Wind observations of mixed magnetosheath‐plasma sheet ions deep inside the magnetosphere

We have analyzed in detail a Wind spacecraft crossing of the high‐latitude magnetosheath, magnetopause, lobe, and the high‐ and low‐latitude plasma sheet on November 27, 1998. The crossing occurred during the first perigee pass of Wind's (new) high‐inclination (45°) petal orbit. Between the lobe and the hot plasma sheet on the duskside, an extended region ( x GSM ∼ −2.8 to −7.8 R E , y GSM ∼ 7.8 R E , z GSM ∼ −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 strong (∼ 40 nT) and steady lobe‐like magnetic field and ceased to exist when the spacecraft approached the neutral sheet. The mixed ion populations are remarkably similar (in their thermal properties) to the distributions detected in the adjacent magnetosheath and plasma sheet proper, except that 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 high‐energy component is unmodified. Near the neutral sheet the ion distributions are dominated by a single, high‐energy population. The electron behavior is significantly different from that of the ions. In the mixed ion region the electrons consist of a single population with energy between those of the magnetosheath and the plasma sheet proper. Electron pitch angle information suggests that the entire mixed region is on closed field lines. However, the large reduction of high‐energy plasma sheet electrons in this region 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 discerned with single‐point observations, especially since the mixed ions are stagnant. However, it is possible (for this event) to rule out the mantle as a source for the low‐energy component of the mixed ions. The similarity between the thermal property of the low‐energy component of the mixed ions and the magnetosheath population suggests that the magnetosheath plasma had direct access to this region without significant heating along its path. The presence of nearly unmodified magnetosheath plasma deep inside the magnetosphere raises questions concerning the processes of plasma entry across the magnetopause. Finally, the mixed region in this event was detected during an extended period of persistent northward and duskward interplanetary magnetic field, which makes this event ideal for model comparisons.

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