Wind observations of mixed magnetosheath-plasma sheet ions deep inside themagnetosphere

We have analyzed in detail a Wind spacecraft crossing of the high-latitude magnetosheath, magnetopause, lobe, and the high- and low-latitude plasma sh eet on November 27, 1998. The crossing occurred during the first perigee pa ss of Wind's (new) high-inclination (45 degrees) petal orbit. Between the l obe and the hot plasma sheet on the duskside, an extended region (x(GSM)sim ilar to -2.8 to -7.8R(E), y(GSM)similar to 7.8 R-E, z(GSM)similar to -11.6 to -4.3 R-E) of mixed low-energy magnetosheath and high-energy plasma sheet ion populations was detected. This region was detected in a region of stro ng (similar to 40nT) and steady lobe-like magnetic field and ceased to exis t when the spacecraft approached the neutral sheet. The mixed ion populatio ns are remarkably similar (in their thermal properties) to the distribution s detected in the adjacent magnetosheath and plasma sheet proper, except th at the mixed ions are nearly stagnant. As the neutral sheet is approached, the low-energy component of the mixed ions is gradually heated, while the h igh-energy component is unmodified. Near the neutral sheet the ion distribu tions are dominated by a single, high-energy population. The electron behav ior is significantly different from that of the ions. In the mixed ion regi on the electrons consist of a single population with energy between those o f the magnetosheath and the plasma sheet proper. Electron pitch angle infor mation suggests that the entire mixed region is on closed field lines. Howe ver, the large reduction of high-energy plasma sheet electrons in this regi on may indicate that the field lines threading this regions were once open. The exact path of plasma entry to form the mixed region cannot be discerne d with single-point observations, especially since the mixed ions are stagn ant. However, it is possible (for this event) to rule out the mantle as a s ource for the low-energy component of the mixed ions. The similarity betwee n the thermal property of the low-energy component of the mixed ions and th e magnetosheath population suggests that the magnetosheath plasma had direc t access to this region without significant heating along its path. The pre sence of nearly unmodified magnetosheath plasma deep inside the magnetosphe re, raises questions concerning the processes of plasma entry across the ma gnetopause. Finally, the mixed region in this event was detected during an extended period of persistent northward and duskward interplanetary magneti c field, which makes this event ideal for model comparisons.