Simultaneous ground-satellite observations of meso-scale auroral arc undulations

[1] We present simultaneous ground-based and in situ measurements of a train of meso-scale (about 100–300 km) auroral arc undulations, occurring in the postmidnight sector (∼1 MLT) between 0040 UT and 0054 UT on September 21, 2009. The undulations appeared at the auroral poleward boundary, and then moved eastward with a speed of 0.9–2.2 km s−1. Dynamic behaviors of the associated meso-scale ionospheric plasma flows and current systems were also detected with the ground-based magnetometer and radar measurements within the all-sky camera field-of-view. During the interval of interest, simultaneous Cluster observations in the central near tail region (11–14 RE down tail) were available, and especially the ionospheric footprint of Cluster 2 (CL2) was close to the optical auroral forms. CL2 observed strong fluctuations in the in situ magnetic field with amplitude of 5–10 nT whenever a bright arc area, and its trailing adjacent area, of the auroral undulations passed its ionospheric footprint. Such in situ magnetic field changes at CL2 could be considered as a manifestation of localized upward and downward field-aligned current sheets moving eastward at the central near-Earth tail boundary, linked to the meso-scale auroral undulation structures.