A simple polynomial expansion for the components of the magnetic field from the magnetospheric surface currents is derived, based on a solution of the boundary problem for a tilted magnetic dipole field confined within an axially symmetric oblong ellipsoidal cavity. The ellipsoidal representation of the magnetospheric boundary is in excellent agreement with its observed shape up to tailward distances of 30–40RE. The obtained representation of the boundary field can be easily adjusted to arbitrary values of the solar wind ram pressure by means of a simple scaling. A comparison with a paraboloid model is performed, using the fact that the paraboloidal surface can be obtained as a limiting case of the ellipsoidal one by a special choice of its parameters.