Dynamics and Electrodynamics of the Equatorial Zone - The Earth's Ionosphere: Plasma Physics and Electrodynamics - page 94

21.111 Downleg - - Alcåntara, Brazil
Vertical DC electric field

Electron drift velocity

Current density Plasma density

120

115

J/qN e

110

105

100

E/B

95

90
2

10 4 10 5

cm 23

10 5 5 0 5 0 5 024

A/km 2

68 0 5 3 1 3 2 3 0 100 200 300 400 500

(Down) mV/m (Up) m/s

(d)

(a) (b) (c)

Figure 3.17b Electric and magnetic field and plasma density measurements along with
the derived current density and electron drift measured over the magnetic equator. [After
Pfaff et al. (1997). Reproduced with permission of the American Geophysical Union.]

where σ c is the so-called Cowling conductivity. Notice that the local neutral

wind does not enter this calculation at all; the electrojet is set up by the global
tidal winds that create the diurnal zonal electric field component measured at
the equator. In a more complete theory, complications due to the zonal neutral
wind may be included, since it will drive an ion current equal to u. Note that the
meridional wind component does not enter at the magnetic equator as a local
dynamo, since for that component the cross product with the magnetic field
vanishes.
In effect, (3.14ab) shows that the zonal conductivity is enhanced by the large
factor 1

2 2

P , the Cowling conductivity factor, which leads to the intense
current jet at the magnetic equator. This can be seen in Fig. 3.14, in which
the magnetic field contours become very close together at the magnetic equator.
This channel of electrical current is termed the equatorial electrojet. The Cowling
conductivity is also plotted in Fig. 3.15 (divided by 100) and displays a peak at
102 km with a half-width of 8 km.
There have been many rocket measurements of the magnetic field due to the
electrojet current. Some of the data are reproduced here in Fig. 3.17a. The plot
displays the altitude variation of the jet over Peru with each profile normalized to
a 100 nT variation of the magnetic field measured on the ground at Huancayo,
Peru; that is, the actual perturbed magnetic field at Huancayo during each rocket
flight was used to scale all data to a common ground perturbation of 100 nT.
Of more importance to the present text is the generation of large vertical
electric fields in the electrojet, an example of which is presented in Fig. 3.17b,

+ σ

H /σ