Magnetic Flux Rope with a Complicated Core Field by a Cluster Case

The Cluster spacecraft observed a magnetic reconnection event at the Earth magnetotail during 09:50 UT-10:50 UT on 28 August 2002. Cluster observed an evident quadrupole structure of the Hall magnetic field near the magnetic reconnection diffusion region. Due to the existence of the guide field, this quadrupole structure was distorted. During this reconnection event, Cluster observed a few magnetic flux ropes, and the polarity of the core field of most magnetic flux ropes is consistent with the polarity of the guiding field. However, one particular magnetic flux rope has a more complex structure of the core field than the others. In the core region of the flux ropes, the intensity peaks of the core field appeared there and its polarity is agreement with the guide field. Outside the core region, the polarity of the core field is opposite to that of the guide field. This complex structure of the core field within a flux rope has not been reported until now. Using the method of Minimum Directional Derivative (MDD), it is found that the flux rope was curved at the place where Cluster passed through. C1 and C3 satellites crossed the curved flux rope and detected the core field contrary to the guide field. C2 and C4 satellites situated on northward of C1 and C3 satellites, only crossed one part of the curved flux rope and thus detected the core field with a polarity consistent with the ambient magnetic field.