Generation of High-Latitude Pi2 Precursors in Growth-Phase (Downward) Field-Aligned Currents

Details Generation of High-Latitude Pi2 Precursors in Growth-Phase (Downward) Field-Aligned Currents Generation of High-Latitude Pi2 Precursors in Growth-Phase (Downward) Field-Aligned Currents

Dec 2007

adsabs.harvard.edu/cgi-bin/nph-data_query?bibcode=2007agufmsm23a1190l&link_type=abstract

American Geophysical Union, Fall Meeting 2007, abstract #SM23A-1190

Physics

2721 Field-Aligned Currents And Current Systems (2409), 2736 Magnetosphere/Ionosphere Interactions (2431), 2740 Magnetospheric Configuration And Dynamics, 2788 Magnetic Storms And Substorms (7954)

Scientific paper

A variety of mechanisms have been proposed to explain the generation of high-latitude Pi2 pulsations preceding the onset of magnetospheric substorms and pseudobreakups. Here we explore a causal relationship between Pi2 signals and field-aligned currents (FACs) flowing into the ionosphere in association with the diversion of the growth-phase cross-tail current. In situ observations of Pi2 precursors measured concurrently with FACs reveal that the diversion of the cross-tail current into FACs occurs before the excitation of Pi2 signals. We show that results from computer simulations of large-scale FACs and Alfven waves interacting with a reactive ionosphere, including realistic magnetospheric and ionospheric plasma and magnetic inhomogeneities, exhibit a similar causal sequence. The diverted FAC need not be oscillatory, although we consider the implications of both monotonically increasing FAC diversion as well as oscillatory currents increasing in amplitude. Of particular interest is the dynamic behavior of the downward-diverted FAC which depletes the E-region and bottomside ionospheric plasma density. At a critical value, the depletion promotes a spontaneous feedback instability which establishes fast ionospheric Alfven resonator oscillations at low altitudes together with growing, pre-onset Pi2 pulsations that extend upward into the plasmasheet while propagating across L shells in the direction of the electric field and toward the region of upward FACs. Although the Pi2 waves are initiated in the downward current channel, they modulate auroral precipitation and auroral lumninosity because the intense Pi2 FACs achieve amplitudes that typically require field-aligned potential drops to satisfy current continuity.

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