Abstract
Context. It has been reported that the boundary between the umbra and the penumbra of sunspots occurs at a canonical value of the strength of the vertical magnetic field, independently of the size of the spot. This critical field strength is interpreted to be the threshold for the onset of magnetoconvection. Aims. Here we investigate the reasons why this criterion, also called the Jurčák criterion in the literature, does not always identify the boundary between the umbra and the penumbra. Methods. We performed a statistical analysis of 23 sunspots observed with Hinode/SOT. We compared the properties of the continuum intensity and the vertical magnetic field between filaments and spines and how they vary between spots of different sizes. Results. We find that the inner boundary of the penumbra is not related to a universal value of the vertical magnetic field. The properties of spines and filaments vary between spots of different sizes. Both components are darker in larger spots and the spines exhibit a stronger vertical magnetic field. These variations of the properties of filaments and spines with the spot size are also the reason for the reported invariance in the averaged vertical magnetic field at 50% of the mean continuum intensity. Conclusions. The formation of filaments and the onset of magnetoconvection are not related to a canonical value of the strength of the vertical magnetic field. The seemingly unique magnetic field strength is rather an effect of the filling factor of spines and penumbral filaments.
Highlights
The penumbrae of sunspots exhibit a complex structure
Based on Hinode observations, Kitai et al (2014) could distinguish between three different scenarios leading to the formation of a penumbra; the active accumulation of magnetic flux involving the accumulation of more magnetic flux in the pore, which leads to a larger inclination at the outer boundary of the flux tube; the rapid emergence of magnetic fields that have the same polarity as the pore; or the appearance of twisted or rotating magnetic flux tubes, which leads to a strongly twisted penumbra
We present an alternative explanation for the observed invariance in Bz, which is based on the variation of the continuum intensity and of the strength of the vertical magnetic field between spots of different sizes
Summary
The penumbrae of sunspots exhibit a complex structure. They consist of bright filaments with a horizontal magnetic field, which is interlaced with dark spines with a more vertical magnetic field (Title et al 1993; Solanki & Montavon 1993; Lites et al 1993; Tiwari et al 2013). The average vertical magnetic field seems to assume a canonical value of Bthr = 1867 G at the UP boundary for all spots, which is defined as where IC = 0.5. Parts of the umbra of some sunspots have a weaker Bz than allowed by the Jurcák criterion These regions were interpreted by Jurcák et al (2018) to be unstable against magnetoconvection and about to be transformed into penumbra. There are some observational indications for such a behavior, such as the formation of penumbra in an emerging sunspot (Jurcák et al 2015), or a pore with a weak magnetic field that gets transformed into orphan penumbra (Jurcák et al 2017). B. Löptien et al.: No universal connection between the vertical magnetic field and the umbra-penumbra boundary in sunspots. All other parameters (including the magnetic field vector) were assumed to be independent of height
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