Instabilities and Structure in the High-Latitude Ionosphere - The Earth's Ionosphere: Plasma Physics and Electrodynamics - page 468

10.1.1 Convection and Production as Sources of Planetary Scale
Structure in the High-Latitude Ionosphere
We have already discussed variations of plasma flow and plasma density varia-
tions at planetary scales in Chapters 8 and 9. We briefly summarize the material
here for completeness. The velocity field applied to the ionosphere from the
solar wind-magnetosphere interaction has a number of discernible patterns at
planetary scales. However, which of these patterns applies at a given time is
highly dependent on conditions in the interplanetary medium. The most crucial
parameter is the sign of the north-south component ( B z ) of the interplanetary
magnetic field (IMF). When the IMF is southward for any extended time (e.g.,
tens of minutes) the classic two-celled convection pattern is imposed on the iono-
sphere. The magnetic field lines that thread the high-latitude ionosphere spread
out enormously as they recede from the earth. This means that the pattern in the
ionosphere represents a focused version of the electrodynamic processes that cre-
ate the electric field pattern. Unfortunately, the topological mapping that occurs
is quite complicated, since some field lines are open and some are closed.
For steady B z south, the two-cell ionospheric flow pattern is more or less fixed
with respect to the sun-earth line. The earth and its neutral atmosphere rotate
under this plasma flow pattern once a day (ignoring tidal forcing and acceleration
of the neutrals by the plasma). This creates a planetary scale, diurnally varying
plasma flow field in the ionosphere. Now, even with B z held south, the classic
symmetric two-celled flow shifts with respect to the sun-earth line as the other
components of the IMF vary (particularly the component parallel to the earth's
orbit, B y ). This shifting of the flow field can occur within one or two Alfvén travel
times from ionospheric altitudes to the generator. Likewise as B z and the velocity
of the solar wind change, the rate of energy transfer to the magnetosphere varies
fromminute to minute and a flow field which is highly variable in space and time
results.
When B z changes sign, the major source of energy transfer ceases but other
effects take over. A viscous interaction seems to create a small two-cell pattern
and the connection of field lines to the IMF in regions of the magnetosphere far
from the ecliptic plane also creates multiple cells with planetary scales (Burke
et al., 1979).
To gauge the effect of these complex flow fields on plasma content we first need
to discuss the processes which create and destroy plasma on planetary scales. The
most important source is photoionization by sunlight. In a nonrotating frame
this region is bounded by the terminator, which moves across the polar region
on a seasonal basis. On the dark side of this line, recombination rapidly destroys
plasma below 200 km but only very slowly erodes the F-peak region. The time
constant is roughly 1 hour at 300 km.
Now when the planetary scale flow is imposed on this source and loss pattern,
it is clear that solar plasma can be transported for vast distances into and clear
through regions of total darkness before recombination can play much of a role.