OBSERVATIONS OF SUBARCSECOND BRIGHT DOTS IN THE TRANSITION REGION ABOVE SUNSPOTS WITH THE INTERFACE REGION IMAGING SPECTROGRAPH

Abstract

Observations with the Interface Region Imaging Spectrograph (IRIS) have\nrevealed numerous sub-arcsecond bright dots in the transition region above\nsunspots. These bright dots are seen in the 1400\\AA{} and 1330\\AA{} slit-jaw\nimages. They are clearly present in all sunspots we investigated, mostly in the\npenumbrae, but also occasionally in some umbrae and light bridges. The bright\ndots in the penumbrae typically appear slightly elongated, with the two\ndimensions being 300--600 km and 250--450 km, respectively. The long sides of\nthese dots are often nearly parallel to the bright filamentary structures in\nthe penumbrae but sometimes clearly deviate from the radial direction. Their\nlifetimes are mostly less than one minute, although some dots last for a few\nminutes or even longer. Their intensities are often a few times stronger than\nthe intensities of the surrounding environment in the slit-jaw images. About\nhalf of the bright dots show apparent movement with speeds of\n$\\sim$10--40~km~s$^{-1}$ in the radial direction. Spectra of a few bright dots\nwere obtained and the Si~{\\sc{iv}}~1402.77\\AA{} line profiles in these dots are\nsignificantly broadened. The line intensity can be enhanced by one to two\norders of magnitude. Some relatively bright and long-lasting dots are also\nobserved in several passbands of the Atmospheric Imaging Assembly onboard the\nSolar Dynamics Observatory, and they appear to be located at the bases of\nloop-like structures. Many of these bright dots are likely associated with\nsmall-scale energy release events at the transition region footpoints of\nmagnetic loops.\n