Ring current

A comparison of the Polar magnetic field data with the Tsyganenko [1996] (T96) model that includes various current systems in the magnetosphere shows that the residuals in the inner magnetosphere on May 11 are those expected from a weak ring current. We do not expect the magnetopause and tail currents to contribute to the residuals because they are proportional to the square root of the solar wind dynamic pressure and we see no such contribution at high altitudes. On May 11, the Dst index ranged from -4 nT to +10 nT and the daily average was 2 nT. At face value one might take this to mean that the ring current was nearly absent at this time, but we find that we need to put in a significant ring current in the T96 model in order to obtain the best fit between the data and the model. Figure 4 shows the residuals on May 11 from the T96 model that uses solar wind and IMF conditions shown in Figure 1 and a ring current with Dst=-10 nT as inputs. By comparing Figure 4 with Figure 3, it is clear that the residual in the magnetic field strength around the Polar orbit perigees (0433 UT and 2229 UT) is greatly reduced. Thus, the ring current did not disappear on May 11 when the solar wind almost disappeared. Again this is expected because although the ring current derives its energy ultimately from the solar wind [Burton et al., 1975], the mass appears to be from the ionosphere and once energized the ring current persists for many hours.