Physics Department - University of Alaska



Slow Quasi-Static Convection of Magnetic Flux Tubes in the
Magnetosphere and its Implications for Current Sheet Thinning in the Late Growth Phase of Magnetospheric Substorms


Fred Hall

Physics Deptartment/GI, UAF



The magnetospheric substorm is a period of enhanced interaction between the solar wind and the magnetosphere. A substorm can be sub-divided into three phases: growth, expansion, and recovery. During the growth phase, energy is transferred from the solar wind and stored in the magnetotail lobes of the magnetosphere. That stored energy is released into the auroral ionosphere and down the magnetotail during the expansion phase. The magnetosphere returns to a `quasi-equilibrium' state during the recovery phase. The transition in the nature and dynamics of the magnetosphere as it progresses from the growth phase to the expansion phase is an area of ongoing research. The thinning of the current sheet in the near-Earth magnetotail, observed toward the end of the growth phase, is interesting since it establishes the conditions for the onset of the substorm expansion phase. Erickson and Wolf (1980) were the first to identify entropy and mass conservation constraints imposed on the slow quasi-static convection of magnetic flux tubes from the mid- and far-tail regions. We propose that these constraints can be used to show that the region of current sheet thinning corresponds to a `reservoir' of magnetic flux in the near-Earth magnetotail. Magnetic flux tubes convecting from this `flux reservoir' replenish magnetic flux eroded at the dayside magnetosphere during the growth phase. We argue that the depletion of this `reservoir' leads to the observed current sheet thinning. The aforementioned entropy and mass conservation constraints will be discussed. We present results from a plausibility study showing the mapping of the region of dayside magnetopause erosion to the region where current sheet thinning is observed. We describe a three-dimensional self-consistent MHD simulation of the proposed mechanism for current sheet thinning. The modeled configuration of the magnetosphere will be discussed.


Friday, Dec. 3, 2004
Globe Room, Elvey building
3:45 pm