High-Latitude Electrodynamics - The Earth's Ionosphere: Plasma Physics and Electrodynamics - page 380

8.1.2 A Qualitative Description of Convection for Southward IMF
Some fundamental properties of the coupling of electric fields, currents, and
energy in the ionosphere, magnetosphere, and solar wind system can be under-
stood as follows. Consider the case where magnetic field lines in the ionosphere
connect to a southward IMF. This defines an area at high latitudes in the iono-
sphere called the polar cap. The geometry is shown schematically in Fig. 8.1a.
This interaction is described qualitatively in Chapter 1, which the reader is
encouraged to revisit at this time. Since the solar wind plasma is collisionless
and expands radially outward from the sun, the electric field in the solar wind
vanishes, while in the earth's frame it is given by E sw

B sw . For a
southward IMF the field will have a component pointing from dawn to dusk.
The electric potential across the connected field lines will be applied across the
magnetosphere and will map down to the polar cap ionosphere, where a dawn-
to-dusk directed ionospheric electric field, E I , will also result. This electric field
drives the ionospheric F-region plasma in the antisunward direction at a speed
V I =

=− V sw ×

B I . The magnetic flux density is higher in the ionosphere than in
the solar wind and, since the equipotential surfaces converge, the electric field in
the ionosphere will be larger than in the solar wind. Typical numbers are

E I × B I /

10 4 B I / B sw = 50 , 000 nT / 5nT =

50mVm − 1 2mVm − 1

E I / E sw = / = 125

1kms − 1 400 km s − 1 10 − 3 V I / V sw = / = 2 . 5 ×

Returning to (8.1) and ignoring the neutral wind in the ionosphere for the
present, the electric field will drive a current at ionospheric heights given by

J = σ · E I (8.7)

The Pedersen component of this ionospheric current is parallel to E I and so is
such that J 0. In this interaction, then, the ionosphere is a load and we must

ascertain where the electrical energy originates. Referring to Fig. 8.1a, suppose
that the solar wind slows down slightly when it is in contact with the region
shown and that we can ignore the gravity and pressure gradients. Then (8.4)
shows that J

· E >

B 2

0.
The solar wind acts as an MHD generator, feeding energy to the ionosphere
as a load. Note that in this region the J

⊥ = (ρ/

) B × d V / dt is antiparallel to E sw —that is, J sw · E sw <

B force is directed back toward the
sun, which shows consistency with the concept that the solar wind is slowing
down due to the interaction. These same principles hold in any electromotive
generator in which kinetic energy is converted to electrical energy. Finally, since
the interaction region where the solar wind slows down is bounded, J sw has a
divergence which provides the field-aligned currents feeding the load.

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