Time development of field‐aligned currents, potential drops, and plasma associated with an auroral poleward boundary intensification

[1] We present a detailed case study of the plasma and fields measured by the Cluster spacecraft fleet at the high‐altitude auroral zone (∼3.5 RE) across the plasma sheet boundary layer and into the polar cap. This event, which occurred during quiet geomagnetic conditions (Kp = 1, AE = 50 nT), is of particular interest in that Cluster provides measurements at key instances during the time development of a new large‐scale auroral arc system. Central to the formation of the arc system is the depletion of ionospheric plasma through a region of small‐scale, field‐aligned currents having the properties of Alfvén waves. This depletion occurred prior to the growth of and ultimately bounded a well‐defined equatorward moving, upward and downward current sheet pair. In association with the transverse scales approaching the electron inertial scale, the Alfvénic currents have amplitudes that appear to be attenuated subsequent to the formation of the cavity. Potential structures essentially time invariant over particle transit times (quasi‐static) associated with the current pair are identified and observed to drive a poleward boundary intensification (PBI) identified in coincident IMAGE satellite far ultraviolet measurements. The PBI formed in association with a local thickening of the plasma sheet via the injection of new magnetospheric plasma, which may be the result of a bursty, patchy reconnection process. Estimates of the ionospheric equatorward velocity and thickness of the PBI are consistent with their ionospheric mapped cavity counterparts, suggesting that the motion and thickness are controlled by the plasma and electrodynamic features at or above the altitude sampled by Cluster. The magnitude of the upward and downward current region parallel potentials is correlated with the temperature of the newly injected electrons suggesting that the electron temperature is an important controlling factor. These novel observations indicate that quasi‐static systems of field‐aligned currents do form out of the highly dynamic Alfvénic region at the plasma sheet boundary layer, and perhaps suggest that the Alfvénic region can be the initial stage in the development of quasi‐static systems. The observed time sequence of the currents is qualitatively similar to the expectations of transient response models of magnetospheric‐ionospheric coupling, however, such models may need to be modified to account for the attenuation of electron inertial scale currents/Alfvén waves.