Laboratory precision measurements of optical emissions from coronal iron

Total solar eclipses, as the recent one seen across North America, are rare opportunities for optical spectroscopy of the corona. In view of the dearth of accurate rest-frame wavelength data, we measured 11 of the strongest optical coronal lines belonging to Fe X-XIV thereby proving the existence of the Fe XII line at 290.385(8) nm. Four lines, such as the green coronal line at 530.28113(13) nm, were measured with unprecedented precision, allowing in principle for absolute velocity determinations of plasmas with uncertainties of 0.08 km ${\mathrm{s}}^{-1}$. These results furthermore stringently benchmark the theory of complex open-$3p$-shell ions.

Figure 1

Spectral map of highly charged iron ions obtained by interpolating spectra obtained at 20 V intervals of $U_{\mathrm{acc}}$, and at several grating angles. To make weak signals more discernible, the color map is scaled with the square root of the intensity (in arbitrary units). Line identifications are indicated where possible, those marked with an asterisk are due to second order diffraction of the grating.

Figure 2

Top: Calibration spectrum of the Pt-Ne hollow cathode lamp. The lines used for the calibration are indicated with orange circles. Here $r_{530}=40\times {10}^3$. Middle: The pixel positions of the calibration lines are plotted against their known wavelengths [37, 38]. A parabola (blue) is fitted to the data to obtain the calibration function. Higher order polynomials did not improve the quality of the fit, nor did the resulting calibration function change the end result for the wavelength determination. In gray, a single 30 min acquisition of the ${\mathrm{Fe}}^{13+}$ spectrum is shown to indicate the position of the Zeeman split 530 nm line relative to the calibration lines. Bottom: Fit residuals, and the 1-$\sigma$ confidence band.

Figure 3

Precision spectra of the Fe lines (blue circles). The Zeeman model fit and residuals are shown in red, together with its $\mathrm{\Delta }{m}_J=0,\pm 1$ components (green triangles). The fit with several independent Gaussians is shown in black. Due to its close agreement with the red line, differences between the fits are best seen in the residuals.

Figure 4

Comparison between wavelengths presented in this work (cyan) and selected literature values (purple). Laboratory measurements are represented by squares, coronal ones by circles, and theory predictions by triangles. The top axis shows the magnitude of relative velocities when deviations from this work are interpreted as Doppler shifts.