ON POTENTIAL-FIELD MODELS OF THE SOLAR CORONA

The current-free approximation has been widely used to infer the magnetic structure of the solar corona from magnetograph observations. In such calculations, the potential field is usually matched directly to the observed line-of-sight component of the photospheric field. However, this procedure is invalid because the magnetograph measurements apply to deep atmospheric layers where the magnetic field is nonpotential and (for observations made in the Fe I lambda-5250 line) nearly radial. A better approach is to match only the radial component of the potential field to the photospheric data (corrected for line-of-sight projection on the assumption that the field lines are radially oriented), while allowing a discontinuity in the tangential field components. The implied current sheet is a mathematical idealization of the finite "boundary layer" within which the rapidly diverging field lines make their transition to a potential configuration. This procedure yields much stronger polar fields than the conventional line-of-sight method and resolves a number of discrepancies between earlier potential field calculations and observations of polar coronal holes, the brightness distribution of the K-corona, and the shape of the interplanetary current sheet.