Dynamics and Electrodynamics of the Equatorial Zone - The Earth's Ionosphere: Plasma Physics and Electrodynamics - page 91

Sq currents

90

60

255

30

0

228

230

x

2 60

2 90 00

03 06 09 12 15 18 21 24

Local time

Figure 3.14 Average contours of vertical magnetic field in nT due to the Sq system mea-
sured during the International Geophysical Year. [After Matsushita (1969). Reproduced
with permission of the American Geophysical Union.]

The diurnal variations of the electric field components measured at Jicamarca
have already been pointed out in Section 3.1. The global vertical magnetic pertur-
bation pattern measured by ground-based magnetometers is shown in Fig. 3.14
and gives further evidence for this simple picture of the electric field. Such data
have been used to construct the pattern of electrical currents in the entire iono-
sphere that is referred to as the Sq current system. (S stands for solar and q for
quiet in this notation.) As noted in Chapter 2, ground magnetometers respond
primarily to horizontal Hall currents, since the magnetic field due to field-aligned
currents is canceled out by the Pedersen currents that link the field lines horizon-
tally through the ionosphere. Since the electric field must be nearly the same at
both ends of a magnetic field line, any differences between the two hemispheres
create currents that flow between the hemispheres to maintain the field lines as
equipotentials. However, to first order, at equinox the tidal modes are symmet-
rical and the field-aligned currents are therefore minimal. In this case the Sq
pattern due to Hall currents represents the true total currents in the ionosphere
quite well.
Although the Sq current deduced from data such as those shown in Fig. 3.14
flows primarily on the dayside (where the conductivity is high), this does not
mean that the electric field is confined to the dayside. In fact, the charge density
that builds up at the terminators to force

0 creates the diurnal zonal
electric field pattern observed at Jicamarca. In this chapter we consider the effect
of the zonal electric field at the equator. The effect of the E-region dynamo at
midlatitudes is discussed in Chapter 5.
Typical plasma densities and conductivities at the dayside equator are shown
in Fig. 3.15. The zonal electric field component will drive a small Pedersen current

∇· J =