Ring Current Error Analysis Using Curlometry

Significant discrepancy exists between ring current density calculated from plasma pressure and by using the curlometer technique and Cluster spacecraft data. Literature suggests a ring current density of ~20-30 nA/m^2 using the curlometer technique, well above calculations using plasma pressure yielding ~2-10 nA/m^2. In an effort to identify sensitivities and possible error sources, the curlometer technique was subjected to a series of parameter perturbations of spacecraft data, as well as simulated currents, to assess performance. Curlometry is sensitive to current density gradients across the tetrahedron volume where linearization assumptions break down, creating false currents in other components, but the effect is too small to have significant impact on total current density. This analysis was unable to identify any noteworthy source of error in the curlometer technique or Cluster dataset, but was also unable to reproduce high current density results of previous curlometer studies. For example, in the perigee pass of 18 March 2002, previous papers suggest a curlometer-derived azimuthal current of 20 nA/m^2, whereas this study finds the value to be only 7 nA/m^2 using the same technique. Curlometry also shows large current components aligned with the magnetic field in this region. Thus, a careful reanalysis of curlometer results is required to reach consensus. The magnitude of the ring current has important implications for space weather modeling, including forecasting of Dst index and understanding plasma population transport and exchange between magnetospheric regions.