Dual Current Sheet Formation in the Earth's Magnetotail: Implications for Substorm Growth and Expansion

Characteristic for the substorm growth phase is enhanced dayside reconnection with the implications of lowering the total closed magnetic flux and increasing the open flux in the tail lobes. It is demonstrated that the depletion of closed flux in the near Earth magnetotail and the increase of open lobe magnetic flux can lead to the evolution of two separate very thin current sheets in the near Earth and the mid tail regions of the magnetosphere. The relative intensity and width of these current sheets depends on the details of the flux circulation and additional solar wind properties. Both of these current sheets can lead to the onset of magnetic reconnection. We discuss the physics of the dual current sheet formation and implications for the properties of magnetic reconnection in either current sheet. It is suggested that only reconnection onset in the near Earth current sheet may be consistent with substorm expansion because the flux tube entropy depletion of mid tail reconnection appears insufficient to cause geosynchronous particle injection and dipolarization. Therefore reconnection in the mid tail current sheet is more likely associated with flux tube entropy depleted bursty bulk flows.