The influence of magnetic flux depletion on the magnetotail and auroral morphology during the substorm growth phase

Abstract

AbstractThe substorm growth phase is characterized by the equatorward motion of the growth phase arc close to or even into the region of diffuseaurora characteristic for a dipolar magnetic field. The presented study examines changes of the near‐Earth current sheet and of the mapping of magnetotail feature into the auroral ionosphere based on midnight magnetic flux depletion (MFD) in the near‐Earth tail. Midnight MFD is caused by sunward convection to replenish magnetic flux that is eroded on the dayside by magnetic reconnection during periods of southward interplanetary magnetic field. The results demonstrate that MFD causes the formation of a very thin current sheet in the near‐Earth tail. It is found that the removal of magnetic flux in the near‐Earth tail causes a contraction of the ionospheric footpoints of this tail region such that all of the mapped magnetotail structures move equatorward by about 2 to 3°. The thin current layer is mapped into the region where magnetic flux is strongly depleted and in close proximity with strong and narrow regions 1 and 2 sense field‐aligned currents. The ionospheric maps show a sharp transition between the dipole and stretched magnetic field and an evolution of thinning and convergent motion of field‐aligned currents in the late growth phase. The results are obtained without loading of magnetic flux and energy into the tail lobes, demonstrating that many typical growth phase properties can be attributed to the depletion of near‐Earth closed magnetic flux.