Giving birth to a solar prominence.

Giving birth to a solar prominence

Solar prominences, which often appear as thick ribbons of gas swirling high above the sun's surface, do more than make a pretty picture. When these elongated gas clouds, normally held in place for days or hours by magnetic fields, erupt in the same region as solar flares, they spew out material that carries the sun's high-energy particles into interplanetary space. Researchers know solar prominences can cause geomagnetic storms when they intercept Earth's magnetic field, interfering with communications and electric power transmission. But how the prominences form remains a matter of debate.

The difficulty lies in understanding how solar prominences condense from the sun's corona. To create prominences, coronal gas must somehow cool to less than one-hundredth of its original temperature. Some scientists have proposed that the gas cools as magnetic-field activity reduces heating throughout the corona. But such theories contain a major flaw: Gravity would quickly cause much of the cooled, denser coronal gas to fall to the solar surface, and the small amount of material left behind in the corona would be insufficient to create the massive prominences observed.

Spiro K. Antiochos of the Naval Research Laboratory in Washington, D.C., and James A. Klimchuk of Stanford University now propose an alternative theory that avoids this pitfall. They base their work on a study of magnetic heating of the corona and on the concept of radiation cooling, in which photons carry energy away from hot objects.

The researchers suggest that certain low-lying portions of the corona's magnetic field -- indicated on the photo by the solid "legs" extending toward the coronal base -- heat the corona more than the looping portions do. According to their computer model, the extra heating from the magnetic legs initially prompts a slow temperature rise throughout the corona. But this stronger heating also causes gas to swirl upward from the solar surface beneath the corona, carrying new material into the corona and increasing its density. The higher density forces the entire corona to emit more radiation and thus to cool. Near the coronal base, the increased magnetic heating counterbalances the extra cooling, but less heating occurs near the top, allowing enough gas to cool and condense into prominences.

Antiochos says this theory could be tested by future X-ray and optical studies of the sun.