Effect of the density perturbation inside a vertical magnetic flux tube F_T (indicated by the purple stub) in a stellar convection zone. For the purpose of evaluating the outer stellar gravitational potential, the mass deficit inside the tube is equivalent to adding two point masses m_A at the pointsA and A′ on a plane perpendicular to the tube axis here taken to be coincident with the y axis. The x-axis of our reference frame is taken along the line joining the two masses that are symmetrically situated with respect to the barycenter of the star O at the abscissas ± x_A. These two masses produce a gravitational potential outside the star that is no longer spherically symmetric and can be described by a quadrupole term. It depends on the distance OP = r of a given point P from the barycenter O and the angle α between the directions OP and OA of the line joining the two point masses (see the text).