Geoscience Reference
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from a rotating garden sprinkler. At the earth's location this “garden hose angle”
is such that the y component and the x component of the interplanetary magnetic
field
are roughly equal and usually have opposite signs. The z component
may be positive or negative, and the terms “ B z positive” or “northward” and
B z negative” or “southward” are used to describe the orientation of the IMF
with respect to the z -axis.
We now briefly describe the interplanetary electric field. To first order the solar
wind blows radially outward from the sun, so that in the ecliptic plane
(
B sw )
=−|
|
V sw
V sw
a x
In a reference frame fixed to the earth, there will be an electric field given by
E sw =−
V sw ×
B sw
This will generate a potential difference across the earth's magnetosphere
given by
V m =−
E sw ·
L a y
where L is the effective size of the magnetosphere perpendicular to V sw . Note that
when B sw has a positive z component, E sw has a dusk-to-dawn direction (negative
y component). When B sw has a negative z component, the interplanetary electric
field is in the dawn-to-dusk direction across the earth. For typical values of
V sw =
10 9 tesla, E sw y =
m. Integrating this
electric field across the front of the magnetosphere, which is roughly a distance
L
500 km
/
s and B sw z =
5
×
2
.
5mV
/
20 R e across (where R e stands for earth radii), yields an available potential
difference of the order of 300,000 volts. The magnitude of this voltage and its
polarity fluctuate along with the parameters V sw and B sw z . The earth is thus
immersed in a magnetohydrodynamic electrical generator capable of hundreds
of kilovolts of potential.
We now turn to the problem of getting this energy into the magnetosphere.
Continuing our analogy with an MHD electrical generator, we need to tap
this power by closing the electrical circuit associated with the solar wind-
magnetosphere interaction. This is accomplished by the connection of magnetic
field lines attached to the earth with those in the interplanetary medium. Cur-
rent can then flow down into the magnetosphere and close the circuit through
the conducting ionosphere. Since the magnetic field lines are equipotentials, the
interplanetary electric field maps into the near-space region of the earth, into the
ionosphere, and even deep into the stratosphere.
This “connection” can occur most easily when the IMF is southward, as illus-
trated in Fig. 1.16, where the cross-sectional view is now in the noon-midnight
plane. This viewpoint was first suggested by Dungey (1961). The numbers
=
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