1. High Altitude Observatory, National Center for Atmospheric Research, PO Box 3000, Boulder, Colorado 80307, USA
2. National Solar Observatory, Sunspot, New Mexico 88349, USA

Correspondence to: Correspondence and requests for materials should be addressed to M.P.R. (e-mail: Email: mprast@ucar.edu).

Abstract

There are two possible explanations for why sunspots are dark: the partial suppression by the sunspot magnetic fields of convective energy transport from the underlying layers¹, or the removal of energy from the sunspot by enhanced hydromagnetic wave radiation². Both processes would reduce the energy emitted radiatively. The first explanation is currently favoured³, and predicts that the blocked energy should show up as a bright ring around the spot², with the actual brightness of the ring sensitive to details of solar convective transport and sunspot structure⁴. Previous searches⁵ for these bright rings were inconclusive because of the presence of bright, vertical magnetic flux tubes near the spots, and a lack of sufficient precision in the observations. Here we report high-photometric-precision observations of bright rings around eight sunspots. The rings are about 10 K warmer than the surrounding photosphere and extend at least one sunspot radius out from the penumbra. About 10% of the radiative energy missing from the sunspots is emitted through the bright rings. We also report observations of a second set of sunspots, for which simultaneous magnetic field measurements demonstrate that the rings are not associated with vertical flux tubes.