Abstract
Abstract. Using three-dimensional magnetohydrodynamic simulations, we investigate the propagation of low-entropy magnetic flux tubes ("bubbles") in the magnetotail. Our simulations address fundamental properties of the propagation and dynamics of such flux tubes rather than the actual formation process. We find that the early evolution, after a sudden reduction of pressure and entropy on a localized flux tube, is governed by re-establishing the balance of the total pressure in the dawn-dusk and north-south directions through compression on a time scale less than about 20s for the typical magnetotail. The compression returns the equatorial pressure to its original unperturbed value, due to the fact that the magnetic field contributes only little to the total pressure, while farther away from the equatorial plane the magnetic field compression dominates. As a consequence the pressure is no longer constant along a flux tube. The subsequent evolution is characterized by earthward propagation at speeds of the order of 200-400km/s, depending on the initial amount of depletion and the cross-tail extent of a bubble. Simple acceleration without depletion does not lead to significant earthward propagation. It hence seems that both the entropy reduction and the plasma acceleration play an important role in the generation of fast plasma flows and their propagation into the near tail. Earthward moving bubbles are found to be associated with field-aligned current systems, directed earthward on the dawnward edge and tailward on the duskward edge. This is consistent with current systems attributed to observed bursty bulk flows and their auroral effects.Key words. Magnetospheric physics (magnetospheric configuration and dynamics; magnetotail; plasma sheet)nguage:
Highlights
The transport of plasma through the magnetosphere, and the magnetotail, governs magnetospheric structure as well as its dynamics
Consistent with a lack of significant steady transport, observations show that the plasma flow speed in the magnetotail is highly variable, with brief periods of fast plasma flow providing much of the sunward transport of mass, energy, and magnetic flux (e.g. Baumjohann et al, 1990; Angelopoulos et al, 1992)
We fully investigate the initial phase after the depletion, which was bypassed by Chen and Wolf by assuming that pressure equilibrium in y and z was re-established, while the pressure remained constant along the depleted field line
Summary
The transport of plasma through the magnetosphere, and the magnetotail, governs magnetospheric structure as well as its dynamics. Steady-state models of such transport or convection for realistic tail configurations are shown to be inconsistent with the combined conditions of frozen-in magnetic flux, and mass and entropy conservation (Erickson and Wolf, 1980; Schindler and Birn, 1982). It appears that the depletion of closed magnetic flux tubes by some process(es) plays a crucial role in permitting their transport from higher to lower latitudes or from the distant to the closer tail. We further consider the effects of anisotropy through a double-adiabatic approach and investigate the role of the cross-tail width of the depleted flux tube and the mechanism of generation of field-aligned currents
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