Statistical Study on Two Types Of Flux Transfer Events Observed By MMS Spacecraft
The magnetic and plasma structures on the dayside magnetopause are examined with unprecedented detail with the exceptionally high temporal resolution measurements of both magnetic field and plasma properties as well as the capability to disentangle the spatial and temporal variations. This capability is enabled by the closely-formed four-spacecraft-tetrahedron of the Magnetospheric Multiscale (MMS) mission. With this detailed examination using definitive criteria, we have been able to distinguish and identify different current structures in the vicinity of magnetopause, namely: the magnetopause current layer, flux transfer events (FTEs), and the magnetosheath field enhancement (MFE). The criteria are based on not only the direct measured magnetic and plasma parameters, but also the derived quantities including the electric current, radius of curvature of the magnetic field and the magnetic and plasma forces.
Furthermore, with observations from MMS, the detailed properties of flux ropes (FRs) are studied for their better understanding. The geometric information of the FR such as the radius of the cross-sectional plane, impact parameter and axial direction are obtained definitively with multi-spacecraft methods. The core field strength and magnetic flux content are extrapolated based on the derived geometric information and magnetic field measurement. The force-freeness and expansion of the FR are also determined with the calculation of velocity of FR motion and the calculation of forces.
With above detailed analyses, we then show that there are two types of flux ropes, one with mixed plasma originated from two regimes (type A) and the other without the mix of signatures (type B). Then the statistical analysis on these two types of FRs shows that they are basically similar, and thus are the same phenomena. Both of them are FTEs generated by dayside magnetic reconnection and are responsible for the magnetic flux transfer.