Spectral lines for polarization measurements of the coronal magnetic field. I. Theoretical intensities

Infrared emission lines are potentially sensitive probes of components of the coronal vector magnetic field, through the Zeeman effect, and on its direction projected onto the plane of the sky, through fluorescent polarization of scattered photospheric light. Prompted by the advent of sensitive infrared array detectors, existing atomic data were reexamined to compile a complete list of coronal lines that may yield a detectable Zeeman effect, through careful differential measurements of Stokes profiles, at typical coronal field strengths of order 10 G. "Average" intensities were computed for a subset of promising forbidden coronal lines. A representative coronal density structure was used. The distribution of plasma with temperature was, at all heights in the corona, assumed to be that described by a standard differential emission measure from extreme-ultraviolet observations of the solar disk. Effects of excitation by photospheric radiation were included, as well as cascades from collisionally excited higher levels having the same principal quantum number as the ground levels. The largest source of error in the computed intensities lies in the form assumed for the emission measure distribution. The assumed density and temperature structure is too simple for detailed comparisons with observations of a particular coronal structure. Nevertheless, existing observed intensities are consistent with the calculations, which suggests that the theoretical intensities of (as yet) unobserved lines can be used as a basis for further study. The strongest predicted lines arise from magnetic dipole transitions within the ground terms of the 2s(m)2p(n) and 3s(m)3p(n), m = 1, 2, n = 1,..., 5, configurations. The most promising lines lie between 1 and 10 mu m, the lower limit being set by the need to detect small held strengths. The upper limit is set by the small Einstein A-coefficients and the smaller intensities of the exciting photospheric light, both of which lead to smaller forbidden line intensities. The most promising lines include [Fe XIII] 1.0747, 1.0798 mu m; [Si X] 1.43 mu m; [Si IX] 2.58, 3.93 mu m; [Mg VIII] 3.03 mu m; and [Mg VII] 5.50, 9.03 mu m. An aircraft experiment is being prepared to obtain targeted portions of the coronal spectrum between 1 and 10 mu m during the 1998 February 26 eclipse, with the goal of detecting some of these promising lines. This work will help toward the planning and development of efficient magnetographs, perhaps space-borne, for the routine measurement of coronal magnetic fields in the quiet and active Sun.