A new concept for modeling a planetary magnetosphere is implemented by juxtaposing the sunward portion of the magnetosphere (in which the magnetic field B is expressed formally as an analytical expansion) and the tail region (in which field lines are constructed geometrically). The expansion coefficients for the sunward region are determined by means of a variational principle, according to which a user-specified linear combination of the mean-square normal component of B on the prescribed magnetopause and the mean-square tangential component of B on the innermost cross-magnetospheric surface normal to the tail field lines must be minimized. The contour of vanishing normal component of B on this latter surface corresponds to the inner edge of the neutral sheet in the planetary magnetotail. In its present form the source-surface model accommodates an arbitrary angle of incidence between the solar-wind velocity and the planetary magnetic-dipole moment. ¿ American Geophysical Union 1987