Magnetospheric electric fields deduced from drifting whistler paths

Abstract

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.

Quantitative simulation of a magnetospheric substorm 3. Plasmaspheric electric fields and evolution of the plasmapause

“…The most extensive investigations of plasmaspheric electric fields during magnetically disturbed periods are based on incoherent scatter radar results fromn St. Santin [Testud et al, 1975;Blanc et al, 1977;Blanc, 1978] and on the cross-L motions of whistler ducts [Park, 1978;Carpenter et al, 1972Carpenter et al, , 1979. Evans (1972) has presented Millstone Hill radar results showing westward F-region ion drifts of almost 200 m/sec in the afternoon sector on 14 May, 1969.…”

Numerical considerations in simulating the global magnetosphere

“…Therefore, the conductance is an intermediate variable that can be tuned, such that, even if the field-aligned currents output from the MHD code are incorrect, the potential can still more or less match the empirical potential values (e.g., Ridley et al, 2002). One justification for doing this is that the ionospheric potential is of primary importance for the ring current and plasmasphere (e.g., Carpenter et al, 1972;Sazykin et al, 2002;Liemohn et al, 2002Liemohn et al, , 2005. Therefore, with some tuning of the ionospheric conductances, such that the potential pattern is more accurate, ring current injections and plasmaspheric erosion events can be modeled better.…”

In situ observations of low-density regions inside the plasmasphere

“…Plasmapause excursions toward the Earth, detected close to 05 MLT, were explained by the rotation under the satellite of the structures formed in earlier local time sectors (around midnight) under the action of the inferred unsteady electric fields during the substorm. Even earlier whistler research (Carpenter et al, 1972) found that during weak to moderate magnetic storms enhanced cross-L inward drifts close to plasmapause occur preferentially in the 2300-0200 MLT sector and may sometimes occur in a wider region extending to local dawn. Above analyzed MAGION 5 data suggest that these inward drifts possibly result in formation of depleted regions-notches-in the plasmasphere.…”