Magnetic properties of orphan penumbrae

TL;DR

This study investigates orphan penumbrae, their magnetic structures, formation mechanisms, and decay processes, using spectropolarimetric data from Hinode and HMI, revealing their similarities to sunspot penumbrae and diverse formation conditions.

Contribution

It provides a comprehensive statistical and observational analysis of orphan penumbrae, highlighting their magnetic configurations, formation pathways, and chromospheric connections, which were not thoroughly characterized before.

Findings

OPUs have Omega-shaped magnetic fields with opposite polarities at ends.

Most OPUs form from penumbra separation or flux emergence near polarity inversion lines.

OPUs' magnetic fields extend into the chromosphere, as indicated by fibrils.

Abstract

Orphan penumbrae (OPU) are features resembling sunspot penumbrae, but are not connected to an umbra. Here we compare OPUs and sunspot penumbrae, including their filaments. We also identify and describe the main mechanisms for the formation of OPUs and we characterise their decay process. Our study is based on spectropolarimetric inversions of active regions observed with the Hinode spectropolarimeter. We manually identified 80 individual OPUs, allowing us to study them statistically. In addition, we analysed the time-evolution of selected OPUs using data provided by the Helioseismic and Magnetic Imager. Orphan penumbrae display a broad range of shapes, associated with typically $\Omega$-shaped magnetic field configurations, where opposite polarity fields predominate at the two ends of the OPU. In addition, the properties of the OPU filaments are remarkably uniform between different OPUs, resembling the ones in sunspot penumbrae. Most OPUs form by either a patch of a penumbra separating from a sunspot, or by new magnetic flux emerging close to the polarity inversion line of an active region. We observe chromospheric fibrils above almost all OPUs in Hinode H$\alpha$ images, indicating that a part of the magnetic field of the OPUs extends to the chromosphere. Our results show that OPU filaments can form given a broad range of boundary conditions for the magnetic field.