The amplitude of the E‐W component Ew of the convection electric field in the nightside magnetosphere has been inferred from the observed cross‐L motions of whistler ducts within the plasmasphere. Several ducts distributed over 1–2 RE in L space and over ±15° around the longitude of the Eights, Antarctica, whistler station have been tracked simultaneously. The method appears capable of resolving fluctuations in Ew with period T ∼ 15 min and rms amplitude as low as 0.05 mv/m. For variations with T > 1 hour the method has a sensitivity of the order of 0.01 mv/m. Three case studies are presented, two of which illustrate convection activity associated with relatively isolated substorms. In these two cases Ew reversed from westward to eastward for a period following the decay of substorm bay activity. In the third case the substorm bay activity was prolonged, and Ew remained westward and at enhanced levels until local dawn. Evidence was found that, at least in a limited longitudinal sector, perturbing substorm Ew fields can penetrate deep within the plasmasphere. In two of the case studies comparisons of Ew and the interplanetary magnetic‐field θ component show evidence of a possible relation based on brief (≤ 1 hour) southward excursions but not on long preceding southward events. The growth of Ew can take the form of an initial brief (<15 min) positive surge followed by a larger surge that is simultaneous with the most active phase of the substorm. Certain of the pronounced increases in Ew were found to be coincident with activation or spreading of electrojet or auroral activity. In one instance low‐amplitude (<0.1 mv/m) presubstorm fluctuations in Ew with periods of the order of 30 min were found to correlate closely with ground‐observed midlatitude fluctuations in the magnetic H component. Calculated values of E‐field power spectral density from the tracking of two long‐lived (∼6 hours) whistler paths reveal considerable fine structure. The falloff with frequency roughly as f−2 agrees approximately with results from balloons, but the calculated spectral amplitudes appear lower than the balloon results by a factor of ∼4. The amplitudes from whistlers appear to be within the range identified by other workers as sufficient to drive radial diffusion in the radiation belts. The present research agrees with balloon measurements on the general presence of a westward field during substorms, but there is apparent disagreement on a number of details, including the post‐substorm reversal in Ew.

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A case study is presented of the concurrent and apparently related behavior of four magnetospheric phenomena: (1) whistler ‘ducts,’ (2) cross‐L drifts of tubes of ionization, (3) an abrupt upper intensity cutoff of ducted whistlers at ƒHo/2, where ƒHo is the minimum electron gyrofrequency along the path, and (4) artificial triggering of VLF whistler‐mode noise at ƒ ∼ ƒHo/2. The event occurred on June 17, 1965, near local midnight, and involved the inward drift of a whistler duct through ∼0.2 L near L = 3. Triggering of magnetospheric noise by NAA transmissions at 17.8 kHz occurred when the minimum gyrofrequency on the drifting path became twice the NAA frequency.