IONOSPHERIC RESPONSE TO MAGNETIC FORCING - MAGNETIC CLOUD PASSAGE OF OCTOBER 18-20, 1995

On October 18-20, 1995 a magnetic cloud was observed, first by the WIN D spacecraft upstream in the solar wind, then at the Earth. Inside the cloud, plasma density, velocity and temperature, and magnetic field s trength were relatively steady for almost 24 hrs. However, the magneti c 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-v arying conditions. This paper reports on the nightside ionospheric res ponse to a long interval with B-z < 0 in the interplanetary medium. Te mporal variations in the integrated energy deposited in the nightside auroral Bone within 3 hrs of local midnight are similar to variations in epsilon, the energy coupling function, and dissimilar to auroral ma gnetograms. While substorm effects are observed in the ionospheric ele ctron flux, the integrated energy is dominated by diffuse precipitatio n at low latitudes. We also find that the integrated energy curve lags the epsilon parameter by 1-2 hrs. This delay represents the time requ ired for convective electric fields to heat particles adiabatically in the plasma sheet.