Dynamics and Electrodynamics of the Equatorial Zone - The Earth's Ionosphere: Plasma Physics and Electrodynamics - page 103

cap and auroral zones but can also be detected at the equator. The sensitivity of
radars to electric fields has, in fact, increased our knowledge of how the solar
wind affects the magnetosphere/ionosphere system (Gonzales et al., 1979, 1983;
Somayajulu et al., 1985, 1987; Earle and Kelley, 1987).
A classical event (Nishida, 1968) illustrating a clear relationship between the
interplanetary magnetic field (IMF), which is of solar origin, and the magnetic
field created by the equatorial electrojet is reproduced in Fig. 3.21. The two
magnetic field measurements are clearly correlated, although the magnetometers
were separated by 10 5 km! Thus, although the magnetosphere, ring current, and
plasmasphere all act to shield out interplanetary and solar wind effects from low
latitudes, the protection is not perfect. Some insight into the complex relationship
between interplanetary and equatorial ionospheric phenomena has come from
simultaneous measurements of electric fields at Jicamarca and in the auroral zone.
Since, as discussed elsewhere in this text, the much shorter chain of cause and
effect between the IMF and auroral fields is itself not yet entirely understood, we
can be satisfied here with some plausible explanations for the equatorial effects.
Forty-eight hours of Jicamarca electric field data are presented in the lower
panel of Fig. 3.22. The eastward electric field component is plotted along with a
lighter line that shows the average quiet-day value for the same solar cycle con-
ditions and season. The top trace is a superposition of magnetic field data from
a number of auroral zone magnetometer stations, which are used to generate the
AU and AL auroral zone magnetic indices (see Appendix B). A series of six bursts
of auroral substorm activity are clear in this data set. The northward component
of the interplanetary magnetic field is also plotted. The latter, unfortunately, has

100

Ionospheric magnetic field

Interplanetary magnetic
field

2508

08

508

16 18 20

Figure 3.21 Correlation of horizontal geomagnetic fluctuations observed at Huancayo
on December 3, 1963, with changes in the direction of the interplanetary magnetic field
component perpendicular to the sun-earth line observed from the satellite IMP-1 versus
UT. [After Nishida (1968). Reproduced with permission of the American Geophysical
Union.]