Upwelling O^+ ion source characteristics

Recent observations from the Dynamics Explorer 1 (DE 1) spacecraft have shown that the dayside auroral zone is an important source of very low‐energy superthermal O + ions for the polar magnetosphere. When observed at 2000‐ to 5000‐km altitude, the core of the O + distribution exhibits transverse heating to energies on the order of 10 eV, significant upward heat flux, and subsonic upward flow at significant flux levels (exceeding 10 8 cm −2 s −1 ). The term “upwelling ions” has been adopted to label these flows, which stand out in sharp contrast to the light ion polar wind flows observed in the same altitude range in the polar cap and subauroral magnetosphere. We have chosen a typical upwelling ion event for detailed study, correlating retarding ion mass spectrometer observations of the low‐energy plasma with energetic ion observations and local electromagnetic field observations. The upwelling ion signature is colocated with the magnetospheric cleft as marked by precipitating energetic magnetosheath ions. The apparent ionospheric heating is clearly linked with the magnetic field signatures of strong field‐aligned currents in the vicinity of the dayside polar cap boundary. Electric field and ion plasma measurements indicate that a very strong and localized convection channel or jet exists coincident with the other signatures of this event. These observations indicate that transverse ion heating to temperatures on the order of 10 5 K in the 2000‐ to 5000‐km ionosphere is an important factor in producing heavy ion outflows into the polar magnetosphere. This result contrasts with recent suggestions that electron heating (to temperatures of order 10 4 K) is the most important parameter with regard to O + outflow.