OBSERVATIONS OF PLASMAS AND MAGNETIC-FIELDS IN EARTHS DISTANT MAGNETOTAIL - COMPARISON WITH A GLOBAL MHD MODEL

We are reporting the first direct comparison of in situ observations o f plasmas and magnetic fields in Earth's distant magnetotail with the results of a time-dependent, global magnetohydrodynamic (MHD) simulati on of the interaction of the solar wind with the magnetosphere. The ma gnetotail observations of were taken with the Geotail spacecraft durin g the period 0300-0630 UT on October 27, 1992 at a position near the d awnside magnetopause at a downstream distance of about 81 R(E). During this period a dense, cold ion stream similar in density and speed to that expected for the magnetosheath plasmas was intermittently observe d. When the cold ion stream was not present, the spacecraft was locate d in the northern magnetotail lobe. The dense, cold ion stream differe d from that expected for the magnetosheath in the Y and Z components o f ion bulk flow and in the Y component of the magnetic field. These co ld ion streams are associated with a magnetopause accommodation region positioned just outside the classical magnetopause, as identified by a well-defined transition from magnetic fields typical of those found in the lobe to the lesser and more fluctuating fields in the magnetosh eath. This accommodation region exhibits perturbations in plasma flows and magnetic fields that appear to be related to the complex topology of the magnetopause at these large downstream positions. Simultaneous observations of the solar wind ions and the interplanetary magnetic f ield (IMF) with the IMP 8 spacecraft upstream from Earth provided the driving input for a global MHD model. The solar wind ion flow was stea dy during this period, and the IMF exhibited a series of rotations fro m northward to duskward. The dynamics of the magnetotail were controll ed by the Y and Z components of the IMF. When this By was strongly pos itive, the magnetotail lobe appeared at the downstream Geotail positio n. Examination of the modeled plasma parameters in the Y-Z plane throu gh the spacecraft position shows that this By provides a torque on the magnetotail about its central axis. The MHD model also accurately pos itions the spacecraft alternately in the magnetopause accommodation re gion and the magnetotail lobe as the IMF clock angle varied from north ward to duskward, respectively. The temporal variations of modeled par ameters, i.e., ion densities, temperatures, and bulk flow velocities a nd the magnetic field components, are directly compared with the Geota il measurements. This first comparison of the Geotail observations wit h the modeled plasma parameters and magnetic fields provides substanti al encouragement that a global MHD model can provide a valid descripti on of important aspects of the large-scale topology and dynamics of th e magnetotail.