Spectral lines for polarization measurements of the coronal magnetic field. IV. Stokes signals in current-carrying fields

We present the first theoretical, forward calculations of the Stokes profiles of several magnetic dipole ("M1") coronal emission lines produced in current-carrying magnetic structures. An idealized coronal model of Low, Fong, and Fan is used, which describes a spherically symmetric, hydrostatic background atmosphere, isothermal at a coronal temperature of 1.6 × 10⁶ K. Embedded is a global, axisymmetric magnetic field that is everywhere potential except at a quiescent prominence, consisting of an infinitesimally thin, equatorial current sheet whose weight is supported by the outward discrete Lorentz force in the sheet. This model contains a physically nontrivial, localized magnetic structure, although the atmospheric plasma is thermally of the simplest stratification possible. The calculated M1 coronal lines show clear and distinct signatures of the presence and magnitude of this localized magnetic structure, in both linear and circular polarizations, even though the thermal structure is almost homogeneous. The morphology of maps of linear polarization is particularly sensitive to the existence and strength of the current sheets, as field lines wrap around them according to the Biot-Savart law, and the linear polarization responds to different projections of field line directions onto local radius vectors. Of the Ml lines studied, those of Fe XIII (1074.7 nm) and Si x (1430.1 nm) are especially promising because of their relatively strong linear polarization. These forward calculations provide a basis for optimism that emission-line measurements may reveal the presence and nature of current systems in the corona, and provide motivation for developing instruments capable of routinely measuring polarized light in forbidden coronal lines.