Despite a considerable scatter of the theoretical predictions of the M1/E2 transition rate of the "red iron line" (Fe X) in the solar corona, there is disagreement of all the results with the single measurement that used an electrostatic ion trap. Employing a heavy-ion storage ring for measuring the same transition in isoelectronic ions of Co, Ni, and Cu, the situation has been clarified, and a new, accurate data point for Fe X can be determined by extrapolation. This result agrees with the

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... c atomic structure prediction for the line strength in combination with the experimental transition energy. For the "green iron line" (Fe XIV), a recent measurement with an electron beam ion trap has resolved similar discrepancies.