Reduced Microwave Brightness Temperature in a Sunspot Atmosphere due to Open Magnetic Fields

Arturs Vrublevskis, Ventspils International Radio Astronomy Centre (VIRAC)

Motivated by dark coronal lanes in SOHO / EIT 284 Å EUV and Yohkoh / SXT soft X-ray observations we construct an atmosphere model of the AR 8535 sunspot by adding a cool and less dense component in the volume of plasma along open field lines which are determined using the Potential Field Source Surface (PFSS) method. Our model matches the observed reduced microwave brightness temperature in the northern part of the sunspot in the VLA observations from May 13, 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 radio observations to determine the significance of open field regions. The connection between open fields and the resulting plasma temperature and density decrease is of relevance for slow solar wind source investigations.