Deciphering magnetospheric cross-field currents

A single near‐tail magnetic field line can be part of a variety of cross‐field current systems, making the interpretation of such currents difficult. It is shown that global, coupled‐model simulation results from the 22 October 1999 storm include a field line crossing downtail at L = 8 during the main phase that contains partial ring current, symmetric ring current, and tail current simultaneously. Such field lines with multiple currents are common in the near‐Earth tail. Another time from the same event showed two closely‐spaced field lines (L = 6.0 and 6.5) with completely different current systems on them (one entirely symmetric ring current and the other entirely tail current). It is shown that, for this storm from this simulation, the tail current inner edge systematically shifts inward then outward during the storm main phase and that most of the Dst perturbation is from the ring current (both partial and symmetric). Caution is advised when analyzing observational or numerical cross‐field currents and when making conclusions about which type of current system dominates the distortion of the near‐Earth magnetosphere. Key Points Deciphering magnetospheric cross‐field current systems is difficult Tail‐ring current boundary shifts systematically with storm phase For this storm, the ring current dominates the Dst perturbation