Title: Structure of sunspot light bridges in the chromosphere and transition region
by Dr. Reza Rezaei,
Light bridges are elongated structures with enhanced intensity embedded in sunspot umbra and pores. Using IRIS spectral data, we study properties of a sample of 60 light bridges in the chromosphere and transition region. Traces of photospheric light bridges are found in the near ultra violet Mg II h and k lines as well as in the far ultra violet emission lines of the O I, C II, Si IV and O IV. These set of spectral lines cover the entire chromosphere and lower part of the transition region. We created a sample of mean values for the continuum and line intensities, line shifts and widths. A majority of photospheric light bridges in our sample have an associated enhanced emission in the chromosphere and transition region. In most cases, morphology of light bridges in the chromosphere and transition region are similar to the photospheric light bridge such that a 1-1 correspondence can be established. We measured a systematic shift between position of the light bridges in the transition region and photosphere, which illustrates the three-dimensional structure of light bridges, where the Si IV signal originates from about 3.5 Mm above the photosphere. The line widths increases with the formation temperature in the light bridge atmosphere, similar to quiet Sun. The line profiles have distinctly different shapes in the light bridge and nearby umbra suggesting that the umbral scattered light does not play an important role. Light bridges show red shift in the chromosphere and the transition region. We find the observed co-temporal and co-spatial emission enhancements above photospheric light bridges is a systematic phenomena suggesting that light bridges extend well beyond the photospheric boundary.