It is well known that the dawn-dusk component of the interplanetary magnetic field (IMF) creates asymmetries in the magnetotail. The main effects are a weak nonuniform penetration of IMF By into the tail and a rotation of the whole tail current sheet. In this study we modify the Tsyganenko (1989) model to simulate the IMF By induced tail effects. By field line mapping we examine how different regions of the magnetosphere and ionosphere are influenced by the IMF By tail effects. The plasma sheet is shown to rotate around the central axis of the magnetotail. The mapping from the plasma sheet to the ionosphere for nonzero IMF By shows a change in the geometry of the auroral oval. The whole oval rotates, a local bulge occurs near midnight and the thickness of the oval is different in the morning and the evening sector. Simple geometrical considerations show that the tilting of the field lines caused by IMF By leads to a rotation of the oval, whereas the rotation of the tail current sheet is reponsible for the thickness variations along the oval. These results fit very well with the observed auroral distribution for an IMF with a dawnward or duskward direction. ¿ American Geophysical Union 1996