The three-dimensional structure of atmospheric magnetic fields in two active regions

The magnetic field above two unrelated active regions on 11 and 12 September, 1974 has been studied using magnetograms obtained in C I 9111, Fe I 8688, Ca II 8542, and H. In C I 9111, originating low in the photosphere, the fields are strong and sharply defined. In Ca II 8542 and H they are very diffuse, with significant diffuseness also in Fe I 8688, due to the spreading of the field with height to form almost horizontal magnetic canopies over regions free of field at lower levels.Within a region between two small sunspots some 140 Mm apart, the canopy height found is typically 300–400 km. Within a small superpenumbra, the canopy height is 150–250 km. In extensive areas surrounding the active regions, over one-half the canopy bases are less than 400–500 km above the _c = 1 level, and over 80% less than 700 km.Arguments are given that the chromospheric fibrils (e.g., in H), taken to delineate the field configuration, are not due primarily to lateral variations in field but rather to differences in density or excitation of gas across the lines of force.Operated by the Association of Universities for Research in Astronomy, Inc., under contract with the National Science Foundation.