Geomagnetic storm particles in the high-latitude magnetotail.

Nearly monoenergetic positive ions flowing outward along magnetic-field lines in the high-latitude magneto tail at ∼18 RE, outside the plasma sheet, have been observed with Vela satellites. These ions, probably mainly protons, are detected only during geomagnetic storms. The ‘storm particles’ have average energies per charge ranging from about 0.3 to 3 kv, but at any instant the energy distribution is quite narrow, sometimes <10%. Their angular distribution is usually narrow, sometimes ∼6°. Particles with storm-particle characteristics are not observed in the plasma sheet. Storm-particle fluxes cannot be determined with high accuracy because of their narrow energy distribution, but they are estimated to reach 105 particles cm−2 sec−1 or more. Possible sources of the storm particles are considered, including the solar wind, magnetosheath, polar cusps, polar wind or ionosphere, plasma sheet, solar neutral hydrogen streams, reconnection transfer of plasma into the magneto tail from the polar cusps, polar wind, or polar ionosphere, and parallel electric-field acceleration of the polar wind or ionosphere ions along polar-cap magnetic-field lines. Of these possibilities, the electric-field acceleration is favored. No matter what their source, storm particles appear to be a new feature of geomagnetic storms.