Configuration of the magnetic field after 19 days of evolution using the model of Mackay & van Ballegooijen (2006a). The figure shows a sample of magnetic field lines that illustrate the structure of the magnetic bipoles, the overlying arcade, and the newly formed magnetic flux rope along the PIL of the left-hand side bipole. The surface boundary is coloured according to the polarity of the magnetic field: negative polarity is black and positive is white.