On the 3‐dimensional structure of plasmoids

We examine the magnetic topology that results from reconnection at a new near‐earth neutral line when the magnetic field in the plasma sheet has a finite dawn‐dusk component. A dawn‐dusk magnetic field component has been observed in the plasma sheet which correlates with the IMF direction. If we include such a component in the standard model, the resulting magnetic structures are very different from the conventional plasmoid model which (with the exception of Hones et al. 1982) has invariably been drawn as a two‐dimensional structure in the noon‐midnight meridional plane. In the three‐dimensional model, reconnected plasma sheet field lines form a magnetic flux‐rope like structure which stretches across the tail but whose ends are connected to the auroral ionosphere, one end in each hemisphere. When open lobe field lines reconnect at the near‐earth neutral line, they form loops of flux that are topologically linked around the flux rope and which will tend to pull the flux rope tailwards. Nevertheless the flux ropes remain attached to the earth until further reconnection near the flanks of the tail releases them. The particle signatures expected in such structures are similar to those observed in plasmoids. Furthermore, the model explains other magnetic signatures seen in the tail, suggesting that plasmoids are one of a complete spectrum of magnetic structures that can arise from near‐earth reconnection.