The umbral--penumbral boundary in sunspsots in the context of magneto-convection

Abstract

Jurcak et al (2018) have reported that, in a sample of more than 100 umbral cores in sunspots, the umbral-penumbral boundary (UPB) is characterized by a remarkably narrowly-defined numerical value (1867 G) of the vertical component of the magnetic field. Gough and Tayler (1966), in their study of magneto-convection, showed that the onset of convection in the presence of a magnetic field is controlled by a parameter δ which also depends on the vertical component of the field. Combining the Jurcak et al result with various empirical models of sunspots leads us to propose the following hypothesis: the UPB occurs where the vertical field is strong enough to increase the effective adiabatic temperature gradient by at least 100% above its non-magnetic value.

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References in corpus (1)

• Magnetic modeling of inflated low-mass stars using interior fields no larger than ~10 kilogauss

Cited by in corpus (5)

• A distinct magnetic property of the inner penumbral boundary III. Analysis of simulated sunspots
• Characterization of magneto-convection in sunspots -- The Gough & Tayler stability criterion in MURaM sunspot simulations
• No universal connection between the vertical magnetic field and the umbra-penumbra boundary in sunspots
• Coronal magnetic field evolution over the cycle 24
• Clarifying Physical Properties of Magnetic Fields in Sunspots