Magnetotail flows can consume as much solar wind energy as a substorm

We examine an event on 17 December 1997, during which the Wind and ACE spacecraft measured an extended period of southward interplanetary magnetic field. The high values of the epsilon parameter were interpreted as strong energy input into the magnetotail. Despite this energy input, the polar cap potential and area remained constant over a period of several hours, the inner magnetotail was very quiet, and there were no substorm signatures either in the magnetotail or in the ionosphere. Comparison of data and MHD simulation results show that the energy input to the magnetosphere was enhanced and that this event was not a case where the solar wind monitor would have trouble predicting the interplanetary field reaching the Earth orbit. Both Geotail observations in the tail and the MHD simulations show that a substantial amount of energy was being consumed in the tail flow activity during this period. Order of magnitude estimates indicate that the flows indeed were sufficient to consume the incoming energy in a quasi-continuous even if bursty manner. Observations from the IMAGE meridional magnetometer network reveal that the substorm that followed several hours later was relatively small, and its size was proportional to the energy input after the substorm onset, not to the total energy input since the southward turning of the interplanetary field. It is argued that the continuous flow activity disturbed the formation of the thin current sheet in the inner magnetotail, which led to the delay in the substorm development. However, the controlling factor which finally led to the global instability remains an open issue.