SUMER measurement of the fex 3p⁴3d ⁴D₅/2,7/₂ energy difference

Recent studies have shown that magnetic fields in the solar corona are strong enough to significantly mix the two 3p⁴3d ⁴D₅/2,7/₂ levels in Cl-like Fe X. This mixing gives rise to a magnetically induced transition (MIT) component in the bright Fe X 257.3 Å line, commonly observed by current instrumentation, that can be used for coronal magnetic field diagnostics. This line, commonly observed by the still operational EIS spectrometer on board the Hinode satellite since 2007, opens a new window into the coronal magnetic field. However, the strength of this MIT transition depends on the square of the energy difference ΔE of the two ⁴D₅ 2,7 ₂ levels, so that an accurate determination of ΔE is of critical importance to accurately measure coronal magnetic field strengths. In the present work we present a new measurement of ΔE obtained determining the separation of the two component of the Fe X doublet close to 1603.3 Å from deep-exposure spectra of a quiescent streamer at the solar limb taken with the SUMER instrument on board SoHO. Our measurement of ΔE = 2.29 ± 0.50 cm^-1 agrees with, and improves upon, an earlier measurements by Judge et al. by decreasing its uncertainty from 80% to approximately 20%, improving the attainable accuracy of magnetic field strength measurements obtainable with the Fe X 257.26 QA line.