Modeling the inner magnetosphere: The asymmetric ring current and Region 2Birkeland currents revisited

A quantitative model is developed of the inner magnetospheric magnetic fiel d, combining the effects of the azimuthally asymmetric ring current with th ose of held-aligned currents, due to azimuthal variation of the plasma pres sure. The axisymmetric part of the model ring current was derived from aver age observed radial profiles of the particle pressure and anisotropy and wa s analytically represented by a vector potential, fitted by least squares t o one derived by the Biot-Savart integral. The contribution of the asymmetr ic part, including the field of Birkeland currents, closed via the ionosphe re, was represented as a divergenceless combination of expansions for the c omponents of the B vector, and was also fitted by least squares to the corr esponding Biot-Savart field. The total field-aligned current, associated wi th the noon-midnight pressure asymmetry of the outer ring current, was foun d to be significantly smaller than the total Region 2 current, observed at low altitudes. It is therefore concluded that most of that current is gener ated in the near-Earth plasma sheet. The goal of this work is a realistic a nd computationally efficient description of the asymmetric ring current, to be included in an advanced model of the external geomagnetic field.