Hydro- and Electrodynamics of the Midlatitude Ionosphere - The Earth's Ionosphere: Plasma Physics and Electrodynamics - page 234

10 − 5 T, E c / B eq = B eq equals the zonal rotation speed of the earth at the

equator. To relate this field to the magnitude of the magnetospheric electric field,
we treat the magnetic field lines as equipotentials. Following the discussion in
Mozer (1970; see also Chapter 2), the meridional electric field component maps
from the ionosphere (I) to the magnetosphere (M) (and vice versa) as

3 . 1 ×

2 L L

1 / 2

3
4

E I / E M = − (5.19)

where L is the equatorial crossing distance of a dipole field line measured in earth
radii. In Fig. 5.6 the corotation field from (5.18) is plotted, along with dashed
lines giving the ionospheric electric field associated with two different magni-
tude magnetospheric source fields of 1 and 0.4mV/m, mapped to the ionosphere
using (5.19). In comparing the magnitudes and effects of these two electric field
sources (rotation versus solar wind), it is important to note that the relevant
reference frame is the one fixed with respect to the sun. In that frame the solar
wind blows by and interacts with the magnetosphere, creating the electric field
of magnetospheric origin discussed above. The earth rotates in that reference
frame, generating the corotation electric field we refer to. The plasma at the
equatorial plane E

× B then drifts at the whim of whichever source of electric field

dominates.

L -Value

1 1.5 248 6

100

1 mV/m

0.4 mV/m

Corotation

10

Magnetospheric
origin

Ionospheric
dynamo

1.0

0.1

0 30 60 90

Latitude (degrees)

Figure 5.6 Typical ionospheric level electric fields observed in a nonrotating frame of
reference that arise from corotation, the interaction of tidal neutral winds with the iono-
sphere, and the interaction of the solar wind with the terrestrial magnetic field. [After
Mozer (1973). Reproduced with permission of the American Geophysical Union.]