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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 field-aligned potential drops (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 necessary magnetic field-aligned electric fields are supported in the
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,
- 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.