High-Latitude Electrodynamics - The Earth's Ionosphere: Plasma Physics and Electrodynamics - page 413

Electrostatic potential (kV) Potential (kV) Millstone Hill 1978 K P 5 3 to 9

12 12

50° 50

0

70

4

2 20

80°

2 32

18 06 18 06

28

25

2 26

15

2 35

225

2

5

2 15

2 5

0

00

00

Figure 8.19 Radar data fromChatanika andMillstone Hill used to derive the convection
pattern in the auroral zone (sunward-flow regions). [After Foster (1983) and Oliver et al.
(1983). Reproduced with permission of the American Geophysical Union.]

Here θ is magnetic co-latitude, φ 0 is the potential at the polar cap boundary

where θ = θ 0 , and the index n typically takes values between − 2 and − 4. Inside

the polar cap boundary, Volland (1975) showed that if

φ = φ 0 (

θ/ θ 0 ) ( )

sin sin sin H (8.18b)

where H is the local hour angle (MLT), then the lines of electrostatic equipoten-
tial depict convective flow that is antisunward inside the polar cap. This model
can be modified to allow

φ 0 to take different values on the dawn- and dusk-side
polar cap boundaries and to allow the origin of the potential coordinate system
to be displaced from the magnetic pole. It has been used extensively in F-region
modeling studies where the offset between the geomagnetic and geographic poles
has also been taken into account.
To reproduce such observations as the crescent-shaped cell in the dusk sector
and the predominance of eastward or westward flow at local noon, some mod-
ifications to the mathematical model can be made. These include the removal
of any local time dependence of the potential in certain local time sectors and
the removal of the latitudinal discontinuity in the potential gradient at the polar
cap boundary. A variety of convection geometries are then available with real-
istic field-aligned current densities similar to those described in Section 8.5. The
challenge of producing mathematical models that adequately describe the effects
of changing B y when the IMF is southward and the four-cell convection pattern
when the IMF is northward still remains.
In our description in previous sections we have shown how the IMF exercises
a large degree of control on the geometry or shape of the convection cells. This