The hourly values of solar wind parameters and AE indices at times when the interplanetary magnetic field has a strong northward component are used to study dependence of AE on the value of the interplanetary magnetic field (B) and on the temporal trends of the solar wind velocity (V). In any randomly chosen statistical set of data, the dependence of AE on B is substantially modified by spurious effects related to the structure of the solar wind streams. In particular, the leveling off of AE at high values of B is spurious: d(AE)/dB is positive for all values of B. AE values corrsponding to negative dV/dt are lower than AE values corresponding to positive dV/dt. This effect is caused by physical processes involving the solar wind-magnetosphere coupling. It is suggested that slow variations of the magnetospheric structure, which are due to slow changes of the solar wind pressures upon the magnetospheric boundary, are likely cause of the amplification and damping of the MHD perturbations existing in the magnetosphere: they are also a likely cause of the observed dependence of AE on dV/dt. ¿ American Geophysical Union 1989