Dominant role of the asymmetric ring current in producing the stormtime <i>Dst</i>*

Three storms are examined to determine the contribution to the Dst * index from the symmetric and asymmetric (partial) components of the ring current. The storms (September 24–25, 1998, October 18–19, 1998, and May 14–15, 1997) all have a similar solar wind trigger (an initial shock followed by a coronal mass ejection with southward interplanetary magnetic field) and placement in the solar cycle (rising phase). The near‐Earth ion distribution function is simulated for each storm using a kinetic transport model. The use of a McIlwain magnetospheric electric field description improves the simulation results over the Volland‐Stern field used previously. It is found that most of the main phase magnetic field depression is due to the asymmetric component of the ring current (≥80% at the Dst * minimum for the three storms). Note that this is a minimum asymmetric ring current contribution, because the closed‐trajectory ions may also be spatially asymmetric. Ions in the partial ring current make one pass through the inner magnetosphere on open drift paths that intersect the dayside magnetopause. Changes in the density of the inner plasma sheet are transmitted directly along these open drift paths. For a steady convection field, an increase in the source population produces a decrease (more intense perturbation) in Dst *, while a decrease produces a Dst * recovery. As the storm recovery proceeds, a decrease in the electric field results in a conversion of open to closed drift paths, forming a trapped, symmetric ring current that dominates Dst *. The mostly H + composition of the ring current for all three storms rules out the possibility of differential charge exchange being the cause of the fast and slow decay timescales, confirming that outflow is the main loss of ring current‐generated Dst * during the early phase decay. The slow decay timescale in the late recovery, however, is dominated by charge exchange with the hydrogen geocorona. The symmetric‐asymmetric ring current is also placed in the context of the solar wind and plasma sheet drivers.