Role of ionospheric effects and plasma sheet dynamics in the formation of auroral arcs

At the ionospheric level, the substorm onset (expansion phase) is marked by the initial brightening and subsequent breakup of a pre-existing auroral a rc. According to the field line resonance (FLR) wave model, the substorm-re lated auroral arc is caused by the field-aligned current carried by FLRs. T he FLRs are standing shear Alfven wave structures that are excited along th e dipole/quasi-dipole lines of the geomagnetic field. The FLRs (that can ca use auroral arc) thread from the Earthward edge of the plasma sheet and lin k the auroral arc to the plasma sheet region of 6-15 R-E. The region is ass ociated with magnetic fluctuations that result from the nonlinear wave-wave interactions of the cross-field current-instability. The instability (exci ted at the substorm onset) disrupts the cross-tail current which is built u p during the growth phase of the substorms and results in magnetic fluctuat ions. The diversion of the current to polar regions can lead to auroral arc intensification. The current FLR model is based on the amplitude equations that describe the nonlinear space-time evolution of FLRs in the presence o f ponderomotive forces exerted by large amplitude FLRs (excited during subs torms). The present work will modify the FLR wave model to include the effe cts arising from magnetic fluctuations that result from current disruption near the plasma sheet (6-15 R-E). The nonlinear evolution of FLRs is couple d with the dynamics of plasma sheet through a momentum exchange term (resul ting from magnetic fluctuations due to current disruption) in the generaliz ed Ohm's law. The resulting amplitude equations including the effects arisi ng from magnetic fluctuations can be used to study the structure of the aur oral arcs formed during substorms. We have also studied the role of feedbac k mechanism (in a dipole geometry of the geomagnetic field) in the formatio n of the discrete auroral arc observed on the nightside magnetosphere. The present nonlinear dispersive model (NDM) is extended to include effects ari sing from the low energy electrons originating from the plasma sheet bounda ry layer. These electrons increase the ionospheric conductivity in a locali zed patch and enhance the field-aligned current through a feedback mechanis m. The feedback effects were studied numerically in a dipole geometry using the the NDM. The numerical studies yield the magnitude of the field-aligne d current that is large enough to form a discrete auroral arc. Our studies provide theoretical support to the observational work of Newell et al. that the feedback instability plays a major role in the formation of the discre te auroral arcs observed on the nightside magnetosphere.