It is unfortunate that coronal magnetic fields cannot be easily measured, even in the case of the Sun. Except for a few measurements of magnetic fields in the transition region above sunspots, made using the conventional Zeeman effect, and except for the possibility of inferring the direction – not the intensity – of coronal magnetic fields using optical forbidden lines, direct measurements of coronal fields are virtually non-existent. The most promising method appears to be the use of the Hanle effect, i.e. the modification of polarization characteristics of spectral lines induced by magnetic fields. This method has been proposed for future space missions in solar physics, for instance for the European satellite SOHO, but its feasibility depends on the strength of the fields to be measured, which in any case must be higher than a few tens of Gauss.
Coronal Magnetic Fields