High-Latitude Electrodynamics - The Earth's Ionosphere: Plasma Physics and Electrodynamics - page 425

function is determined by taking the divergence of (8.23) and

solving the two-dimensional Poisson equation

ϕ . The function ϕ

2

∇ ϕ =−∇· δ B (8.24)

The right-hand side of this equation is given by a global distribution of ground
observations with suitable interpolation at each grid point.
These two quite different techniques yield very similar results for the hor-
izontal equivalent ionospheric current. In principle, only the equivalent cur-
rent can be derived from ground magnetic perturbations, and some model of
ionospheric conductivity is required to proceed further. However, many fea-
tures of the equivalent current system are useful in establishing self-consistency
among the electrodynamic parameters and we will now describe these
characteristics.
Our purpose here is to describe the dependence of the equivalent current dis-
tribution on magnetic activity and the interplanetary magnetic field orientation.
In this way a self-consistent picture of the electrodynamic parameters J

⊥ ,

J ,

||

and E can be established. The effects of magnetic activity on the horizontal

equivalent current manifest themselves principally in small-scale features that
we will discuss later. The large-scale features of the horizontal current and its
dependence on the IMF are shown in Fig. 8.28. A two-cell pattern of current
flow exists for all orientations of the IMF. In each of the pictures, the current
flows in closed loops around a focus labeled “

⊥

+ ,” located on the nightside near

midnight, and around a focus labeled “ ,” located on the dayside near noon.

The sense of circulation of the current is clockwise around the

−

+ focus and

anticlockwise around the − focus. When the IMF has a southward component

( the current is much larger and the two-cell pattern is more well defined

than when the IMF is northward. Examining the southward IMF case in more
detail, we note that at the low-latitude extremes of the current system there exist
bands of current flowing, respectively, eastward on the dusk side and westward
on the dawn side. These current bands are collocated with the diffuse auro-
ral zones and are called the eastward and westward electrojets. Note that the
direction of the electrojet currents is opposite to that of the F-region plasma
flow.
The fact that a two-cell current pattern is retained for all orientations of the
IMF indicates that some element of the current pattern may be independent of
the IMF. Statistical analysis of ground-based magnetograms shows this to be the
case, and this portion of the current system is called the S q (“solar quiet polar”)
current system. Detailed analysis shows that there may also exist semidiurnal
contributions to this current that may be attributable to lunar variations, as is
the case in the equatorial region.
For a southward IMF during times of relatively low magnetic activity (i.e.,
K p <

B z < 0 )

3), the total current system is made up of the S q system plus another system
called the DP2 system; the DP2 current system is made up of the DPY and DPZ