in Greenbelt, Md. "Before, whenever we saw an eruptive prominence, we'd usually see it continue outward or else just dissolve. That's the classic groundbased observation of an eruptive prominence."

If the plasma that is the stuff of a prominence gets hot enough, it will no longer emit the hydrogen-alpha light that makes it visible, says David M. Rust of the Johns Hopkins University Applied Physics Laboratory in Laurel, Md. The sun's magnetic field lines apparently hold prominences in place, and Rust suggests that if the field lines containing a prominence "unravel," this may allow hot electrons of the surrounding plasma to ionize the neutral hydrogen atoms of the prominence itself, thus making the prominence invisible to Solar Max.

Rust notes that it seems to take an unusually "tight" magnetic field to contain a prominence long enough for it to extend as far as did the June 20 event and still remain visible as some of its hydrogen-alpha-glowing material starts back down.

Solar Max's more recent observations include the July 8 discovery of its ninth "sun-grazing" comet, which apparently passed so close to the solar disk that it never emerged on the other side. Scientists think the comet may have vaporized in the sun's heat.