The magnetometer aligned along the spin axis of the Isis 2 satellite has been used to study magnetic field perturbations in the dawn-dusk sectors of the auroral oval and in the polar cap. The field perturbations have been compared with simultaneous particle measurements and with measurements of the interplanetary magnetic field (IMF). Northern hemisphere passes were selected during a period when the satellite orientation could be determined to an accuracy of about ¿0.1¿. In about 80% of the passes examined, there are significant and systematic deviations from a model field in the oval and in the polar cap. The magnitude of the perturbations, ΔB, in both the oval and the polar cap, is correlated with the z component of the IMF. The perturbations in the polar cap also suggest a dependence on the y component of the IMF, being larger on the morning side than on the afternoon side when By is positive and larger on the afternoon side than on the morning side when By is negative. The perturbations are discussed in terms of the tilt of the magnetic field direction, either toward or away from the sun, relative to the direction of the model field, and it is suggested that the tilt direction can be related to the direction of convection in the magnetosphere. When the poleward edge of the plasma sheet is compared with the latitude where the direction of the perturbation changes from sunward to antisunward, it is found that the former typically extends poleward of the latitude where ΔB changes sign. It is suggested that the region between the poleward edge of the plasma sheet and the latitude where ΔB changes sign is the low-altitude signature of the magnetospheric boundary layer.