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Ann. Geophys., 22, 1773-1786, 2004
www.ann-geophys.net/22/1773/2004/
© European Geosciences Union 2004
On the propagation of bubbles in the geomagnetic tail
J. Birn1, J. Raeder2,5, Y. L. Wang1,2, R. A. Wolf3, and M. Hesse4
1Los Alamos National Laboratory, Los Alamos, New Mexico, USA
2University of California, Los Angeles, USA
3Rice University, Houston, Texas, USA
4NASA Goddard Space Flight Center, Greenbelt, Maryland, USA
5University of New Hampshire, Durham, New Hampshire, USA
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:
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