Geoscience Reference
In-Depth Information
140
B
z
.
0
120
100
80
60
40
20
0
0
1
2
3
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5
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8
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10
Hours after B
z
. 0
Figure 8.21
The polar cap potential decreases slowly after the IMF turns northward. It
finally reaches a value between 20 and 15 kV that is perhaps attributable to the viscous
interaction process. [After Wygant et al. (1983). Reproduced with permission of the
American Geophysical Union.]
electric field might be produced in the ionosphere at high latitudes by a flywheel
effect during transitions from previously stable IMF orientations. In such a case,
J
0 in the ionosphere and the Poynting flux is upward. Rocket data indicate
that shrinkage of the polar cap for
B
z
north occurs in discrete steps accompanied
by new auroral arc generation poleward of the instantaneous auroral oval (Berg
et al., 1994). Alternatively, this decay may be the time constant required to dis-
sipate stored magnetic energy by Joule heating at ionospheric altitudes (Kelley
et al., 2009).
The observations of the flow geometry when the IMF has a northward com-
ponent are not sufficiently advanced to make quantitative analysis possible. As
mentioned earlier, it is likely that processes other than merging (e.g., viscous
interaction) may be important in producing the ionospheric motion in this case,
and advances in this area are occurring at the time of this writing. An excel-
lent climatological model for magnetospheric convection has been developed by
Weimer (1995, 2001).
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8.5 Observations of Field-Aligned Currents
Field-aligned currents are essential to the linkage between the solar wind-
magnetosphere system and the ionosphere. The ionosphere is not a passive ele-
ment in the electric field mapping process between these regions and, in fact, the
electric field can be modified at the driver and throughout the system by the iono-
sphere. This is particularly true in the auroral zone, where the ionosphere and
magnetosphere are linked and the hot plasma sheet particles can determine the
ionospheric conductivity through particle precipitation. Study of these currents
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