Discovery of inward moving magnetic enhancements in sunspot penumbrae

¹ National Astronomical Observatories, Chinese Academy of Sciences, Beijing 100012, PR China e-mail: zjun@ourstar.bao.ac.cn
² Max-Planck-Institut für Sonnensystemforschung, 37191 Katlenburg-Lindau, Germany e-mail: [solanki;woch]@mps.mpg.de

Received: 9 May 2007
Accepted: 3 September 2007

Abstract

Context.Sunspot penumbrae show a fine structure in continuum intensity which displays considerable dynamics. The magnetic field, in contrast, although also highly structured, has been shown to be relatively static in earlier studies.

Aims.We report on the discovery of inward moving magnetic enhancements in the penumbrae of two regular sunspots, and characterize their fundamental properties.

Methods.Using continuum images, longitudinal magnetograms and Dopplergrams, recorded in high resolution mode by the Michelson Doppler Imager (MDI) instrument on the Solar and Heliospheric Observatory (SOHO), we have probed the evolution of magnetic features in the inner penumbra. The relationship between magnetic features, corresponding brightness in continuum images and Dopplergrams is also considered.

Results.Local enhancements of the line-of-sight (LOS) component of the magnetic field in the inner part of the penumbral region move inward towards the umbra-penumbra boundary with a radial speed of about 0.3 km s^-1. These local inward-moving enhancements of the LOS component of the magnetic fields appear to be relatively common. They are associated with dark structures and tend to display downflows relative to the penumbral background. We also confirm the presence of outward moving magnetic enhancments in the outer half of the penumbra. Possible explanations are discussed.