A distinct magnetic property of the inner penumbral boundary

Abstract

Context. We recently presented evidence that stable umbra-penumbra boundaries are characterised by a distinct canonical value of the vertical component of the magnetic field, B stable ver . In order to trigger the formation of a penumbra, large inclinations in the magnetic field are necessary. In sunspots, the penumbra develops and establishes by colonising both umbral areas and granulation, that is, penumbral magneto-convection takes over in umbral regions with B ver < B stable ver , as well as in granular convective areas. Eventually, a stable umbra-penumbra boundary settles at B stable ver . Aims. Here, we aim to study the development of a penumbra initiated at the boundary of a pore, where the penumbra colonises the entire pore ultimately. Methods. We have used Hinode/SOT G-band images to study the evolution of the penumbra. Hinode/SOT spectropolarimetric data were used to infer the magnetic field properties in the studied region. Results. The penumbra forms at the boundary of a pore located close to the polarity inversion line of NOAA 10960. As the penumbral bright grains protrude into the pore, the magnetic flux in the forming penumbra increases at the expense of the pore magnetic flux. Consequently, the pore disappears completely giving rise to an orphan penumbra. At all times, the vertical component of the magnetic field in the pore is smaller than B stable ver ≈ 1.8 kG. Conclusions. Our findings are in an agreement with the need of B stable ver for establishing a stable umbra-penumbra boundary: while B ver in the pore is smaller than B stable ver , the protrusion of penumbral grains into the pore area is not blocked, a stable pore-penumbra boundary does not establish, and the pore is fully overtaken by the penumbral magneto-convective mode. This scenario could also be one of the mechanisms giving rise to orphan penumbrae.

Normal and counter Evershed flows in the photospheric penumbra of a sunspot

“…In our case, we do not have enough information on the magnetic field configuration in the pore during the formation process of the anomalous penumbra due to the lack of SP scans during those stages. Nonetheless, its evolution on the G-band images looks in some aspects similar to the case studied in Jurčak et al (2017): the filaments seem to grow at the expense of the adjacent pore. However, an important difference is that in the case studied by Jurčak et al (2017), the penumbra ends up as an orphan penumbra once the pore has disappeared, i. e. the filaments are not connected to any umbral region, while the anomalous penumbra in our study is continually attached to the umbra of the main sunspot.…”

High-resolution imaging and near-infrared spectroscopy of penumbral decay

“…Based on a study of ten sunspots, Jurčák (2011) concluded that the inner penumbral boundaries are defined by the critical value of the vertical component of the magnetic field, i.e., B stable = 1.8 kG. Recently, Jurčák et al (2017) studied penumbra formation around a pore, which supports the scenario earlier proposed by Jurčák (2011) and Jurčák et al (2015), i.e., the stable vertical component of magnetic field is needed to establish the umbra-penumbra boundary.…”

The Formation and Decay of Sunspot Penumbrae in Active Region NOAA 12673

“…If the vertical component of magnetic field (B ver ) in pore was smaller than B ver stable , the penumbral magneto-convective mode in the pore would be not hindered and a stable pore-penumbra boundary would not be established. Jurčák et al (2017) further analyzed the penumbral formation of a pore and confirmed the necessity of B ver stable for establishing a stable umbra-penumbra boundary (Jurčák et al 2015). They found that the penumbra grew at the expense of magnetic flux of the pore, which supports the result proposed earlier by Watanabe et al (2014).…”