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

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 R E 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.