Far Tail (255 Re) Fast Response to Very Weak Magnetic Activity

We present an original study of the dynamical changes measured in the far tail at XGSM~ -255 RE, onboard STEREO-B, related to very weak substorm activity (AE<100 nT). Three weak auroral electrojet perturbations are well correlated with motions of the far tail in which the spacecraft passes from the lobe to the boundary layer or to the magnetosheath. These boundary motions can hardly be related to a plasmoid as a widening of the tail is expected from such a high pressure structure. Furthermore, for one of the AE enhancements, ground measurement of auroral luminosity provided a precise timing of the substorm onset, thus allowing to estimate the propagation speed of the tail disturbance, supposing it is initiated at 15-20 RE in the mid-tail. The computed velocity, 1805 km/s, much greater than the typical plasmoid propagation velocity implies that the tail disturbance is due to a large scale wave propagating inside the lobe, initiated inside the inner plasma sheet at substorm onset and linked to current disruption and/or magnetic reconnection.