A magnetic field model with a linearly superposed uniform external field is used to study the effects of the magnetotail field intensity BT on the relationship between the convection pattern in the polar cap and the interplanetary magnetic field (IMF). It is found that as BT increases, the polar cap size increases, overall electric field intensity decreases, and the dependence of the total ''open'' flux on the magnitude and direction of the IMF weakens. In addition, the tail field introduces significant differences between the convection patterns corresponding to ''toward'' and ''away'' IMF sectors, respectively: in the northern (southern) polar cap, features such as polar cap boundary displacements and electric field intensity gradients are more pronounced when the earth is in an ''away'' (''toward'') sector. On the other hand, in the case of a northward-tilted IMF, sunward convection in the north (south) polar cap is extended over a larger area for a ''toward'' (''away'') sector. Since dipole tilt was not taken into account in the model, these results should mainly apply to periods around equinox.