The Multi-Component Photospheric Magnetic Field

Wednesday, February 5, 2014

The Spectro-Polarimeter aboard the Hinode spacecraft was developed jointly by HAO and the Lockheed Martin Solar and Astrophysics Laboratory. This instrument provides continuous, high resolution, high precision measurements of polarization of the Sun. Shown in the figure below is an example of polarization spectra (Stokes parameters I, Q, U, V) taken at the extreme edge of the Sun. It reveals for the first time a very thin layer of emission in the neutral iron spectrum lines that are normally in absorption on the solar disk. The image at left is the spectral continuum image from the Spectro-Polarimeter map showing the position of the spectrograph slit as a vertical dashed line. The corresponding Stokes I,Q,U,V spectra are shown at right. The Stokes I spectrum appears in absorption on the disk, giving way to emission just above the limb.

Observed over several days the Doppler shifts of the emission line profiles are clearly visible along the slit. With the exception of very few points, the observed Stokes Q,U spectral images reveal linear polarization signal was predominantly radially oriented, that is, perpendicular to the natural direction of polarization expected for scattering around a spherical source, that is, nearly opposite to that expected from classical scattering polarization.

The authors were able to reproduce the anomalous polarization orientation arises from the complex atomic structure of the neutral iron atoms giving rise to these spectrum lines (see figure below). The degree of polarization observed in this emission indicates the presence of a weak magnetic field in the upper solar atmosphere having a strength approximately 2 Gauss. Further, this groundbreaking work demonstrates the need to implement complex multi-level atom models to provide a correct interpretation of the subtle polarization signatures observed in the radiation-dominated chromospheric layers, the next frontier of solar physics.

radial scattering polarization
Modeling of the radial scattering polarization observed on Sept. 9, 2007 near the solar limb with Hinode SOT/SP, at a height of 0.3 inches. The atomic model adopted for the these calculations included 80 levels, for a total of 532 radiative transitions between 200nm and 26,000nm.The top row of the figure shows the normalized intensity (Stokes I: left) and Stokes Q/ Stokes I ratio (right) assuming an atmosphere free from magnetic field. Note that the maximum amplitude of the scattering polarization is about 3 times larger than the observed value. The center row shows the same quantities as the top row, but assuming that the terms a5P, a5D, and a5F in the model atom are completely depolarized by collisions. We note how the ratio between the scattering polarizations in the two lines is strongly dependent on the presence of these depolarizing collisions. The bottom row again shows the same quantities, but for a horizontal, random-azimuth (i.e., "canopy-like") magnetic field of 2 Gauss, and no depolarizing collisions. In this case, we obtain scattering polarization amplitudes that are in agreement with the observed values.
From Lites et al. 2010 "Scattering Polarization in the Fe I 630 nm Emission Lines at the Extreme Limb of the Sun" (