Magnetograms

The Zeeman effect states that the emission from an atom in an external magnetic field will be split into several components. The shift of these components from the normal wavelength of emission is directly proportional to the field strength. Viewed along the magnetic field, these shifted components are left- (L) and right- (R) circularly polarised. Hence, by imaging the L and R circularly polarised components, both the Stokes I (=R+L) and Stokes V =(R-L) signal can be easily calculated for each pixel. The degree of polarisation, P (=V/I), is directly proportional to the magnetic field.

For this dataset, both circularly polarised components of an Fe I line (λ = 6302.5 Å) were imaged in the UBF cycle. This was achieved by initially passing the light through a λ/4 plate in order to change the polarisation from circular to linear. Then a linear polariser only accepts light polarised parallel to its optical axis. Reversing the λ/4 retardation will change the direction of the two polarised components. Unfortunately the time delay (a few secs) between the acquisition of L and R images means that large scale seeing effects dominate the data. These can be reduced by firstly destretching the data, aligning to sub-pixel accuracy, then combining the R and L images, integrating over several of these combinations and applying a spatial smoothing. This meant that it took ~ 30 minutes of data to create one magnetogram, and hence time series analysis of magnetic field information was not possible. However it was possible to identify photospheric magnetic features with increased intensity in the chromosphere Chapter 4).