Observations of auroral precipitation characteristics (and the resulting ionospheric ionization profiles) have shown that, within discrete auroras, field-aligned acceleration of the precipitating electrons plays an important role (inverted-V electron precipitation). It seems likely that upward directed electric fields are formed to create this effect (e.g., Weimer and Gurnett, 1993), although different wave-related schemes have also been suggested (e.g., Bryant et al., 1991). The problem is that it is not clear how the magnetic field-aligned electric fields are supported in the magnetospheric plasma.
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 Fälthammar, 1990). However, although double layers are asymmetric and correspond to net potential drops individually, statistically one seems to gain a zero potential drop!
- Block, L. P. and C.-G. Fälthammar, The role of magnetic-field-aligned electric fields in auroral acceleration, J. Geophys. Res., 95, 5877-5888, 1990.
- Bryant, D. A., A. C. Cook, Z.-S. Wang, U. de Angelis, and C. H. Perry, J. Geophys. Res., 96, 13829-13839, 1991.
- Mozer, F. S., C. W. Carlson, M. K. Hudson, R. B. Torbert, B. Parady, J. Yatteau, and M. C. Kelley, Observations of paired electrostatic shocks in the polar magnetosphere, Phys. Rev. Lett., 38, 292, 1977.
- Mozer, F. S., C. A. Cattell, M. K. Hudson, R. L. Lysak, M. Temerin, and R. B. Torbert, Satellite measurements and theories of low altitude auroral particle acceleration, Space Sci. Rev., 27, 155-213, 1980.
- Mozer, F. S., andd C. A. Kletzing, Direct observations of large, quasi-static, parallel electric fields in the auroral acceleration region, Geophys. Res. Lett., 25, 1629-1632, 1998.
- Torbert, R., and F. S. Mozer, Electrostatic shocks as the source of discrete auroral arcs, Geophys. Res. Lett., 5, 135, 1978.
- Weimer, D. R. and D. A. Gurnett, Large-amplitude auroral electric fields measured with DE 1, J. Geophys. Res., 98, 13557-13564, 1993.