Drifts
The convection of plasma is vital in understanding the large-scale convection
of plasmas in the magnetosphere, the creation of unstable particle distributions and associated
wave growth and particle scattering, and the motions of individual particles. Reconsider
Figures 14.4 - 14.8 from the point-of-view of drifts.
Suppose first that the
solar wind electric field is directed from dawn to dusk (i.e., eastwards) or out of the paper.
Then in the plasmasheet the drift is Earthwards from the tail and sunwards from
Earth, so that tail plasma is brought Earthwards, drifts around the Earth due to curvature and
gradient drift, and then drifts to the magnetopause where it is lost. Similarly, plasma leaving the
cusp tailward or plasma moving in the tail is convected down toward the plasma sheet, leading to cutoff
distributions and a denser and thinner plasma sheet.
Notice now the effect of the solar wind electric field reversing in sign. (How could this
happen? Consider the definition of 
and the usual
dominance of the radial or 
component of the solar wind velocity.) Now all the
drift velocities are reversed and so are the
corresponding convection patterns. It can now be seen that solar wind activity can be expected to have
major consequences on the plasma and ``space weather'' in Earth's magnetosphere. This subject is
discussed in more detail in the next lecture.