Simulating the Solar Corona in the Forbidden and Permitted Lines with Forward Modeling. I. Saturated and Unsaturated Hanle Regimes

The magnetic field in the corona is important for understanding solar activity. Linear polarization measurements in forbidden lines in the visible/IR provide information about coronal magnetic direction and topology. However, these measurements do not provide a constraint on coronal magnetic field strength. The unsaturated, or critical regime of the magnetic Hanle effect is potentially observable in permitted lines for example in the UV, and would provide an important new constraint on the coronal magnetic field. In this paper we present the first side-by-side comparison of forbidden versus permitted linear polarization signatures, examining the transition from the unsaturated to the saturated regime. In addition, we use an analytic 3D flux rope model to demonstrate the Hanle effect for the line-of-sight versus plane-of-sky (POS) components of the magnetic field. As expected, the linear polarization in the unsaturated regime will vary monotonically with increasing magnetic field strength for regions where the magnetic field is along the observer's line of sight. The POS component of the field produces a linear polarization signature that varies with both the field strength and direction in the unsaturated regime. Once the magnetic field is strong enough that the effect is saturated, the resulting linear polarization signal is essentially the same for the forbidden and permitted lines. We consider how such observations might be used together in the future to diagnose the coronal magnetic field.