Sunspot Bright Rings: Evidence from Case Studies

We present evidence, from both the Precision Solar Photometric Telescope and the Advanced Stokes Polarimeter, for a ring of enhanced continuum intensity surrounding large isolated sunspots. We do not attempt to evaluate the frequency of the phenomenon based on a large sample of spots but instead concentrate on illustrative best-case examples. The rings are about 0.5%-1.0% brighter in red and blue continuum (10 K warmer) than the surrounding photosphere and extend about one sunspot radius outward from the outer penumbral boundary. Most of the excess radiation is not directly associated with the strongest regions of Ca II K emission surrounding the spots or with measurable vertical magnetic field when such measurements are available. Moreover, the temporal evolution of the Ca II K and continuum emission in the ring differ, with the continuum intensity evolving on a shorter timescale. This suggests a convective origin for the bright ring, although a role for weaker, more diffuse magnetic fields cannot be ruled out. While we have inferred that only about 10% of the radiant energy missing from the sunspot is emitted through the bright ring, even this small enhancement may be significant to our understanding of subsurface sunspot structure and energy transport.