Simultaneous measurements of ionospheric and magnetospheric electric fields in the outer plasmasphere

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Abstract
The eastward component of the electric field was measured simultaneously at ionospheric heights and in the equatorial plane of the magnetosphere by using the incoherent scatter radar at Millstone Hill and whistler data from Siple, Antarctica, respectively. The locations of the two data sets correspond to approximately magnetically conjugate areas within the outer plasmaphere, and thus a unique opportunity arises for testing the electric field mapping hypothesis and for intercomparing the measurement techniques. The whistler results are averaged for L = 3.5‐4.7, while the radar data correspond to L = 4.4. The measurements represent time averages over about 20 minutes. There is excellent correlation during most of the 12 hour period of simultaneous data on 10 July 1978. The period was one of moderate disturbance (Kp = 2‐3). A 500 gamma substorm developed around 0830 UT (0330 LT). During this substorm, both of the electric field measurements showed a large westward perturbation, in agreement with previous observations. The meridional component of the electric field was also measured at Millstone Hill. Taken together, these data indicate an enhanced flow with components inward and towards dawn during the substorm. Except for a time near magnetic midnight the magnitude of the zonal field scales from the ionosphere to the equator in direct ratio with the spreading of magnetic field lines in the IGRF 1975 model. The whistler data clearly show that the measurements were obtained inside the plasmasphere. The middle latitude substorm perturbations appeared to be related to a reconfiguration of the magnetospheric current system.