Field‐aligned currents, convection electric fields, and ULF‐ELF waves in the cusp

Abstract

Nearly simultaneous observations from the Triad and Hawkeye spacecraft at low altitudes over the south polar cap show that in and near the polar cusp there exist several relationships between field‐aligned currents, convection electric fields, ULF‐ELF magnetic noise, broad‐band electrostatic noise and interplanetary magnetic fields. The most important relationships are: (1) The region 1 permanent field‐aligned currents are located inside the region of sunward (eastward) convection and therefore on closed field‐lines. (2) The polar cusp field‐aligned currents are located inside the regions of antisunward (westward) convection. (3) The observations are consistent with a two‐cell convection pattern symmetric in one case and asymmetric in another with stronger convection on the dusk side. This is explainable by models which require interconnection of interplanetary magnetic field lines with those of the geomagnetic field. Furthermore, this region of maximum merging shifts to the morning side in the southern hemisphere for away sectors of the interplanetary magnetic field. (4) In and near the polar cusp, field‐aligned currents and convection velocity gradients (separated only by two hours in MLT) nearly coincide with regions of ULF‐ELF magnetic noise and broadband electrostatic waves. We offer this as evidence consistent with the idea that field‐aligned currents with fine structure are responsible for the presence of ELF noise in the polar cusp.