Temporal development of field-aligned current systems near and within the plasma sheet boundary layer observed by Cluster

We present Cluster multi-point observations at key instances within and above the acceleration region (> 3 Re) of auroral arc current systems. We will primarily focus attention on the temporal development of field-aligned current systems near and within the plasma sheet boundary, associated potentials, and the reorganization of the source and ionospheric plasma that transpire in such systems. We show examples that indicate current system formation/reconfiguration is a process associated with the occurrence of small-scale, field-aligned currents having the properties of Alfven waves. At the plasma sheet boundary layer this may be manifested as the so-called Alfven region. Well-defined, quasistationary upward-downward current sheet paired systems are observed to grow out of such Alfven wave dominated systems of small-scale, field-aligned currents in the span of a few minutes. Characteristic of this evolutionary process is the attenuation of the Alfvenic transients coincident with the growth of the large-scale current system. A preliminary evaluation of one example revealed that the attenuation was associated with the transverse scales approaching the electron inertial scale, indicating the Alfvenic currents became dispersive as the quasistatic arc system developed and thus were susceptible to damping. The data are qualitatively consistent with the predictions of transient response models for magnetospheric-ionospheric coupling. However, our results suggest these models need to be modified to account for dissipation along magnetic field-line for sufficiently small-scale structures.