The magnetic nature of umbra-penumbra boundary in sunspots

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.

A selection of sunspots from our analysed sample of sunspots is displayed. All maps have the same spatial scale marked in the lower left panel. The white contours mark the intensity threshold of 50% of the quiet-Sun intensity. The red contours are independently defined and outline isocontours of 1867 G of Bver. Only contours encircling regions larger than 3 Mm2 are shown. Sunspots marked (L) clearly show a systematic displacement of the white and red contours that is due to line-of-sight effects. Sunspots marked (U) have regions where the red contours lie within the intensity (white) contours and these umbral regions are unstable and prone to be transformed into a different mode of magneto-convection like a penumbra or a light bridge. The arrows point to the centre of the solar disk. The numbers denote the heliocentric angle of the sunspot, i.e. denote the distance of the spot from disk center.

In a forthcoming article of the European journal Astronomy & Astrophysics, an international team including two KIS scientists give statistical proof of an empirical law governing the boundary between umbra and penumbra in stable sunspots: The vertical component of the magnetic field, Bver, is invariant.

Contours of intensity at 50% of the granular quiet-sun value and contours of Bver=1867 G match. This is illustrated in the figure, in which contours for intensity (white) and Bver (red) are displayed for some of the spots that where included in the sample. Deviations were studied and shown to be non-significant.

This empirical law applies for all sunspots of the sample with different sizes, morphologies, evolutionary stages, and phases of the solar cycle. Thereby we have unveiled the magnetic property that discriminates between the umbral and penumbral modes of magneto-convection. This discovery carries strong consequences for the understanding of the fundamental processes of energy transport occurring in the magnetic Sun and are subject of further investigations at KIS.



"The magnetic nature of umbra-penumbra boundary in sunspots",
Jurcak, Rezaei, Bello Gonzalez, Schlichenmaier, Vomlel, 2018,
A&A forthcoming,