Evaluation of twist helicity of flux transfer event flux tubes

Abstract

Flux transfer events are increasingly being viewed as the result of time‐dependent localized reconnection at the dayside magnetopause. As such, it is necessary to consider the three‐dimensional nature of the reconnection process. While the microphysics of reconnection is still somewhat uncertain, information about the final state of the reconnection process can be obtained by considering quantities which are conserved through the reconnection process. One such quantity is the magnetic helicity, which is related to the topology of magnetic field lines. The magnetic helicity of a closed magnetic flux tube can be due to the internal structure of the flux tube, such as the twist of field lines in the flux tube or kinks or knots in the flux tube itself, or due to external conditions, such as the linkage of the flux tube with another flux tube. Three‐dimensional magnetic reconnection can only redistribute internal and external magnetic helicity while nearly conserving the total helicity (Berger, 1982); therefore a twisting of the flux tube can be obtained by transforming the external (mutual) helicity due to magnetic field linkage or by releasing the internal helicity due to kinking of the flux tube through the reconnection process. An evaluation of twist helicity for previous flux transfer event models is presented. The theory of three dimensional reconnection is briefly discussed. Three‐dimensional magnetic reconnection is defined as a self‐organized evolution process of a non equilibrium dissipative system. Flux transfer events are characterized by a twisted flux tube containing highly correlated fluctuating velocity and magnetic fields. It is suggested that flux transfer events represent a mixed state, with a minimum energy state combined with a maximum cross helicity state.