Ionospheric electric field and ground magnetic field measurements at high, middle and low latitudes are used to study the global pattern of the convection electric field on March 26, 1979. The IMF Bz component was southward and relatively steady over a prolonged period of time. The data set is thus representative of a steady magnetospheric convection. A semi-analytical time-dependent convection model is used to interpret the observations. During the daytime the model steady state results roughly reproduces the mean observed convection pattern. Fluctuations of the observed electric field from the model predictions are interpreted as resulting from (1) the transient responses of the ionosphere to small time variations of the IMF, (2) substorm effects, (3) neutral wind disturbances. Only the first type of fluctuations can be reproduced by the initial state prediction of the convection model. ¿American Geophysical Union 1987