The magnetic nature of umbra–penumbra boundary in sunspots

Abstract

Context. Sunspots are the longest-known manifestation of solar activity, and their magnetic nature has been known for more than a century. Despite this, the boundary between umbrae and penumbrae, the two fundamental sunspot regions, has hitherto been solely defined by an intensity threshold. Aims. Here, we aim at studying the magnetic nature of umbra-penumbra boundaries in sunspots of different sizes, morphologies, evolutionary stages, and phases of the solar cycle. Methods. We used a sample of 88 scans of the Hinode/SOT spectropolarimeter to infer the magnetic field properties in at the umbral boundaries. We defined these umbra-penumbra boundaries by an intensity threshold and performed a statistical analysis of the magnetic field properties on these boundaries. Results. We statistically prove that the umbra-penumbra boundary in stable sunspots is characterised by an invariant value of the vertical magnetic field component: the vertical component of the magnetic field strength does not depend on the umbra size, its morphology, and phase of the solar cycle. With the statistical Bayesian inference, we find that the strength of the vertical magnetic field component is, with a likelihood of 99%, in the range of 1849-1885 G with the most probable value of 1867 G. In contrast, the magnetic field strength and inclination averaged along individual boundaries are found to be dependent on the umbral size: the larger the umbra, the stronger and more horizontal the magnetic field at its boundary. Conclusions. The umbra and penumbra of sunspots are separated by a boundary that has hitherto been defined by an intensity threshold. We now unveil the empirical law of the magnetic nature of the umbra-penumbra boundary in stable sunspots: it is an invariant vertical component of the magnetic field.

Magnetic properties of a long-lived sunspot

“…They found that a developing penumbra completely cannibalized the pore, thus supporting the assertion that in umbral areas with |B ver | < B stable ver , the penumbral mode of magneto-convection takes over the umbral mode. Jurčák et al (2018) extends the analysis of Jurčák (2011) to 88 scans of 79 different active regions again using Hinode/SP and showed that the I c = 0.5I qs contours match mostly the |B ver | = 1867 G contours. A Bayesian linear regression showed that a model with constant |B ver | is more likely to explain the data than a first or second order polynomial with log area as independent variable.…”

“…Using I c = 0.5I qs to define the umbral boundary, we obtain |B ver | = 1693 G ± 15 (1σ t -error). Jurčák et al (2018) used Hinode/ SP data to find |B ver | = 1867 +18 −16 G (99%-error) at I c = 0.5I qs . The values for |B ver | differ by some 175 G. In general, a difference is expected due to differences in the experimental setup and analysis methods.…”

A Fractal Analysis of Magnetograms Within Active Regions

“…These field groups were taken from the HMI Magnetic Field Color Table (Hoeksema, 2014) obtained from the JSOC. These outlines are analogous to the magnetic fields thresholds used by (Jurčák, et al, 2018), Meunier (1999) as well as, the magnetic field contours used by (Ioshpa, Mogilevskii, Obridko and Rudenchik, 2005), Régnier and Canfield (2006). By using the color magnetograms and the magnetic field groups outlined in the method, the magnetic fields can be compared by strength and polarity to their corresponding umbral and penumbral structures.…”

Observation of Differential Rotation Within a Sunspot Umbra During an X-Class Flare

“…This is a reliable method in cases where the sunspot umbra can be defined well by an ellipse. Often, the ellipse is fit to the boundary region of the umbra, which is found at around 50% quiet-Sun intensity (Jurčák et al, 2018). In this paper we describe a technique that uses ellipses at multiple levels of quiet-Sun intensity, making it possible to investigate how distance from the darkest point affects the rotation rate as measured by the ellipse fitting method.…”