Magnetopause currents

We first compare the Polar magnetic field observations [Russell et al., 1995] on May 11 with the control day May 14. Figure 3 shows the residual of the magnetic field by subtracting the IGRF model field from the Polar MFE data. The residuals on May 14 (right panel) represent typical values under normal solar wind conditions. A comparison of the magnetic field for these two days shows that the residuals from the internal field model on May 11 (right panel) were much smaller than those under normal solar wind condition, i.e., the magnetic field agreed well with the IGRF internal magnetic field model. This is especially true at high altitudes from $\sim$ 08 to 19 UT, an indication of little magnetopause currents. Thus, only weak external current systems were present and the region of the magnetosphere accessed by Polar became much more dipolar than usual under the unusually tenuous solar wind condition on May 11.