Simulating the solar corona in the forbidden and permitted lines with forward modeling I: Saturated and unsaturated Hanle regimes

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Thursday, October 3, 2019

The magnetic field in the corona is important for understanding solar activity. Linear polarization measurements inforbidden lines in the visible/IR provide information about coronal magnetic direction and topology.

Graphic depicting Linear polarization azimuth
Linear polarization azimuth (Ly-alpha coronal emission line) can be seen to increase relative to the tangential direction (90 degrees) with increasing line of sight magnetic field strength. The black line segments show the local magnetic field POS direction.

However, thesemeasurements do not provide a constraint on coronal magnetic field strength. The unsaturated, or critical regime ofthe magnetic Hanle effect is potentially observable in permitted lines for example in the UV, and would provide animportant new constraint on the coronal magnetic field. In this paper we present the first side-by-side comparison offorbidden vs. permitted linear polarization signatures, examining the transition from the unsaturated to the saturatedregime. In addition, we use an analytic 3D flux-rope model to demonstrate the Hanle effect for the line-of-sight vs.plane-of-sky components of the magnetic field. As expected, the linear polarization in the unsaturated regime willvary monotonically with increasing magnetic field strength for regions where the magnetic field is along the observer’sline of sight. The plane-of-sky component of the field produces a linear polarization signature that varies with boththe field strength and direction in the unsaturated regime. Once the magnetic field is strong enough so that the effectis saturated, the resulting linear polarization signal is essentially the same for the forbidden and permitted lines. Weconsider how such observations might be used together in future to diagnose the coronal magnetic field.

Link to article

Publication Name: Astrophysical Journal
First HAO Author's Name: Sarah Gibson