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Keywords:

  • dipolarization;
  • magnetic reconnection;
  • retreat;
  • substorm

[1] Magnetic reconnection is the key process of plasma transport in the Earth's magnetotail. The ‘X-line’ where magnetic field lines reconnect often moves away from the Earth. However, the precise cause of the X-line motion remains unclear. Here we present data from five THEMIS probes positioned along the Sun-Earth line and show that a tailward retreat motion of the X-line (detected by the outermost probe P1) occurred when the dipolarized inner magnetosphere started to return to a more stretched, tail-like configuration (observed by the inner probes P3, P4, and P5). At an intermediate location (P2), the total pressure was increasing. These observations are consistent with the idea that the pressure increase in the inner magnetosphere eventually causes the X-line to retreat tailward.