SUMER Measurement of the Fe x 3p ⁴3d ⁴D_5/2,7/2 Energy Difference

Abstract

Recent studies have shown that magnetic fields in the solar corona are strong enough to significantly mix the two 3p 43d 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 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 Å line.

First Laboratory Measurement of Magnetic-field-induced Transition Effect in Fe x at Different Magnetic Fields

“…The calculation of the response curve requires ΔE as an input parameter. The ΔE presented above is close to the most recent calculated value (Li et al 2021), but much larger than the result of 2.29 ± 0.5 cm −1 given by Landi et al (2020b) as shown in Table 4. Therefore, we calculated two series of the response curves of 257/226 to the magnetic fields at different densities with the CRM method.…”

“…Li et al (2016) measured the MIT line of Fe X with an estimated electron density in the EBIT and derived a ΔE of 3.5 cm −1 with a possible upper limit of 7.8 cm −1 . Judge et al (2016) and Landi et al (2020b) analyzed the astronomical observation of spectral line 1603 Å decaying from an upper level 3p 4 3d 2 G 7/2 to 4 D 5/2, 7/2 , and proposed ΔE of 3.6 ± 2.9 cm −1 and 2.29 ± 0.50 cm −1 , respectively. Recently, Li et al (2021) employed four calculation schemes and considered that the computation of ΔE exceeds the precision limits of current theoretical calculations, and therefore recommended the value of 2.29 ± 0.50 cm −1 .…”

“…There are also some nonnegligible uncertainties regarding the key parameter ΔE and the simulation method itself (Landi et al 2020a;Li et al 2021). The ΔE value varies by one order of magnitude in different calculations and is acquired from different measurements with at least 20% uncertainty, e.g., 3.6 ± 2.9 cm −1 (Judge et al 2016) and 2.29 ± 0.50 cm −1 (Landi et al 2020b). Simulation of the level population equilibrium in an atomic system will be highly dependent on the accuracy of the atomic data, and the simulation strategy itself will also give rise to large uncertainties if not corrected by experimental data (Gu 2008).…”

Application of a Magnetic-field-induced Transition in Fe x to Solar and Stellar Coronal Magnetic Field Measurements

“…Judge et al (2016) obtained a value of ΔE = 3.6 ± 2.9 cm −1 by an analysis of S082B observations of UV spectra close to 1603.2 Å, which decay from 3p 4 3d 2 G 7/2 to these two lower levels (see Table 1). More recently, Landi et al (2020b) reported a new measurement of ΔE utilizing the 1603.2 Å spectra taken with the Solar Ultraviolet Measurement of Emitted Radiation (SUMER, Wilhelm et al 1995) onboard the Solar and Heliospheric Observatory (SOHO), and obtained ΔE = 2.29 ± 0.5 cm −1 . This new value agrees with and improves the measurements by Judge et al (2016) and has been adopted for some recent studies for magnetic field measurements using the MIT method.…”

CHIANTI -- an atomic database for emission lines -- Paper XVI: Version 10, further extensions

“…For the 3s3p 3 3d levels, the four previous tentative identifications proposed by Del Zanna (2010) have been revised following Wang et al (2018b). The energy separation of the 4 D 7/2 and 4 D 5/2 has been modified by using the recent measurement by Landi et al (2020), decreasing it from the previous 5.5 cm −1 value (see the discussion in Del Zanna et al 2004) to 2.29 cm −1 . This change has negligible effects on the wavelengths of the lines emitted by these levels around 257.26 Å, but it has a critical importance for the measurement of magnetic field strength using the magnetically induced transition from the 4 D 7/2 level to the ground (Landi et al 2020b, in press).…”