Are solar eruptions triggered a loopy way?

Several times a day, the seething cauldron known as the sun undergoes a major eruption, shooting billions of tons of electrically charged gas into interplanetary space. Some of these parcels of gas, called coronal mass ejections, strike Earth and can damage sensitive satellite instruments and knock out power grids. Such temper tantrums are expected to peak in frequency this year as the sun reaches the maximum of its 11-year activity cycle, yet astronomers understand precious little about the origin of these explosive events.

X-ray emission (center) from the sun’s corona recorded by Yohkoh is surrounded by a picture of the outer corona taken by SOHO. Dark area (arrow) indicates where an X-ray-emitting loop has disappeared. Gopalswamy et al.

This week, new clues emerged from movies made by Yohkoh, a Japanese satellite that records X rays from the corona, the sun’s hot outer atmosphere. Reviewing a sequence of Yohkoh images from May 1998, Josef I. Khan of University College London noticed something strange. Magnetically confined loops of material that straddle the sun’s equator and arc far out into the corona disappeared just after the sun emitted a flare, a concentrated outpouring of X rays and other radiation. Khan saw the same pattern on three occasions—May 6, 8, and 9, 1998.

Pulses of radio waves detected by a ground-based telescope revealed shock waves from each flare, Khan and Hugh S. Hudson of the Solar Physics Research Corp. in Tucson, Ariz., found. Khan and Hudson speculated that these shock waves had rammed into the loops, breaking them up and releasing a torrent of hot, X-ray-emitting material that became a coronal mass ejection. The researchers then tracked these eruptions far into the corona with a detector aboard another spacecraft, the Solar and Heliospheric Observatory. They found that the coronal mass ejections indeed took off soon after the flares.

The order of the events—flare, then shock wave, and lastly coronal mass ejection—suggests that when a wave encounters a loop, the loop erupts, Hudson says. The shock wave’s propagation could explain why none of the flares that the team examined was directly below the coronal mass ejections that followed.

The researchers stress that their mechanism can only account for some coronal mass ejections because many ejections occur directly above flares. They describe their work in the April 15 Geophysical Research Letters.

Not everyone agrees with the findings. In an earlier study, N. Gopalswamy of NASA’s Goddard Space Flight Center in Greenbelt, Md., and his colleagues calculated that flares and their shock waves occur minutes after coronal mass ejections rather than minutes before.

“The flare and its shock wave are the consequence not the cause” of an ejection, Gopalswamy asserts. In his team’s scenario, the loop still plays a key role. The loop lifts off the visible surface of the sun and creates instabilities in the underlying material, triggering a coronal mass ejection and then a flare.

Gopalswamy estimated the onset time of a coronal mass ejection from its initial velocity and height. Hudson and Khan say they dispute his calculation but that an argument over a matter of just minutes represents progress in the highly turbulent field of solar physics.

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