Dynamics and Electrodynamics of the Equatorial Zone - The Earth's Ionosphere: Plasma Physics and Electrodynamics - page 105

consistent with the postmidnight “substorm recovery” characteristics seen 24 h
later (i.e., 0700-1200 UT on January 19). These eastward perturbations in the
postmidnight sector are very commonly reported in the various studies of dis-
turbance fields using the Jicamarca data base. In part, this is due to the fact that
they are easily recognizable in that sector since the quiet-time field is usually
steady and westward. (Notice, for example, that the July 3, 1968, event identi-
fied in Fig. 3.3 is of this type.) A local time dependence may exist due to the ion
conductivity postmidnight, which leads to enhanced perturbations in this sector.
The westward perturbation in the daytime Jicamarca data (1700-2400UT on
Jan. 18) is now also interpreted as the same (between substorm) type of event
viewed on the opposite side of the system. The transition in sign is clearly seen at
0 UT (1900 LT) when B z turned north: substorm activity ceased and Jicamarca
recorded an eastward perturbation.
Another event of this class is shown in Fig. 3.23. The shaded period is char-
acterized as follows:

1. The IMF turned northward (panel 2) and magnetic activity decreased (panel 1).
2. The auroral zone electric field measured over Alaska (Chatanika data) and eastern
Canada (Millstone Hill) decreased dramatically.
3. An eastward electric field perturbation occurred in the postmidnight sector over Peru
(Jicamarca data).

A qualitative explanation for such events was proposed by Kelley et al. (1979).
As we will discuss in more detail in subsequent chapters, it is known that if a
steady magnetospheric electric field exists, divergence of the ring current near
L

4 will eventually create a charge separation in such a way that the magneto-
spheric electric field will be shielded from low latitudes. This charge separation
region is called the Alfvén layer (Vasyliunas, 1972). Now if the magnetospheric
field rapidly increases or decreases, the charges will be temporarily out of balance
with the new configuration, and a brief low-latitude disturbance will result. An
example of the effect of a rapid decrease in the external electric field is illustrated
in Fig. 3.24a. The transient state inside the plasmasphere when the external field
vanishes is an eastward perturbation on the nightside and a westward perturba-
tion during the day, just as observed in the data shown in Figs. 3.22 and 3.23.
In other words, the low-latitude electric field may be due to Alfvén layer charges
that still exist, even though no external field is left in the outer regions to shield
out! In circuit terminology, the finite inductance of the ring current maintains
the voltage in the inner region when the source is turned off. This effect is also
called overshielding.
Perhaps a better way to understand these penetrating electric fields is illustrated
in Fig. 3.24b. Here the physics is described by a current source rather than a
voltage source. In the middle panel we illustrate the dayside voltage response to
enhanced Region 1 (high latitude) currents (increasing magnetic activity) and at
the bottomwe illustrate the opposite effect when Region 2 (low latitude) currents
exceed those in Region 1, which occurs when the solar wind-magnetosphere

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