On October 18--20, 1995 a magnetic cloud was observed, first by the WIND spacecraft upstream in the solar wind, then at the Earth. Inside the cloud, plasma density, velocity and temperature, and magnetic field strength were relatively steady for almost 24 hrs. However, the magnetic field direction rotated slowly from -Z to -Y and finally to +Z (GSE coordinates). This magnetic cloud passage allows us to study the solar wind-magnetosphere-ionosphere interaction under nearly ideal slowly-varying conditions. This paper reports on the nightside ionospheric response to a long interval with Bz<0 in the interplanetary medium. Temporal variations in the integrated energy deposited in the nightside auroral zone within 3 hrs of local midnight are similar to variations in &egr;, the energy coupling function, and dissimilar to auroral magnetograms. While substorm effects are observed in the ionospheric electron flux, the integrated energy is dominated by diffuse precipitation at low latitudes. We also find that the integrated energy curve lags the &egr; parameter by 1--2 hrs. This delay represents the time required for convective electric fields to heat particles adiabatically in the plasma sheet. ¿ 1998 American Geophysical Union