Magnetic properties on the boundary of an evolving pore

¹ Astronomical Institute of the Czech Academy of Sciences, Fričova 298, 25165 Ondřejov, Czech Republic
e-mail: rivas@asu.cas.cz
² Astronomical Institute, Charles University, V Holešovickách 2, 18000 Praha, Czech Republic
³ Leibniz-Institut für Sonnenphysik (KIS), Schöneckstrasse 6, 79104 Freiburg, Germany

Received: 12 October 2020
Accepted: 8 February 2021

Abstract

Context. Analyses of the magnetic properties on umbrae boundaries have led to the Jurčák criterion, which states that umbra-penumbra boundaries in stable sunspots are equally defined by a constant value of the vertical magnetic field, B_ver^crit, and by a 50% continuum intensity of the quiet Sun, I_QS. Umbrae with vertical magnetic fields stronger than B_ver^crit are stable, whereas umbrae with vertical magnetic fields weaker than B_ver^crit are unstable and prone to vanishing.

Aims. We aim to investigate the existence of a critical value of the vertical magnetic field on a pore boundary and its role in the evolution of the magnetic structure.

Methods. We analysed SDO/HMI vector field maps corrected for scattered light and with a temporal cadence of 12 min during a 26.5-hour period. A continuum intensity threshold (I_c  =  0.55 I_QS) is used to define the pore boundary and we study the temporal evolution of the magnetic properties there.

Results. We observe well-defined stages in the pore evolution: (1) during the initial formation phase, total magnetic field strength (B) and vertical magnetic field (B_ver) increase to their maximum values of ∼1920 G and ∼1730 G, respectively; (2) then the pore reaches a stable phase; (3) in a second formation phase, the pore undergoes a rapid growth in terms of size, along with a decrease in B and B_ver on its boundary. In the newly formed area of the pore, B_ver remains mostly below 1731 G and B remains everywhere below 1921 G; (4) ultimately, pore decay starts. We find overall that pore areas with B_ver < 1731 G, or equivalently B < 1921 G, disintegrate faster than regions that fulfil this criteria.

Conclusions. We find that the most stable regions of the pore, similarly to the case of umbral boundaries, are defined by a critical value of the vertical component of the magnetic field that is comparable to that found in stable sunspots. In addition, in this case study, the same pore areas can be similarly well-defined by a critical value of the total magnetic field strength.