High-Latitude Electrodynamics - The Earth's Ionosphere: Plasma Physics and Electrodynamics - page 394

day, the associated flux tubes fill from below with ionospheric plasma produced
on the dayside of the earth (see Chapter 5). To first order, the plasmasphere
lies just inside the ring current and the plasma sheet field lines. Outside of this
region, the flow is more or less toward the sun in the equatorial plane. Where a
flow line meets the magnetopause, it is assumed either to make contact with the
interplanetary field and flow back over the top of the magnetopause or to flow
back down the flanks of the tail in the boundary layer. Figures 8.5 and 8.6 are
in the sun-fixed frame.
On the magnetic field lines in contact with the corotating plasmasphere, the
ionosphere to first order corotates with the earth just as the atmosphere does.
In Chapters 3 and 5 we worked in this earth-fixed frame where the corotation
field vanishes and discussed the ionospheric “weather.” This involved electric
fields and consequent plasma motions in the rotating frame, which had typical
magnitudes of 1-10mV s. For reference, in the

equatorial plane the earth's rotation speed at the surface is 434m

/ m and velocities of 20-200m /

/ s, while the

corotation speed at L = 4 in the equatorial plane is near 2 km / s.

8.2 Observations of Ionospheric Convection

Our knowledge of the large-scale motion of the high-latitude F-region plasma
has come from satellite, rocket, balloon, and ground-based radar and magnetic
field measurements. These and other measurement techniques have also been
used extensively to study smaller-scale features of the plasma density and plasma
motion associated with the aurora. This topic is dealt with in Chapter 10. No
single measurement can provide a complete description of the large-scale motion
of F-region plasma. Over a 24-hour period, for example, a satellite measurement
can be repeated 10 to 15 times over the entire high-latitude range in the iono-
sphere but only in a very limited local time region. Over the same time period a
ground-based radar measurement can be made over the entire local time region
but in a limited latitude range. A description of the high-latitude plasma motion
is therefore made up from a synthesis of these complementary measurements,
taken over a period of many years.
For a time-independent system, ∇ × E =− ∂ B /∂ t = 0 and the electric field may

be described by an electrostatic potential . The electric field

is perpendicular to a line of constant potential and is also perpendicular to the
E

φ such that E =− ∇ φ

B motion of the plasma. Thus, the low temperature plasma flows along lines
of constant potential and a pattern of equipotentials also represents the plasma
flow pattern. Each measurement of electric field or plasma velocity therefore
provides a signature of some portion of the convection pattern. These measure-
ments show that the convection pattern is highly dependent on the orientation
and the magnitude of the IMF. Recall from Chapter 1 that when discussing the
IMF, its direction can be described in a number of ways. The z component can be
negative or positive and the IMF is often referred to as southward or northward,

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