Ionospheric Response to Electric Fields - The Earth's Ionosphere: Plasma Physics and Electrodynamics - page 446

ion temperature and ion velocity can be seen. Keep in mind, however, that such
a correlation can occur only when the neutral gas velocity is much smaller than
the ion velocity. At high latitudes the same ion-neutral collisions that produce
frictional heating of the ion gas also tend to set the neutral gas in motion, leading
to an equalization of the ion and neutral velocities in a steady state. When this
occurs, the ion gas can be moving quite rapidly with almost no frictional heating.
Figure 9.11 shows such an event where the ion temperature maintains its equi-
librium value throughout a region of antisunward flow where the ion velocity is
in excess of 1 km/s. In this example, simultaneous measurement of the neutral
gas velocity shows that it is also moving at high speed and in the same direction
as the ions. The momentum transfer between the plasma and the neutral gas is
described in more detail in Section 9.3. As we shall see, if the high-latitude ion
convection pattern remains stable for several hours, then the ion-neutral colli-
sions can, in fact, set up a similar convection pattern in the neutral gas. However,
if we see evidence of frictional heating in the high-latitude polar cap region, as
shown in Fig. 9.10, then it is reasonable to conclude that the observed ion con-
vection pattern has been established within the last hour or two and the neutrals
are not yet moving with the same velocity as the ions.

DE-2 Orbit

Date

1161
81294

Ions

500 m/s

Ap
33

Kp
5

Neutrals

1

4000

T e

T i

2000

(K)

T n

0
1.E9
1.E8
1.E7
200
100

n(0)

1.E7

(cm 23 )

n(N 2 )

1.E6

ne

1.E5

ni

(Mins)

0

UT
ALT
LAT

8733
367
275.2

8983
337
288.1

9233
327
271.4

9483
339
254.6

9733
373
237.8

(secs)
(km)
(deg)

Figure 9.11 Under stable conditions the ion and neutral velocities can become compa-
rable, as observed in the polar cap here by DE-2. Then, little Joule heating occurs, even
when the ion velocity is large. [After Killeen et al. (1984). Reproduced with permission
of the American Geophysical Union.]