Observation of directional change of core field inside flux ropes within one reconnection diffusion region in the Earth's magnetotail

Two consecutive magnetic flux ropes, separated by less than 30 s (Delta t < 30 s), are observed within one magnetic reconnection diffusion region without strong guide field in the Earth's magnetotail by Cluster multi-spacecraft. The flux ropes are characterized by bipolar signatures of the south-north magnetic field component B (z) accompanied with strong core magnetic field B (y) , intense current J and density depletions inside of them. In spite of the small but non-trivial global scale negative guide field (-B (y) ), there exists a directional change of the core fields of two flux ropes, i.e., -B (y) for the first one, and +B (y) for the second one. The directions of the core fields are the same as the ambient cross-tail magnetic field component (B (y) ) just outside of flux ropes. Therefore, we suggest that the core field of flux ropes is formed by compression of the local preexisting B (y) and that the directional change of core field is due to the change of local preexisting B (y) . Such a change in ambient B (y) might be caused by some microscale physics.