Cross-tail current, field-aligned current, and By

Richard L. Kaufmann, Chen Lu, and Douglas J. Larson

Department of Physics, University of New Hampshire, Durham

Abstract. Orbits of individual charged particles were traced in a one-dimensional magnetic field model that included a uniform cross-tail component Byo. The effects of on the cross-tail current distribution , the average cross-tail drift velocity , and the average pitch angle change ·DaŅ experienced during current sheet encounters were calculated. The addition of a that exceeded several tenths of one nanotesla com- pletely eliminated all resonance effects for odd-N orbits. An odd-N resonance involves ions that enter and exit the current sheet on the same side. Pitch angles of nearly all such ions changed substantially during a typical current sheet interaction, and there was no region of large cross-tail drift velocity in the presence of a modest . The addition of a very large guide field in the direction that enhances the natural drift produces a large jy and small ·DaŅ for ions with all energies. The addition of a modest Byo had less effect near even-N resonances. In this case, ions in a small energy range were found to undergo so little change in pitch angle that particles which originated in the ionosphere would pass through the current sheet and return to the conjugate ionosphere. Finally, the cross-tail drift of ions from regions dominated by stochastic orbits to regions dominated by either resonant or guiding center orbits was considered. The ion drift speed changed substantially during such transi- tions. The accompanying electrons obey the guiding center equations, so electron drift is more uniform. Any difference between gradients in the fluxes associated with electron and ion drifts requires the presence of a Bir- keland current in order to maintain charge neutrality. This plasma sheet region therefore serves as a current gen- erator. The analysis predicts that the resulting Birkeland current connects to the lowest altitude equatorial regions in which ions drift to or from a point at which stochastic orbits predominate. The proposed mechanism appears only in analyses that include non-guiding-center effects.

Paper Number 94JA00490