Flux transfer events: 1. Generation mechanism for strong southward

Abstract. We use a global numerical model of the interac-tion of the solar wind and the interplanetary magnetic field with Earth’s magnetosphere to study the formation process of Flux Transfer Events (FTEs) during strong southward IMF. We find that: (i) The model produces essentially all obser-vational features expected for FTEs, in particular the bipolar signature of the magnetic field BN component, the correct polarity, duration, and intermittency of that bipolar signa-ture, strong core fields and enhanced core pressure, and flow enhancements; (ii) FTEs only develop for large dipole tilt whereas in the case of no dipole tilt steady magnetic recon-nection occurs at the dayside magnetopause; (iii) the basic process by which FTEs are produced is the sequential gener-ation of new X-lines which makes dayside reconnection in-herently time dependent and leads to a modified form of dual or multiple X-line reconnection; (iv) the FTE generation pro-cess in this model is not dependent on specific assumptions about microscopic processes; (v) the average period of FTEs can be explained by simple geometric arguments involving magnetosheath convection; (vi) FTEs do not develop in the model if the numerical resolution is too coarse leading to too much numerical diffusion; and (vii) FTEs for nearly south-ward IMF and large dipole tilt, i.e., near solstice, should only develop in the winter hemisphere, which provides a testable prediction of seasonal modulation. The semiannual mod-ulation of intermittent FTE reconnection versus steady re-connection is also expected to modulate magnetospheric and ionospheric convection and may thus contribute to the semi-annual variation of geomagnetic activity