Small-scale convection signatures associated with a strong plage solar magnetic field

¹ Institute for Solar Physics, Royal Swedish Academy of Sciences, AlbaNova University Center, 106 91 Stockholm, Sweden
e-mail: gautam@astro.su.se; scharmer@astro.su.se
² Stockholm Observatory, Dept. of Astronomy, Stockholm University, AlbaNova University Center, 106 91 Stockholm, Sweden

Received: 8 May 2010
Accepted: 15 July 2010

Abstract

Context. Solar convection in a strong plage, in which the magnetic field is vertical and strong over extended regions, but much weaker than in the umbrae of large sunspots, has so far not been well studied. This has been mostly because of a lack of spectropolarimetric data at adequate spatial resolution. The combination of a large solar telescope, such as the Swedish 1-m Solar Telescope, adaptive optics, powerful image reconstruction techniques, and a high-fidelity imaging spectropolarimeter is, however, capable of producing such data.

Aims. In this work, we study and quantify the properties of strong-field small-scale convection and compare these observed properties with those predicted by numerical simulations.

Methods. We analyze spectropolarimetric 630.25 nm data from a unipolar ephemeral region near the Sun center. We use line-of-sight velocities and magnetic field measurements obtained with Milne-Eddington inversion techniques along with measured continuum intensities and Stokes V amplitude asymmetry at a spatial resolution of 0″̣15 to establish statistical relations between the measured quantities. We also study these properties for different types of distinct magnetic features, such as micropores, bright points, ribbons, flowers, and strings.

Results. We present the first direct observations of a small-scale granular magneto-convection pattern within extended regions of a strong (more than 600 G on average) magnetic field. Along the boundaries of the flux concentrations, we see mostly downflows and asymmetric Stokes V profiles, consistent with synthetic line profiles calculated from MHD simulations. We note the frequent occurrence of bright downflows along these boundaries. In the interior of the flux concentrations, we observe an up/down flow pattern that we associate with small-scale magnetoconvection, appearing similar to that of field-free granulation but with scales 4 times smaller. Measured rms velocities are 70% of those of nearby field-free granulation, even though the average radiative flux is not lower than that of the quiet Sun. The interiors of these flux concentrations are dominated by upflows.