Spectropolarimetry of Fine Magnetized Structures in the Upper Solar Atmosphere

Abstract

One of the earliest indications of magnetic fields acting in the solar atmosphere came at the beginning of the 20th century when George Hale noted a "decided definiteness of structure" in photographs within the Hydrogen Balmer-alpha line core. Fine structure both in the chromosphere and in the corona result from processes that are not well understood but accepted as a consequence of the solar magnetic field. Our knowledge of this field is lacking, and until recently, the assumed relationship between fine thermal structure and the magnetic field remained untested. Here, spectropolarimetric diagnostics of fine structures in the solar chromosphere and cool corona are advanced using the infrared He I triplet at 1083 nm. Precise calibration procedures are developed for the Facility Infrared Spectropolarimeter (FIRS), recently commissioned at the Dunn Solar Telescope. Together with high-order adaptive optics, we simultaneously map fine structures while obtaining a polarimetric sensitivity of up to 2 x 10 ⁻⁴ of the incoming intensity. These instrument improvements result in the first maps of the He I polarized signatures within an active region superpenumbra, where Hale first recognized fine-structuring. Selective absorption and emission processes due to non-equilibrium optical pumping are recognized. Our interpretation, using advanced inversions of the He I triplet, provides confirmation of Hale's initial suspicion--the fine structures of the solar chromosphere are visual markers for the magnetic field. Yet, the fine chromospheric thermal structure is not matched by an equivalently fine magnetic structure. Our ability to measure this field suggests the utility of the He I triplet as an inner boundary condition for the inner heliospheric magnetic field. In the corona itself, we infer the vector properties of a catastrophically-cooled coronal loop, uniting space-based and ground-based instrumentation. We determine how fine loops are anchored in the photosphere via a narrow umbral flare, the consequence of a supersonic downflow of cooled material. A stereoscopic reconstruction as well as full-Stokes inversions of the He I measurements provide the first comparison of the 3D thermal structure and 3D magnetic structure of a fine-scaled coronal loop.