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THE towering loops of plasma in the sun鈥檚 corona 鈥� some of them hundreds of thousands of kilometres high 鈥� are supposed to follow the sun鈥檚 magnetic field. But comparing individual loops with models of the underlying magnetic field lines has revealed that the two don鈥檛 match up.

James Klimchuk of the Naval Research Laboratory in Washington DC and his colleagues analysed images of coronal loops taken by NASA鈥檚 TRACE spacecraft. They found that the loops were uniformly thick and symmetrical at both anchor points on the sun鈥檚 surface. Then they calculated the thickness of the corresponding magnetic flux tubes, or bundles of field lines, using data from the SOHO spacecraft. Surprisingly, the calculations showed that many of the tubes were considerably thicker at one anchor point than at the other, and also expanded twice as much with height as the corresponding coronal loops.

Klimchuk thinks the discrepancy arises because existing models of the sun鈥檚 magnetic field are too simple. 鈥淲ithin the sun the convection currents stir up the plasma, which carries the magnetic [flux tubes] along for the ride,鈥� he says. This causes the tubes to tangle up, and since models don鈥檛 account for this, they don鈥檛 match up with the coronal loops.