A simple model of core field generation during plasmoid evolution

Bipolar magnetic field signatures in the far magnetotail observed by the ISEE 3 spacecraft are commonly interpreted as signatures of a passing magnetic bubble, or plasmoid. A large number of such plasmoid-type variations in the north-south component of the magnetic field are accompanied by large core magnetic fields which are directed primarily in the cross-tail direction, indicating a flux rope like structure. Similar signatures are also found in a recent examination of GEOTAIL deep tail data. The fact that more of these flux ropelike plasmoids are encountered in the far tail than closer to the Earth raises the question whether they are the result of an evolution from no or low core fields to high core fields or whether plasmoids without core fields and flux ropes are entirely different entities. We present a model which explains the evolution of a looplike plasmoid in the near tail to a thinner flux rope in the far tail. The transition is accomplished by magnetic reconnection, which progressively connects the plasmoid magnetic field lines to the colder plasma in the low-latitude boundary layer and magnetosheath. The connection leads to a draining of hot plasma from plasmoid field lines and a subsequent collapse due to the plasma pressure reduction. The collapse causes a strong enhancement of any preexisting cross-tail magnetic field component, until a quasi-force-free state is reached. We also present MHD simulations to demonstrate the process. Last, we show that this mechanism call produce core field enhancements beyond the ambient lobe field strength.