A preliminary exploration of the mechanism for the occurrence of two types of various magnetic structures in the magnetotail

[Abstract] Based on a previous two-dimension and three-component MHD numerical simulation, this paper analyses the evolution of magnetic helicity of various magnetic structures in the magnetotail. The results indicate that, in a driven magnetic reconnection process generated by the dawn-dusk electric field in the magnetotail, the transportation of magnetic helicity flux via the boundaries of system is the direct cause of the change of the total magnetic helicity in the system. The various initial distributions of magnetic helicity density and the transportation of magnetic helicity flux may lead to various evolutions of magnetic helicity density in the neutral sheet region, and could result in the formation of various magnetic structures.

The Evolution of Magnetic Helicity of Various Magnetic Structures in the Magnetotail

Cui¹,

Jin²

2002

Numerical Study on the Earthward Propagating Plasmoids

Yang¹,

Jin²

2002

The correlation between plasmoids and geomagnetic substorms has been demonstrated by satellite observations. In addition to the tailward propagating plasmoids with N‐S bipolar signatures, earthward propagating plasmoids are found, which are characterized by plasma sheet S‐N bipolar events and lobe S‐N bipolar signatures. The occurrence rate of the S‐N bipolar events is much lower than that of the N‐S events. The S‐N bipolar events occur primarily during quiet geomagnetic and IMF Bz north period, and they are correlated with small, isolated geomagnetic substorms. A 2.5 dimensional MHD simulation on the earthward propagating plasmoids has been carried out, on base of the specific distribution of the cross‐tail electric fields Ey during geomagnetically quiet times (IMF Bz is northward and | By |≥ Bz). The simulation results of the two cases showed the major features of the flux rope magnetic structures and plasmoids with complex closed magnetic field lines. The similar signatures to tailward propagating plasmoids could be consistent with IMP 8 observations about the earthward propagating plasma sheet S‐N bipolar events. The results displayed a similar configuration to the schema given by Schindler which may explain the low occurrence frequency of the S‐N events, and also showed that the dynamic growth of the magnetic reconnection becomes considerably suppressed for a strong cross‐tail magnetic field By. From this simulation it could be concluded that the magnetic reconnection can be the genetic mechanism of the plasma acceleration and heating in the magnetotail in both geomagnetically active and quiet time, namely, under both southward and northward (with | By |≥ Bz) IMF conditions.

The Core Field Enhancement Caused by Pressure Loss in the Flux Rope Structures in the Magnetotail

Jin¹,

Yang²

2004

The plasmoids in the Earth's magnetotail, which are closely correlated with magnetospheric substorms, might be the structures with various magnetic topologies. Most plasmoids are flux rope like structures ith large core fields. The MHD simulations show that the occurrence of various magnetic structures in the magnetotail might be related to different distributions of the cross‐tail component By. However, the core field strength of flux rope structures shown in the previous simulations is much lower than that in the ambient lobe. In this paper, we consider that the pressure loss caused by extension of helical field lines to the flank regions and introduce the modificative terms to the MHD equations. In two cases with uniform initial By distribution, the recurrent formation and successive ejection of flux rope structures are illustrated under the action of plasma inflow caused by the dawn‐dusk electric field. It is noticeable that in the cases with the modificative terms, a reduction of the averaging magnitude of peak pressure values and a considerable enhancement of the averaging strength of core magnetic field are exhibited for the multiple magnetic structures. Moreover, in case 1 with a lower background By (By0 = 1nT) the relative amplitude of core field By enhancement is larger than that in case 2 (By0 = 2nT).