The magnetospheric banana current

摘要

It is shown that the banana current, a current system in the inner magnetosphere closing entirely within the magnetosphere (i.e., not through the ionosphere or on the magnetopause) but not circumflowing around the Earth, is a regular feature of near-Earth space. Closure options for the eastward asymmetric current on the inside of a localized pressure peak were explored, with the conclusion that the current must close via westward current around the outside of the high pressure region. It is a current that encircles a pressure peak and, therefore, whenever there is a pressure peak in the inner magnetosphere, a banana current exists. If multiple pressure peaks exist in the inner magnetosphere, then multiple banana currents will also coexist. Its occurrence rate is equal to that of the partial ring current, defined here as westward magnetospheric current that closes through field-aligned currents into and out of the ionosphere. Its magnitude can reach a few mega-amps during the main phase of storms, but drops to <0.1 MA during extended quiet intervals. The magnetic perturbation related to this current is strong within the region of high plasma pressure that it encircles, but is otherwise very weak outside of the banana current loop because the oppositely-directed current flow on either side of the loop largely cancels each other. In general, its related magnetic field is a few nanotesla of northward perturbation for both ground-based and geosynchronous magnetometers, making it difficult to magnetically detect. The banana current is placed in the context of the other near-Earth nightside current systems. Key Points Near-Earth asymmetric eastward current closes around pressure peaks, not FACs This Its magnetic perturbation is large inside the loop, but very small outside of it