He I Vector Magnetometry of Field Aligned Superpenumbral Fibrils

TL;DR

This study uses high-resolution spectropolarimetric observations of the He I triplet to measure the magnetic field geometry of superpenumbral fibrils, confirming their field alignment and exploring flow dynamics.

Contribution

It provides the first direct measurements of the full magnetic field vector in superpenumbral fibrils using the He I triplet with advanced inversion techniques.

Findings

Fibril magnetic fields align within 10 degrees of their projected orientation.

Evidence of an accelerated flow (~40 m/s^2) along a fibril supports the magnetic siphon-flow mechanism.

Connectivity of fibrils to the photospheric magnetic field varies and is often uncertain.

Abstract

Atomic-level polarization and Zeeman effect diagnostics in the neutral helium triplet at 10830 angstroms in principle allow full vector magnetometry of fine-scaled chromospheric fibrils. We present high-resolution spectropolarimetric observations of superpenumbral fibrils in the He I triplet with sufficient polarimetric sensitivity to infer their full magnetic field geometry. He I observations from the Facility Infrared Spectropolarimeter (FIRS) are paired with high-resolution observations of the Halpha 6563 angstroms and Ca II 8542 angstroms spectral lines from the Interferometric Bidimensional Spectrometer (IBIS) from the Dunn Solar Telescope in New Mexico. Linear and circular polarization signatures in the He I triplet are measured and described, as well as analyzed with the advanced inversion capability of the "Hanle and Zeeman Light" (HAZEL) modeling code. Our analysis provides direct evidence for the often assumed field alignment of fibril structures. The projected angle of the fibrils and the inferred magnetic field geometry align within an error of plus or minus 10 degrees. We describe changes in the inclination angle of these features that reflect their connectivity with the photospheric magnetic field. Evidence for an accelerated flow (approx 40 m/s^2) along an individual fibril anchored at its endpoints in the strong sunspot and weaker plage in part supports the magnetic siphon-flow mechanism's role in the inverse Evershed effect. However, the connectivity of the outer endpoint of many of the fibrils cannot be established.