Abstract
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.
Highlights
Helicity integral (Moffatt, 1969) as a parameter to measure the complexity of the topology of a curve has been used to study the structures of vortex line, streamline and DNA (Pohl, 1980)
The study of magnetic helicity has a history of half century and has been broadly applied to magnetic fields in space: remote sensing observation and simulation were performed for the solar active region (Pevtsov et al, 2003), while modeling and in-situ observations have been performed for magnetic clouds in the interplanetary space (Dasso et al, 2003; Hu and Dasgupta, 2005)
This work is based on the THEMIS-C spacecraft (THC) (Angelopoulos et al, 2008) performing in-situ observation in the near-Earth plasma sheet
Summary
Helicity integral (Moffatt, 1969) as a parameter to measure the complexity of the topology of a curve has been used to study the structures of vortex line, streamline and DNA (Pohl, 1980). Magnetic helicity over a volume V is defined as H = A · BdV (Berger and Field, 1984), where. Magnetic field lines in magnetotail become flattened, and especially those in the plasma sheet lean to the equator and are sheared (Shen et al, 2007, 2008). The magnetic field lines with a complicated topology in many key regions of the magnetosphere serve the helicity study. The complexity of the magnetic field lines in these regions has been less investigated so far, and our works aim to apply the concept of magnetic helicity to the magnetosphere. We present a study of the magnetic helicity in a flux rope which is suited for the helicity study due to its twisted field line geometry. This work is based on the THEMIS-C spacecraft (THC) (Angelopoulos et al, 2008) performing in-situ observation in the near-Earth plasma sheet
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