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In addition to downward accelerated electrons, also upward flowing ions are common. Some of most energetic ion beams may be generated by similar upward directed electric fields. Moreover, there are also clear indications of downward directed electric fields producing upward (downward) accelerated electron (ion) beams.
The easiest way to observe the quasi-static, field-aligned potential drops is an indirect way. Auroral arcs have been related to large, perpendicular electric field structures, electrostatic shocks, observed mostly at geocentric distances 1.4 - 1.8 Re (9000 - 12000 km altitude; Mozer et al., 1977; Torbert and Mozer, 1978; Mozer et al., 1980; Weimer and Gurnett, 1993). These indicate the precense, at lower altitudes, of field-aligned potential drops that accelerate the precipitating electrons.
For recent direct measurement of E(parallel), see Mozer and Kletzing (1998).
It is generally thought that electric double layers may play a role in producing the field-aligned electric fields. Many small double layers and other solitary structures may contribute to the total potential drop along the field line (Block and FДlthammar, 1990). However, although double layers are asymmetric and correspond to net potential drops individually, statistically one seems to gain a zero potential drop!
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