Hanle diagnostics of solar magnetic fields : the Sr ii 4078 Å line

The Hanle depolarization and rotation effects in the Sr ii 4078 Å line have been explored with the instrumentation at IRSOL (Istituto Ricerche Solari Locarno) by recording the Stokes I , Q, and U line profiles with high spectral resolution and polarimetric accuracy in a large number of regions across the solar disk. From the extracted line parameters we have constructed “Hanle histograms” showing the statistical distributions of the Hanle rotation and depolarization effects. Comparison with theoretical calculations allow these histograms to be understood in terms of magnetic fields with a strength of about 5–10 G, which is similar to the field strengths previously found through analysis of Q/I Hanle depolarization in the Ca i 4227 Å line. While small-scale magnetic fields with spatially unresolved angular distributions contribute to the observed Hanle depolarization effects, the observed Hanle rotation effects in Stokes U are due to spatially resolved fields with net large-scale orientations (e.g. global or canopy-type fields). We have also for the first time determined empirical “Hanle efficiency profiles”, derived independently for the Hanle rotation and depolarization effects. They show how the Hanle efficiency has its maximum in the Doppler core of the line and then rapidly decreases to become zero in the line wings.