Dynamics and Electrodynamics of the Equatorial Zone - The Earth's Ionosphere: Plasma Physics and Electrodynamics - page 109

the equatorial plane). The power is down by only a factor of four at Jicamarca,
which means the field penetrating to the equator in Fig. 3.25b is reduced by
only 50% from the magnetospheric (auroral zone) value in the equatorial plane.
This is typical of the electric field values in the few-hour frequency range stud-
ied by Earle and Kelley (1987), who also have reported experimental evidence
that the shielding process acts like a high-pass filter, allowing signals with peri-
ods shorter than about8htopenetrate to the equatorial ionosphere. This is in
good agreement with the time constant predicted by ring current shielding theory
(Vasyliunas, 1972). Furthermore, they show that for the zonal electric field com-
ponent at Jicamarca, the signal in the frequency range corresponding to a period
of a few hours exceeds the “geophysical noise” due to gravity waves at a mod-
erate level of magnetic activity ( K p

3). In other words, at frequencies above
the ring current shielding frequency, high-latitude effects may always be present
at all latitudes but cannot be detected unless they are above the fluctuation level
(geophysical noise) due to neutral wind-driven electric fields.
Examples of the low-latitude effect of increasing magnetospheric electric fields
are rare for reasons that are not altogether clear. One good example on March
23-24, 1971, is shown in Fig. 3.26a. The IMF turned southward abruptly at
0700 UT on March 24 after 5 hours of northward field. A strong westward
perturbation occurred over Jicamarca, which was in the postmidnight sector.
Notice that on the next day the more common eastward perturbation was seen
in conjunction with a northward turning of the IMF at 0900 UT. There is an
unfortunate data gap, but it does seem that a westward electric field perturbation
occurred in conjunction with the B z south event at 0500 UT on the 25th. One
of the best documented events (Kelley et al., 2003a) occurred on April 17, 2002.
For example, Fig. 3.26b directly compares the interplanetary electric field (IEF)
in the upper panel to the electric field measured with the incoherent scatter radar
(middle panel). The IEF has been divided by 15 before plotting in the middle panel
and is highly correlated with the equatorial field at Jicamarca. Gonzales et al.
(1979) and more recently Anderson et al. (2002) developed a technique using
magnetic field data from two sites—one on and one off the equator—which
has promise for greatly improving our knowledge of the electric field. A direct
comparison of the Anderson-Gonzales method with the measured electric field
is shown in the lower panel of Fig. 3.26b and is very convincing. Not shown
is a good correlation between the Jicamarca electric field and that measured at
Sondre Stromfjord (Kelley et al., 2003b).
Fejer and Scherliess (1995) created a semi-empirical model and compared it
with the Rice Convectionmodel, which includes both Region 1 and Region 2 cur-
rents. The former corresponds to the highest latitude currents, which are closely
controlled by and linked to the solar wind generator. The Region 2 currents cor-
respond to the inner magnetospheric portion of the auroral current system and
link up with the ring current system. Their results are shown in Fig. 3.27 for an
increase in the Region 1 current without a corresponding Region 2 current, corre-
sponding to increased polar cap convection. For a rapid decrease in convection

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