Magnetic helicity of a flux rope in the magnetotail: THEMIS results

Abstract. The magnetic field in many regions of magnetosphere has a complex topological structure. As a parameter to measure the topological complexity, the concept of magnetic helicity is a useful tool in magnetospheric physics. Here we present a case study of magnetic helicity in the flux rope (FR) in the near-Earth plasma sheet (PS) based on the in-situ observation from THEMIS for the first time. With the help of the Grad-Shafranov reconstruction technique, we determine the spatial distribution of magnetic field and evaluate the magnetic helicity in the flux rope. The conservation of magnetic helicity during multiple X-line reconnections and the transport of magnetic helicity between different magnetic field configurations are also discussed. The further application of helicity in magnetosphere will provide us more knowledge about the topologic property of the magnetic fields there and more attention should be paid to that.