The Physical Origin and Magnetic Sensitivity of the Scattering Polarization Observed in the O I IR Triplet at 777 nm

Monday, January 9, 2017

The linearly polarized solar limb spectrum caused by the absorption and scattering of anisotropic radiation has a very rich diagnostic potential, given its sensitivity to the thermal, dynamic and magnetic structure of the solar atmosphere.

Response functions image
Response functions to a horizontal 1 G magnetic field of the emergent Q linear scattering polarization (top row) and response function of the emergent V circular Zeeman polarization (bottom row), normalized to the emergent intensity I, for a line of sight with cos θ = 0:1, being θ the heliocentric angle.

A crucial first step towards its scientific exploitation is to understand the physical origin of the observed spectral line polarization and its magnetic sensitivity via the Hanle and Zeeman effects. Here we study the linear polarization signals observed in the IR triplet of O I at 777 nm, describing in detail the multilevel radiative transfer calculations that allowed us to decipher their physical origin. We investigate the sensitivity of the calculated scattering polarization signals to various modeling parameters, finding that the observed fractional linear polarization pattern originates mainly in the solar chromosphere, although the intensity profiles of the O I IR triplet come mainly from the lower photosphere. We find that the three lines are sensitive, via the Hanle effect, to magnetic fields with strengths between 0.01 and 10 G, in a extended region of the solar atmosphere. We show this through calculations of the response function to magnetic field perturbations. The dominant response of the linear polarization signals occurs at heights ~1000 km above the visible solar surface, which demonstrates that the scattering linear polarization signals of the oxygen IR triplet encode information on the magnetism of the solar chromosphere.

Submitted to Astrophysical Journal