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

TL;DR

This study models a sunspot atmosphere with open magnetic fields to explain observed reduced microwave brightness, linking magnetic topology to plasma properties and solar wind origins.

Contribution

It introduces a new atmosphere model incorporating open magnetic field lines to explain microwave brightness reductions in sunspots.

Findings

Model reproduces observed microwave brightness reduction.

Open magnetic fields correlate with cooler, denser plasma regions.

Method can be applied to other sunspots for solar wind studies.

Abstract

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.