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Nature 336, 231 - 232 (17 November 1988); doi:10.1038/336231a0

Large plasma velocities along the magnetic field line in the auroral zone

G. O. L. Jones*, P. J. S. Williams*, K. J. Winser, M. Lockwood & K. Suvanto

*Physics Department, University College of Wales, Aberystwyth SY23 3BZ, UK
Rutherford-Appleton Laboratory, Chilton, Didcot OX11 0QX, UK
Imperial College of Science and Technology, London SW7 2AZ, UK

In the auroral zone, ionospheric plasma often moves horizontally at more than 1 km s−1, driven by magnetospheric electric fields, but it has usually been assumed that vertical velocities are much smaller. On occasions, however, plasma has been seen to move upwards along the magnetic field line at several hundred m s−1. These upward velocities are associated with electric fields strong enough to heat the ion population and drive it into a non-thermal state1,2. Here we report observations of substantial upwards acceleration of plasma, to velocities as high as 500 m s−1. An initial upthrust was provided by a combined upwelling of the neutral atmosphere and ionosphere but the continued acceleration at greater heights is explained by a combination of enhanced plasma pressure and the 'hydrodynamic mirror force'3. This acceleration marks an important stage in the transport of plasma from the ionosphere into the magnetosphere.



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