Azimuthal variations of magnetic field strength and inclination on penumbral boundaries

Astronomical Institute of the Academy of Sciences, Fričova 298, 25165 Ondřejov, Czech Republic
e-mail: jurcak@asu.cas.cz

Received: 20 October 2010
Accepted: 5 May 2011

Abstract

Aims. I try to determine the properties of the magnetic field on the inner and outer penumbral boundaries and find out if either magnetic field strength or inclination are constant there and if these plasma parameters depend on the sunspot area.

Methods. The spectropolarimetric data obtained with the Hinode satellite were analysed. Active regions located mostly around the disc centre were selected to compare sunspots of different sizes. The magnetic field strength and inclination were estimated using the inversions of observed Stokes profiles.

Results. Both the magnetic field strength and inclination do not vary along individual outer penumbral boundaries, and the magnetic field probably becomes weaker and more vertical with decreasing sunspot area. The magnetic field strength and inclination are changing along the inner penumbral boundaries and also depend on the umbral area. Weaker magnetic fields are more vertical on the inner penumbral boundaries, which leads to a constant vertical component of the magnetic field on these boundaries. The vertical component of the magnetic field is possibly independent of the umbral area.

Conclusions. The inner penumbral boundaries are defined by the critical value of the vertical component of the magnetic field. This implies that the penumbral filaments have a convective origin.