  Motivated by dark coronal lanes in SOHO / EIT 284 {\AA} EUV observations we
construct and optimize an atmosphere model of the AR 8535 sunspot by adding a
cool and dense component in the volume of plasma along open field lines
determined using the Potential Field Source Surface (PFSS) extrapolation. Our
model qualitatively reproduces the observed reduced microwave brightness
temperature in the northern part of the sunspot in the VLA observations from 13
May 1999 and provides a physical explanation for the coronal dark lanes. We
propose application of this method to other sunspots with such observed dark
regions in EUV or soft X-rays and with concurrent microwave observations to
determine the significance of open field regions. The connection between open
fields and the resulting plasma temperature and density change is of relevance
for slow solar wind source investigations.
