Drawing defining the regions of the parameter space explored. The red line indicates the circular models analyzed in Sect. 3. The MC boundary is elliptical for the blue region and is deformed to a bean shape in the green region. The two blue lines indicate the elliptical models analyzed in Sect. 4. Finally, the purple line indicates an extreme case where the MC boundary is rectangular. n defines the axial electric current and magnetic field component (Eqs. (3), (4)). A cross section elongated orthogonally to the spacecraft trajectory has b > 1 (Fig. 2).