Ionospheric Response to Electric Fields - The Earth's Ionosphere: Plasma Physics and Electrodynamics - page 458

4000

3000

102 km/sec
≅ 7.4 keV

Motion of the upp er
end of Ba 1 - jet // B

2000

1000

10 km/sec ≅ 71eV

0 0

Onset of
aurora

150

100

Electric field ⊥ B

50

0 0

200 400 600 800 1000

Time after injection (sec)

Figure 9.20 Upward velocity of the upper end of the barium ion jet (January 11, 1975)
(upper panel) and transverse electric field projected to the 100 km level as derived from
the low altitude portion of the ion jet (lower panel).

injection. The velocity change corresponds to passage through an acceleration
zone where E points upwards antiparallel to B in the Northern Hemisphere.
Such fields are unexpected due to the high conductivity parallel to the magnetic
field. Early theories appealed to anomalous resistivity supported by ion acoustic
or ion cyclotron waves. The idea was that the electrons would collide with waves
rather than other particles, creating an effective collision frequency,

ν ∗ . Then

σ ∗ =

|| /σ 0 to become large. This is an
attractive idea since both ion cyclotron and ion acoustic waves are unstable
to large J

ne 2 ν ∗ would decrease, allowing E

/ m

|| =

J

(Kindel and Kennel, 1971). Another attraction was that the plasma
density decreases faster than does the area of the magnetic flux tube. Thus,
for a given J

||

neV d , where V d is the parallel differential drift of ions and
electrons, V d can become quite large a few thousand kilometers above the auroral
ionosphere. When V d exceeds the electron thermal speed, the plasma is unstable;
that is, for

|| =

J

||
ne >

V th
e

V d = (9.15)