Thin current sheet formation in response to the loading and the depletion of magnetic flux during the substorm growth phase

The physics of geomagnetic substorms has been under debate for a long time. Although the growth phase has received less attention than the spectacular expansion phase, the state of the magnetotail at the end of the growth phase, i.e., a thin current sheet (CS) formation is of central importance because it provides the magnetotail conditions for the expansion phase onset. This study investigates CSs evolution during the substorm growth phase. The growth phase physics considered here includes the removal of closed magnetic flux in the near‐Earth tail and the addition of magnetic flux into the tail lobes. It is found that two separate CSs evolve at distinct locations in the near‐Earth and the midtail region in response to the depletion and the loading of magnetic flux. The relative current densities in the dual‐current‐sheet configuration depends strongly on whether the depleted or the added magnetic flux dominates. The presented results suggest that substorm expansion onset may be associated only with reconnection onset in the near‐Earth CS, while midtail reconnection causes bursty bulk flows or bubbles. Key Points Growth phase physics includes the removal and the addition of magnetic flux Two separate CSs evolve in the near‐Earth and the midtail regions Expansion onset may be associated only with near‐Earth reconnection onset