Solitary potential structures associated with ion and electron beams near

Small-scale solitary electric potential structures are commonly observed on auroral field lines with the Polar Electric Field Instrument (EFI). This study focuses on observations of solitary structures in the southern hemisphere auroral zone at altitudes between 5500 and 7500 km. Some of the potential structures are similar to those observed previously by the S3-3 and Viking satellites and are inferred to be negative potential pulses traveling upward along the auroral magnetic field lines, associated with upgoing ion beams and upward currents. The velocities of these “ion” solitary potential structures are estimated, using spaced EFI measurements, to be distributed within the range of ∼75 – 300 km s−1. In addition to these structures, a different type of solitary potential structure with.opposite polarity has been observed with faster propagation velocities. These faster structures (termed “electron” solitary potential structures) are distinguishable from the slower, ion solitary structures in that their distinctive bipolar electric field signature, common to both types of solitary structure, is reversed. The ultimate distinction for the electron solitary potential structures is that they are observed on auroral field lines in conjunction with magnetically field-aligned upflowing electron beams. The electron solitary potential structures propagate up the field line in the same direction as the electron beam. An example is shown of the polarity reversal from ion to electron solitary potential structures coincident with a simultaneous shift from upgoing ion beams to upgoing electron beams.

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