Anomalous reversals of the zonal equatorial electric field component have sometimes been observed when the interplanetary magnetic field turns northward from a steady southerly direction. We suggest that this reversal is associated with a sudden change in the convection electric field in the magnetosphere and present measurements to support this explanation. Although slower variations in the convection field are shielded from the low latitude ionosphere by polarization charges at the inner edge of the ring current, these charges may require an hour or more to vary. A sudden decrease in the cross‐tail electric field will thus be accompanied by a dusk‐dawn perturbation electric field across the inner magnetosphere.

Simultaneous auroral and equatorial electric field data are used along with magnetic field data to study anomalous electric field patterns during disturbed times. During some substorms, accompanied by ring current activity, the worldwide equatorial zonal electric field component reverses from the normal pattern. This is interpreted as a partial closure of high latitude field aligned currents in the dayside, low latitude ionosphere. These currents flow westward across the dayside. In several cases the zonal equatorial electric field component was nearly identical in form to the zonal auroral component, indicating the close electrical coupling between these regions. Less certain, but equally intriguing, is the evidence presented for a close relationship between the zonal equatorial electric field and the time derivative of the ring current induced magnetic field. Another class of events seems related to rapid changes of magnetospheric convection and a temporary imbalance between the field external to the plasmasphere and the shielding charges in the Alfven layer. Examples of both rapid increases and decreases are presented. The latter seems often to be related to a northward turning of the interplanetary magnetic field.

Radar measurements of E and F region drift velocities have been used to look for correlations between changes in equatorial electric fields and the interplanetary magnetic field (IMF). The east‐west component of the IMF appears to be unimportant, but the north‐south component has some effect; rapid reversals from south to north are sometimes correlated with reversals of the equatorial east‐west electric field during both daytime and nighttime. This is not always true, however, the IMF may reverse without any apparent effect at the equator. Furthermore, large equatorial field perturbations are sometimes observed when the IMF Bz is large and southward but not varying drastically. In this latter case the equatorial perturbations start nearly simultaneously with the onset of auroral substorms, while in the previous case they usually correlate with the onset of the substorm recovery phase. These observations indicate that the IMF does not affect the equatorial electric fields directly. Rather, it is changes in the magnetospheric electric fields and the auroral zone electric field and conductivity distribution (which may or may not be triggered by IMF changes) which alter the worldwide ionospheric current flow and electric field pattern, of which the equatorial observations are an indication.

A joint alert campaign was organized during the month of October 1980 by the incoherent scatter radars in the American sector: namely, Jicamarca, Arecibo, Millstone Hill, and Chatanika. The campaign, which met with success, was designed to study the behavior of the ionospheric electric field as a function of latitude during magnetically active conditions. The Arecibo data in this campaign support present and previous observations at Jicamarca that suggest that when the convection E field suddenly decreases, the Alfvén layer shielding field becomes unbalanced and penetrates the plasmasphere. While this type of observation is reasonably convincing, others are more difficult to categorize. We suggest that, beside the high‐latitude electric fields, time‐varying auroral conductivity models will have to be considered in order to understand the morphology of the low‐latitude E field disturbances. We present the first correlation analysis and determination of the amplitude ratio of the disturbed zonal electric field at 30° geometric latitude (Arecibo) to the field at 0° (Jicamarca). Other highlights of the paper are a discussion of DP2, which may help clarify the controversy surrounding it, and a discussion of the sensitivity of low‐ and mid‐latitude radars to disturbances of magnetospheric origin. We show that this sensitivity maximizes at the magnetic equator.

F region east-west drifts have observations with other related data (such as the been measured at Jicamarca for almost 10 years, Indian results) and with available theory. using incoherent scatter. The drifts are west-