Solar magnetic fields derived from hydrogen alpha filtergrams

The distribution and polarity of solar magnetic fields can be inferred from photographs taken with narrow band‐pass filters centered on the Hα spectral line. Structures observed in Hα, including filaments, filament channels, fibrils, arch‐filament systems, and plage corridors are used to infer positions of neutral lines in the radial component of the solar magnetic fields. This source of magnetic‐field information, although it is not strictly objective and quantitative, offers a number of important advantages over the solar magnetograph. The inferred data can be obtained with the existing global network of flare patrol observatories at low cost, high speed, and complete reliability from day to day. Patrol films allow the filling of interruptions in magnetograph data and the extension of the magnetic‐field data base back in time to the first spectroheliograms near the beginning of the century. The large‐scale Hα patterns are more precisely outlined than their large‐scale counterparts on magnetograms. The continuity of Hα markers of polarity reversals from active centers to quiescent, large‐scale structures reveals associations and patterns not previously studied. The inferred magnetic data have important applications in solar‐activity forecasting, detection of interplanetary‐sector boundaries, and the inference of structures in the solar corona.