Diagnostics of Closed Magnetic Flux Depletion in the Near-Earth Magnetotail During the Substorm Growth Phase

Based on magnetohydrodynamics simulations, it was recently suggested that magnetotail reconfiguration during the substorm growth phase results from combined action of two large-scale processes: (1) open magnetic flux accumulation (OMFA) in the tail lobes and (2) closed magnetic flux depletion (CMFD) in the near-Earth tail, caused by flux evacuation to the dayside magnetopause. Simultaneous action of uniform along the tail OMFA and strongly nonuniform CMFD leads to different rates of magnetic flux growth in different tail cross sections. According to Global magnetohydrodynamics simulations CMFD is about 35% of OMFA, the corresponding differences of magnetic flux increase in two-tail cross sections at -7R(E) and -20R(E) being as large as 0.1-0.2 GWb. To study this effect on real data, we applied the recent method of magnetotail flux estimation (Shukhtina et al., ) to simultaneous Cluster and Geotail observations in the tail lobes. By finding such rare observations in the inner and middle magnetotail (at -10 R-E and - 21R(E)) we confirm a larger magnetic flux increase in the midtail, with the ratio between OMFA and CMFD comparable to that obtained in magnetohydrodynamics, although with a large scatter. These results confirm the scenario of two decoupled magnetotail regions with different convection regimes during the growth phase. Convection is depressed in the midtail plasma sheet, showing the pressure crisis in action inherent to tail-like 2-D magnetic configuration. At the same time convection is substantially enhanced in the inner magnetosphere, where the pressure crisis is solved due to azimuthal flux transport around the Earth, essentially a 3-D effect. Plain Language Summary The mechanism of magnetospheric substorms is one of the key problems in magnetospheric physics. Magnetosphere is prepared to substorm onset during the growth phase, which is characterized by magnetotail reconfiguration. For a long time this reconfiguration was completely attributed to magnetic flux accumulation in the tail lobes due to magnetic reconnection on the dayside magnetopause. Recently, another mechanism of the inner tail reconfiguration, caused by enhanced convection in the inner magnetosphere, was suggested and verified on MHD simulations. Here we demonstrate this mechanism on real data, using observations of two separated spacecraft in the magnetotail.