Geoscience Reference
In-Depth Information
Using the lowest value of any component of
σ
in the ionosphere, we find the
10
−
6
s. Electric fields thus build up very quickly indeed in
response to any divergence of
J
. Such divergences arise whenever there are spa-
tially varying forces on the plasma or when the conductivity changes in space.
In practice, it is not possible to calculate
largest value for
τ
=
ρ
c
from (2.42) and then
E
from (2.43).
Rather, the electric field is treated as a free parameter that adjusts in magnitude
and direction to fit the requirement that
0. In the next chapters we show
in detail how electric fields arise in the earth's ionosphere.
When an electric field is created by a wind, the process is often called a dynamo
in analogy to a motor-driven electric generator in which a conductor is moved
across a magnetic field. In this case, or any case in which electrical energy is
created, the quantity (
J
∇·
J
=
E
) should be negative, and in addition the electrical
forces must act in opposition to the source of the charge separation.
When electric fields are applied from an external source, such as occurs at
high latitudes due to the solar wind-magnetosphere interaction, it is usually the
case that
J
·
0 in the ionosphere. In this case electrical energy is converted
into mechanical energy in the ionosphere and is released in the form of heat.
Such Joule heating is a very important process at high latitudes and may greatly
affect the thermospheric winds. In this case the ionosphere acts like an electrical
load on some external generator. Likewise, momentum may be transferred to
the thermospheric gas through the “ion drag” term if the ions are driven very
strongly by an externally applied electric field. In such a case the ionosphere-
magnetosphere system acts like a motor with electrical energy converted into
mechanical energy.
·
E
>
2.4 Electric Field Mapping
The high conductivity parallel to the earth's magnetic field,
σ
0
, has important
implications concerning the transmission of electric fields for long distances along
B
. In fact, if
σ
0
were infinite, there would be zero potential drop along the
magnetic field, and the potential difference between any two field lines would be
constant. In such a case any electric field generated at ionospheric heights would
be transmitted along the magnetic field lines to very high altitudes. For example,
an electric field generated at 60
◦
magnetic latitude would be communicated to
the equatorial plane at an altitude over 25
000 km. Likewise, electric fields of
solar wind or magnetospheric origin could be transmitted to ionospheric heights.
This phenomenon can be studied quantitatively as follows (following Farley,
1959, 1960). Suppose first that the conductivity is anisotropic but uniform and
that the neutral wind is absent. If the electric field perpendicular to
B
is
E
,
and
⊥
the field parallel to
B
is
E
the total current is
||
⊥
−
σ
H
E
⊥
×
B
J
=
σ
P
E
+
σ
0
E
(2.45)
||
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