High-latitude electric fields and the three-dimensional interaction between the interplanetary and terrestrial magnetic fields

More than 200 hours of ionospheric electric field measurements taken on balloons flown from three polar cap sites have been analyzed to determine average properties of the large-scale polar cap electric field and its dependencies on the interplanetary magnetic field. The major component of this electric field was directed from dawn to dusk and produced an average polar cap potential drop of about 55 kV. The magnitude of this potential provides an upper limit of about 700 RE for the length of the magnetospheric tail and implies an energy input from the solar wind to the magnetosphere of about 5 × 1019 ergs/sec. The dawn to dusk component of the high-latitude polar cap electric field responds to Bz, the northward component of the interplanetary magnetic field, on a time scale ≲1 hour and with an average increase of about 3 mV/m for each 1 γ decrease of Bz. The hourly averages of the electric field data at each of the three sites are well described by a two-cell convection pattern whose location depends on the y component of the interplanetary magnetic field. When By is positive (negative), the two-cell convection pattern shifts toward dawn (dusk) in the northern hemisphere with the following consequences: the maximum intensity of the northern polar cap dawn to dusk electric field component occurs at local morning (evening), and the auroral zone return flow reaches higher latitudes in the evening (morning). Evidence of the vector nature of the interaction between interplanetary and terrestrial magnetic fields is provided by the observation that the above By dependent signatures are most evident when Bz is most negative.